Nitrous oxide

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Nitrous_oxide 

Not to be confused with nitric oxide (NO), nitrogen dioxide (NO
₂), or generic nitrogen oxide pollutants NOx.

 

Nitrous oxide

Names
IUPAC name Nitrous oxide
Other names Laughing gas, sweet air, protoxide of nitrogen, hyponitrous oxide, dinitrogen oxide, dinitrogen monoxide
Identifiers
CAS Number	10024-97-2
3D model (JSmol)	Interactive image Interactive image
Beilstein Reference	8137358
ChEBI	CHEBI:17045
ChEMBL	ChEMBL1234579
ChemSpider	923
DrugBank	DB06690
ECHA InfoCard	100.030.017
E number	E942 (glazing agents, ...)
Gmelin Reference	2153410
KEGG	D00102
PubChem CID	948
RTECS number	QX1350000
UNII	K50XQU1029
UN number	1070 (compressed) 2201 (liquid)
CompTox Dashboard (EPA)	DTXSID8021066

Properties
Chemical formula	N 2O
Molar mass	44.013 g/mol
Appearance	colourless gas
Density	1.977 g/L (gas)
Melting point	−90.86 °C (−131.55 °F; 182.29 K)
Boiling point	−88.48 °C (−127.26 °F; 184.67 K)
Solubility in water	1.5 g/L (15 °C)
Solubility	soluble in alcohol, ether, sulfuric acid
log P	0.35
Vapor pressure	5150 kPa (20 °C)
Magnetic susceptibility (χ)	−18.9·10−6 cm3/mol
Refractive index (nD)	1.000516 (0 °C, 101,325 kPa)
Viscosity	14.90 μPa·s[2]
Structure
Molecular shape	linear, C∞v
Dipole moment	0.166 D
Thermochemistry
Std molar entropy (So298)	219.96 J/(K·mol)
Std enthalpy of formation (ΔfH⦵298)	+82.05 kJ/mol
Pharmacology
ATC code	N01AX13 (WHO)
Routes of administration	Inhalation
Pharmacokinetics:
Metabolism	0.004%
Biological half-life	5 minutes
Excretion	Respiratory
Hazards
Safety data sheet	Ilo.org, ICSC 0067
GHS pictograms
NFPA 704 (fire diamond)	2 0 0 OX
Flash point	Nonflammable
Related compounds
Related nitrogen oxides	Nitric oxide Dinitrogen trioxide Nitrogen dioxide Dinitrogen tetroxide Dinitrogen pentoxide
Related compounds	Ammonium nitrate Azide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Nitrous oxide, commonly known as laughing gas or nitrous, is a chemical compound, an oxide of nitrogen with the formula N
₂O. At room temperature, it is a colourless non-flammable gas, with a slight metallic scent and taste. At elevated temperatures, nitrous oxide is a powerful oxidiser similar to molecular oxygen.

Nitrous oxide has significant medical uses, especially in surgery and dentistry, for its anaesthetic and pain reducing effects. Its colloquial name "laughing gas", coined by Humphry Davy, is due to the euphoric effects upon inhaling it, a property that has led to its recreational use as a dissociative anaesthetic. It is on the World Health Organisation's List of Essential Medicines, the safest and most effective medicines needed in a health system. It is also used as an oxidiser in rocket propellants, and in motor racing to increase the power output of engines.

Nitrous oxide's atmospheric concentration reached 333 parts per billion (ppb) in 2020, increasing at a rate of about 1 ppb annually. It is a major scavenger of stratospheric ozone, with an impact comparable to that of CFCs. Global accounting of N
₂O sources and sinks over the decade ending 2016 indicates that about 40% of the average 17 TgN/yr (Teragrams of Nitrogen per year) of emissions originated from human activity, and shows that emissions growth chiefly came from expanding agriculture and industry sources within emerging economies. Being the third most important long-lived greenhouse gas, nitrous oxide also substantially contributes to global warming.

Uses

Rocket motors

Nitrous oxide may be used as an oxidiser in a rocket motor. This is advantageous over other oxidisers in that it is much less toxic, and due to its stability at room temperature is also easier to store and relatively safe to carry on a flight. As a secondary benefit, it may be decomposed readily to form breathing air. Its high density and low storage pressure (when maintained at low temperature) enable it to be highly competitive with stored high-pressure gas systems.

In a 1914 patent, American rocket pioneer Robert Goddard suggested nitrous oxide and gasoline as possible propellants for a liquid-fuelled rocket. Nitrous oxide has been the oxidiser of choice in several hybrid rocket designs (using solid fuel with a liquid or gaseous oxidiser). The combination of nitrous oxide with hydroxyl-terminated polybutadiene fuel has been used by SpaceShipOne and others. It also is notably used in amateur and high power rocketry with various plastics as the fuel.

Nitrous oxide also may be used in a monopropellant rocket. In the presence of a heated catalyst, N
₂O will decompose exothermically into nitrogen and oxygen, at a temperature of approximately 1,070 °F (577 °C). Because of the large heat release, the catalytic action rapidly becomes secondary, as thermal autodecomposition becomes dominant. In a vacuum thruster, this may provide a monopropellant specific impulse (I_sp) of as much as 180 s. While noticeably less than the I_sp available from hydrazine thrusters (monopropellant or bipropellant with dinitrogen tetroxide), the decreased toxicity makes nitrous oxide an option worth investigating.

Nitrous oxide is said to deflagrate at approximately 600 °C (1,112 °F) at a pressure of 309 psi (21 atmospheres). At 600 psi, for example, the required ignition energy is only 6 joules, whereas N
₂O at 130 psi a 2,500-joule ignition energy input is insufficient.

Internal combustion engine

In vehicle racing, nitrous oxide (often referred to as just "nitrous") allows the engine to burn more fuel by providing more oxygen during combustion. The increase in oxygen allows for an increase in the injection of fuel, allowing the engine to produce more engine power. The gas is not flammable at a low pressure/temperature, but it delivers more oxygen than atmospheric air by breaking down at elevated temperatures, about 570 degrees F (~300C). Therefore, it often is mixed with another fuel that is easier to deflagrate. Nitrous oxide is a strong oxidising agent, roughly equivalent to hydrogen peroxide, and much stronger than oxygen gas.

Nitrous oxide is stored as a compressed liquid; the evaporation and expansion of liquid nitrous oxide in the intake manifold causes a large drop in intake charge temperature, resulting in a denser charge, further allowing more air/fuel mixture to enter the cylinder. Sometimes nitrous oxide is injected into (or prior to) the intake manifold, whereas other systems directly inject, right before the cylinder (direct port injection) to increase power.

The technique was used during World War II by Luftwaffe aircraft with the GM-1 system to boost the power output of aircraft engines. Originally meant to provide the Luftwaffe standard aircraft with superior high-altitude performance, technological considerations limited its use to extremely high altitudes. Accordingly, it was only used by specialised planes such as high-altitude reconnaissance aircraft, high-speed bombers and high-altitude interceptor aircraft. It sometimes could be found on Luftwaffe aircraft also fitted with another engine-boost system, MW 50, a form of water injection for aviation engines that used methanol for its boost capabilities.

One of the major problems of using nitrous oxide in a reciprocating engine is that it can produce enough power to damage or destroy the engine. Very large power increases are possible, and if the mechanical structure of the engine is not properly reinforced, the engine may be severely damaged, or destroyed, during this kind of operation. It is very important with nitrous oxide augmentation of petrol engines to maintain proper operating temperatures and fuel levels to prevent "pre-ignition", or "detonation" (sometimes referred to as "knock"). Most problems that are associated with nitrous oxide do not come from mechanical failure due to the power increases. Since nitrous oxide allows a much denser charge into the cylinder, it dramatically increases cylinder pressures. The increased pressure and temperature can cause problems such as melting the piston or valves. It also may crack or warp the piston or head and cause pre-ignition due to uneven heating.

Automotive-grade liquid nitrous oxide differs slightly from medical-grade nitrous oxide. A small amount of sulfur dioxide (SO
₂) is added to prevent substance abuse. Multiple washes through a base (such as sodium hydroxide) can remove this, decreasing the corrosive properties observed when SO
₂ is further oxidised during combustion into sulfuric acid, making emissions cleaner.

Aerosol propellant

Food-grade N
₂O whipped-cream chargers

The gas is approved for use as a food additive (E number: E942), specifically as an aerosol spray propellant. Its most common uses in this context are in aerosol whipped cream canisters and cooking sprays.

The gas is extremely soluble in fatty compounds. In aerosol whipped cream, it is dissolved in the fatty cream until it leaves the can, when it becomes gaseous and thus creates foam. Used in this way, it produces whipped cream which is four times the volume of the liquid, whereas whipping air into cream only produces twice the volume. If air were used as a propellant, oxygen would accelerate rancidification of the butterfat, but nitrous oxide inhibits such degradation. Carbon dioxide cannot be used for whipped cream because it is acidic in water, which would curdle the cream and give it a seltzer-like "sparkling" sensation.

The whipped cream produced with nitrous oxide is unstable, however, and will return to a more liquid state within half an hour to one hour. Thus, the method is not suitable for decorating food that will not be served immediately.

During December 2016, some manufacturers reported a shortage of aerosol whipped creams in the United States due to an explosion at the Air Liquide nitrous oxide facility in Florida in late August. With a major facility offline, the disruption caused a shortage resulting in the company diverting the supply of nitrous oxide to medical clients rather than to food manufacturing. The shortage came during the Christmas and holiday season when canned whipped cream use is normally at its highest.

Similarly, cooking spray, which is made from various types of oils combined with lecithin (an emulsifier), may use nitrous oxide as a propellant. Other propellants used in cooking spray include food-grade alcohol and propane.

Medicine

Medical-grade N
₂O tanks used in dentistry

Nitrous oxide has been used in dentistry and surgery, as an anaesthetic and analgesic, since 1844. In the early days, the gas was administered through simple inhalers consisting of a breathing bag made of rubber cloth. Today, the gas is administered in hospitals by means of an automated relative analgesia machine, with an anaesthetic vaporiser and a medical ventilator, that delivers a precisely dosed and breath-actuated flow of nitrous oxide mixed with oxygen in a 2:1 ratio.

Nitrous oxide is a weak general anaesthetic, and so is generally not used alone in general anaesthesia, but used as a carrier gas (mixed with oxygen) for more powerful general anaesthetic drugs such as sevoflurane or desflurane. It has a minimum alveolar concentration of 105% and a blood/gas partition coefficient of 0.46. The use of nitrous oxide in anaesthesia, however, can increase the risk of postoperative nausea and vomiting.

Dentists use a simpler machine which only delivers an N
₂O/O
₂ mixture for the patient to inhale while conscious. The patient is kept conscious throughout the procedure, and retains adequate mental faculties to respond to questions and instructions from the dentist.

Inhalation of nitrous oxide is used frequently to relieve pain associated with childbirth, trauma, oral surgery and acute coronary syndrome (includes heart attacks). Its use during labour has been shown to be a safe and effective aid for birthing women. Its use for acute coronary syndrome is of unknown benefit.

In Britain and Canada, Entonox and Nitronox are used commonly by ambulance crews (including unregistered practitioners) as rapid and highly effective analgesic gas.

Fifty percent nitrous oxide can be considered for use by trained non-professional first aid responders in prehospital settings, given the relative ease and safety of administering 50% nitrous oxide as an analgesic. The rapid reversibility of its effect would also prevent it from precluding diagnosis.

Recreational use

Aquatint depiction of a laughing gas party in the nineteenth century, by Thomas Rowlandson

 

Whippit remnants (the small steel canisters) of recreational drug use, the Netherlands, 2017

Recreational inhalation of nitrous oxide, with the purpose of causing euphoria and/or slight hallucinations, began as a phenomenon for the British upper class in 1799, known as "laughing gas parties".

Starting in the nineteenth century, widespread availability of the gas for medical and culinary purposes allowed the recreational use to expand greatly throughout the world. In the United Kingdom, as of 2014, nitrous oxide was estimated to be used by almost half a million young people at nightspots, festivals and parties. The legality of that use varies greatly from country to country, and even from city to city in some countries.

Widespread recreational use of the drug throughout the UK was featured in the 2017 Vice documentary Inside The Laughing Gas Black Market, in which journalist Matt Shea met with dealers of the drug who stole it from hospitals, although with nitrous oxide canisters being readily available online, the incidents of hospital theft are expected to be extremely rare.

A significant issue cited in London's press is the effect of nitrous oxide canister littering, which is highly visible and causes significant complaint from communities.

Safety

The major safety hazards of nitrous oxide come from the fact that it is a compressed liquefied gas, an asphyxiation risk and a dissociative anaesthetic.

While relatively non-toxic, nitrous oxide has a number of recognised ill effects on human health, whether through breathing it in or by contact of the liquid with skin or eyes.

Nitrous oxide is a significant occupational hazard for surgeons, dentists and nurses. Because nitrous oxide is minimally metabolised in humans (with a rate of 0.004%), it retains its potency when exhaled into the room by the patient, and can pose an intoxicating and prolonged exposure hazard to the clinic staff if the room is poorly ventilated. Where nitrous oxide is administered, a continuous-flow fresh-air ventilation system or N
₂O scavenger system is used to prevent a waste-gas buildup.

The National Institute for Occupational Safety and Health recommends that workers' exposure to nitrous oxide should be controlled during the administration of anaesthetic gas in medical, dental and veterinary operators. It set a recommended exposure limit (REL) of 25 ppm (46 mg/m³) to escaped anaesthetic.

Mental and manual impairment

Exposure to nitrous oxide causes short-term decreases in mental performance, audiovisual ability and manual dexterity. These effects coupled with the induced spatial and temporal disorientation could result in physical harm to the user from environmental hazards. Part of safer use can be to inhale it while seated, because there is an increased risk of injury from falling if you lose consciousness.

Neurotoxicity and neuroprotection

Like other NMDA receptor antagonists, N
₂O was suggested to produce neurotoxicity in the form of Olney's lesions in rodents upon prolonged (several hour) exposure. New research has arisen suggesting that Olney's lesions do not occur in humans, however, and similar drugs such as ketamine are now believed not to be acutely neurotoxic. It has been argued that, because N
₂O has a very short duration under normal circumstances, it is less likely to be neurotoxic than other NMDAR antagonists. Indeed, in rodents, short-term exposure results in only mild injury that is rapidly reversible, and neuronal death occurs only after constant and sustained exposure. Nitrous oxide also may cause neurotoxicity after extended exposure because of hypoxia. This is especially true of non-medical formulations such as whipped-cream chargers (also known as "whippets" or "nangs"), which never contain oxygen, since oxygen makes cream rancid.

Additionally, nitrous oxide depletes vitamin B₁₂ levels. This can cause serious neurotoxicity if the user has preexisting vitamin B₁₂ deficiency.

Nitrous oxide at 75% by volume reduces ischemia-induced neuronal death induced by occlusion of the middle cerebral artery in rodents, and decreases NMDA-induced Ca²⁺ influx in neuronal cell cultures, a critical event involved in excitotoxicity.

DNA damage

Occupational exposure to ambient nitrous oxide has been associated with DNA damage, due to interruptions in DNA synthesis. This correlation is dose-dependent and does not appear to extend to casual recreational use; however, further research is needed to confirm the duration and quantity of exposure needed to cause damage.

Oxygen deprivation

If pure nitrous oxide is inhaled without oxygen mixed in, this can eventually lead to oxygen deprivation resulting in loss of blood pressure, fainting and even heart attacks. This can occur if the user inhales large quantities continuously, as with a strap-on mask connected to a gas canister. It can also happen if the user engages in excessive breath-holding or uses any other inhalation system that cuts off a supply of fresh air. A further risk is that symptoms of frostbite can occur on the lips, larynx and bronchi if the gas is inhaled directly from the gas container. Therefore, condoms or balloons are often used to inhale nitrous oxide out of them.

Vitamin B₁₂ deficiency

Long-term exposure to nitrous oxide may cause vitamin B₁₂ deficiency. It inactivates the cobalamin form of vitamin B₁₂ by oxidation. Symptoms of vitamin B₁₂ deficiency, including sensory neuropathy, myelopathy and encephalopathy, may occur within days or weeks of exposure to nitrous oxide anaesthesia in people with subclinical vitamin B₁₂ deficiency.

Symptoms are treated with high doses of vitamin B₁₂, but recovery can be slow and incomplete.

People with normal vitamin B₁₂ levels have stores to make the effects of nitrous oxide insignificant, unless exposure is repeated and prolonged (nitrous oxide abuse). Vitamin B₁₂ levels should be checked in people with risk factors for vitamin B₁₂ deficiency prior to using nitrous oxide anaesthesia.

Prenatal development

Several experimental studies in rats indicate that chronic exposure of pregnant females to nitrous oxide may have adverse effects on the developing fetus.

Chemical/physical risks

At room temperature (20 °C [68 °F]) the saturated vapour pressure is 50.525 bar, rising up to 72.45 bar at 36.4 °C (97.5 °F)—the critical temperature. The pressure curve is thus unusually sensitive to temperature.

As with many strong oxidisers, contamination of parts with fuels have been implicated in rocketry accidents, where small quantities of nitrous/fuel mixtures explode due to "water hammer"-like effects (sometimes called "dieseling"—heating due to adiabatic compression of gases can reach decomposition temperatures). Some common building materials such as stainless steel and aluminium can act as fuels with strong oxidisers such as nitrous oxide, as can contaminants that may ignite due to adiabatic compression.

There also have been incidents where nitrous oxide decomposition in plumbing has led to the explosion of large tanks.

Mechanism of action

The pharmacological mechanism of action of N
₂O in medicine is not fully known. However, it has been shown to directly modulate a broad range of ligand-gated ion channels, and this likely plays a major role in many of its effects. It moderately blocks NMDAR and β₂-subunit-containing nACh channels, weakly inhibits AMPA, kainate, GABA_C and 5-HT₃ receptors, and slightly potentiates GABA_A and glycine receptors. It also has been shown to activate two-pore-domain K⁺
channels. While N
₂O affects quite a few ion channels, its anaesthetic, hallucinogenic and euphoriant effects are likely caused predominantly, or fully, via inhibition of NMDA receptor-mediated currents. In addition to its effects on ion channels, N
₂O may act to imitate nitric oxide (NO) in the central nervous system, and this may be related to its analgesic and anxiolytic properties. Nitrous oxide is 30 to 40 times more soluble than nitrogen.

The effects of inhaling sub-anaesthetic doses of nitrous oxide have been known to vary, based on several factors, including settings and individual differences; however, from his discussion, Jay (2008) suggests that it has been reliably known to induce the following states and sensations:

• Intoxication
• Euphoria/dysphoria
• Spatial disorientation
• Temporal disorientation
• Reduced pain sensitivity

A minority of users also will present with uncontrolled vocalisations and muscular spasms. These effects generally disappear minutes after removal of the nitrous oxide source.

Euphoric effect

In rats, N
₂O stimulates the mesolimbic reward pathway by inducing dopamine release and activating dopaminergic neurons in the ventral tegmental area and nucleus accumbens, presumably through antagonisation of NMDA receptors localised in the system. This action has been implicated in its euphoric effects and, notably, appears to augment its analgesic properties as well.

It is remarkable, however, that in mice, N
₂O blocks amphetamine-induced carrier-mediated dopamine release in the nucleus accumbens and behavioural sensitisation, abolishes the conditioned place preference (CPP) of cocaine and morphine, and does not produce reinforcing (or aversive) effects of its own. Effects of CPP of N
₂O in rats are mixed, consisting of reinforcement, aversion and no change. In contrast, it is a positive reinforcer in squirrel monkeys, and is well known as a drug of abuse in humans. These discrepancies in response to N
₂O may reflect species variation or methodological differences. In human clinical studies, N
₂O was found to produce mixed responses, similarly to rats, reflecting high subjective individual variability.

Anxiolytic effect

In behavioural tests of anxiety, a low dose of N
₂O is an effective anxiolytic, and this anti-anxiety effect is associated with enhanced activity of GABA_A receptors, as it is partially reversed by benzodiazepine receptor antagonists. Mirroring this, animals that have developed tolerance to the anxiolytic effects of benzodiazepines are partially tolerant to N
₂O. Indeed, in humans given 30% N
₂O, benzodiazepine receptor antagonists reduced the subjective reports of feeling "high", but did not alter psychomotor performance, in human clinical studies.

Analgesic effect

The analgesic effects of N
₂O are linked to the interaction between the endogenous opioid system and the descending noradrenergic system. When animals are given morphine chronically, they develop tolerance to its pain-killing effects, and this also renders the animals tolerant to the analgesic effects of N
₂O. Administration of antibodies that bind and block the activity of some endogenous opioids (not β-endorphin) also block the antinociceptive effects of N
₂O. Drugs that inhibit the breakdown of endogenous opioids also potentiate the antinociceptive effects of N
₂O. Several experiments have shown that opioid receptor antagonists applied directly to the brain block the antinociceptive effects of N
₂O, but these drugs have no effect when injected into the spinal cord.

Conversely, α₂-adrenoceptor antagonists block the pain-reducing effects of N
₂O when given directly to the spinal cord, but not when applied directly to the brain. Indeed, α_2B-adrenoceptor knockout mice or animals depleted in norepinephrine are nearly completely resistant to the antinociceptive effects of N
₂O. Apparently N
₂O-induced release of endogenous opioids causes disinhibition of brainstem noradrenergic neurons, which release norepinephrine into the spinal cord and inhibit pain signalling. Exactly how N
₂O causes the release of endogenous opioid peptides remains uncertain.

Properties and reactions

Nitrous oxide is a colourless, non-toxic gas with a faint, sweet odour.

Nitrous oxide supports combustion by releasing the dipolar bonded oxygen radical, and can thus relight a glowing splint.

N
₂O is inert at room temperature and has few reactions. At elevated temperatures, its reactivity increases. For example, nitrous oxide reacts with NaNH
₂ at 460 K (187 °C) to give NaN
₃:

2 NaNH
₂ + N
₂O → NaN
₃ + NaOH + NH
₃

The above reaction is the route adopted by the commercial chemical industry to produce azide salts, which are used as detonators.

History

The gas was first synthesised in 1772 by English natural philosopher and chemist Joseph Priestley who called it phlogisticated nitrous air (see phlogiston theory) or inflammable nitrous air. Priestley published his discovery in the book Experiments and Observations on Different Kinds of Air (1775), where he described how to produce the preparation of "nitrous air diminished", by heating iron filings dampened with nitric acid.

Early use

"LIVING MADE EASY"
A satirical print from 1830 depicting Humphry Davy administering a dose of laughing gas to a woman

The first important use of nitrous oxide was made possible by Thomas Beddoes and James Watt, who worked together to publish the book Considerations on the Medical Use and on the Production of Factitious Airs (1794). This book was important for two reasons. First, James Watt had invented a novel machine to produce "Factitious Airs" (i.e. nitrous oxide) and a novel "breathing apparatus" to inhale the gas. Second, the book also presented the new medical theories by Thomas Beddoes, that tuberculosis and other lung diseases could be treated by inhalation of "Factitious Airs".

Sir Humphry Davy's Researches chemical and philosophical: chiefly concerning nitrous oxide (1800), pages 556 and 557 (right), outlining potential anaesthetic properties of nitrous oxide in relieving pain during surgery

The machine to produce "Factitious Airs" had three parts: a furnace to burn the needed material, a vessel with water where the produced gas passed through in a spiral pipe (for impurities to be "washed off"), and finally the gas cylinder with a gasometer where the gas produced, "air", could be tapped into portable air bags (made of airtight oily silk). The breathing apparatus consisted of one of the portable air bags connected with a tube to a mouthpiece. With this new equipment being engineered and produced by 1794, the way was paved for clinical trials, which began in 1798 when Thomas Beddoes established the "Pneumatic Institution for Relieving Diseases by Medical Airs" in Hotwells (Bristol). In the basement of the building, a large-scale machine was producing the gases under the supervision of a young Humphry Davy, who was encouraged to experiment with new gases for patients to inhale. The first important work of Davy was examination of the nitrous oxide, and the publication of his results in the book: Researches, Chemical and Philosophical (1800). In that publication, Davy notes the analgesic effect of nitrous oxide at page 465 and its potential to be used for surgical operations at page 556. Davy coined the name "laughing gas" for nitrous oxide.

Despite Davy's discovery that inhalation of nitrous oxide could relieve a conscious person from pain, another 44 years elapsed before doctors attempted to use it for anaesthesia. The use of nitrous oxide as a recreational drug at "laughing gas parties", primarily arranged for the British upper class, became an immediate success beginning in 1799. While the effects of the gas generally make the user appear stuporous, dreamy and sedated, some people also "get the giggles" in a state of euphoria, and frequently erupt in laughter.

One of the earliest commercial producers in the U.S. was George Poe, cousin of the poet Edgar Allan Poe, who also was the first to liquefy the gas.

Anaesthetic use

The first time nitrous oxide was used as an anaesthetic drug in the treatment of a patient was when dentist Horace Wells, with assistance by Gardner Quincy Colton and John Mankey Riggs, demonstrated insensitivity to pain from a dental extraction on 11 December 1844. In the following weeks, Wells treated the first 12 to 15 patients with nitrous oxide in Hartford, Connecticut, and, according to his own record, only failed in two cases. In spite of these convincing results having been reported by Wells to the medical society in Boston in December 1844, this new method was not immediately adopted by other dentists. The reason for this was most likely that Wells, in January 1845 at his first public demonstration to the medical faculty in Boston, had been partly unsuccessful, leaving his colleagues doubtful regarding its efficacy and safety. The method did not come into general use until 1863, when Gardner Quincy Colton successfully started to use it in all his "Colton Dental Association" clinics, that he had just established in New Haven and New York City. Over the following three years, Colton and his associates successfully administered nitrous oxide to more than 25,000 patients. Today, nitrous oxide is used in dentistry as an anxiolytic, as an adjunct to local anaesthetic.

Nitrous oxide was not found to be a strong enough anaesthetic for use in major surgery in hospital settings, however. Instead, diethyl ether, being a stronger and more potent anaesthetic, was demonstrated and accepted for use in October 1846, along with chloroform in 1847. When Joseph Thomas Clover invented the "gas-ether inhaler" in 1876, however, it became a common practice at hospitals to initiate all anaesthetic treatments with a mild flow of nitrous oxide, and then gradually increase the anaesthesia with the stronger ether or chloroform. Clover's gas-ether inhaler was designed to supply the patient with nitrous oxide and ether at the same time, with the exact mixture being controlled by the operator of the device. It remained in use by many hospitals until the 1930s. Although hospitals today use a more advanced anaesthetic machine, these machines still use the same principle launched with Clover's gas-ether inhaler, to initiate the anaesthesia with nitrous oxide, before the administration of a more powerful anaesthetic.

As a patent medicine

Colton's popularisation of nitrous oxide led to its adoption by a number of less than reputable quacksalvers, who touted it as a cure for consumption, scrofula, catarrh and other diseases of the blood, throat and lungs. Nitrous oxide treatment was administered and licensed as a patent medicine by the likes of C. L. Blood and Jerome Harris in Boston and Charles E. Barney of Chicago.

Production

Reviewing various methods of producing nitrous oxide is published.

Industrial methods

Nitrous oxide production

Nitrous oxide is prepared on an industrial scale by careful heating of ammonium nitrate at about 250 C, which decomposes into nitrous oxide and water vapour.

NH
₄NO
₃ → 2 H
₂O + N
₂O

The addition of various phosphate salts favours formation of a purer gas at slightly lower temperatures. This reaction may be difficult to control, resulting in detonation.

Laboratory methods

The decomposition of ammonium nitrate is also a common laboratory method for preparing the gas. Equivalently, it can be obtained by heating a mixture of sodium nitrate and ammonium sulfate:

2 NaNO
₃ + (NH
₄)₂SO
₄ → Na
₂SO
₄ + 2 N
₂O + 4 H
₂O

Another method involves the reaction of urea, nitric acid and sulfuric acid:

2 (NH₂)₂CO + 2 HNO
₃ + H
₂SO
₄ → 2 N
₂O + 2 CO
₂ + (NH₄)₂SO₄ + 2 H
₂O

Direct oxidation of ammonia with a manganese dioxide-bismuth oxide catalyst has been reported: cf. Ostwald process.

2 NH
₃ + 2 O
₂ → N
₂O + 3 H
₂O

Hydroxylammonium chloride reacts with sodium nitrite to give nitrous oxide. If the nitrite is added to the hydroxylamine solution, the only remaining by-product is salt water. If the hydroxylamine solution is added to the nitrite solution (nitrite is in excess), however, then toxic higher oxides of nitrogen also are formed:

NH
₃OHCl + NaNO
₂ → N
₂O + NaCl + 2 H
₂O

Treating HNO
₃ with SnCl
₂ and HCl also has been demonstrated:

2 HNO
₃ + 8 HCl + 4 SnCl
₂ → 5 H
₂O + 4 SnCl
₄ + N
₂O

Hyponitrous acid decomposes to N₂O and water with a half-life of 16 days at 25 °C at pH 1–3.

H₂N₂O₂→ H₂O + N₂O

Atmospheric occurrence

Nitrous oxide atmospheric concentration since 1978.

 

Annual growth rate of atmospheric nitrous oxide since 2000.

Nitrous oxide is a minor component of Earth's atmosphere and is an active part of the planetary nitrogen cycle. Based on analysis of air samples gathered from sites around the world, its concentration surpassed 330 ppb in 2017. The growth rate of about 1 ppb per year has also accelerated during recent decades. Nitrous oxide's atmospheric abundance has grown more than 20% from a base level of about 270 ppb in year 1750.

Important atmospheric properties of N
₂O are summarized in the following table:

Property	Value
Ozone depletion potential (ODP)	0.17 (CCl3F = 1)
Global warming potential (GWP: 100-year)	265 (CO2 = 1)
Atmospheric lifetime	121 years

In October 2020 scientists published a comprehensive quantification of global N
₂O sources and sinks. They report that human-induced emissions increased by 30% over the past four decades and are the main cause of the increase in atmospheric concentration. The recent growth has exceeded some of the highest projected emission scenarios.

Earth's nitrous oxide budget from the Global Carbon Project (2020).

Emissions by source

As of 2010, it was estimated that about 29.5 million tonnes of N
₂O (containing 18.8 million tonnes of nitrogen) were entering the atmosphere each year; of which 64% were natural, and 36% due to human activity.

Most of the N
₂O emitted into the atmosphere, from natural and anthropogenic sources, is produced by microorganisms such as bacteria and fungi in soils and oceans. Soils under natural vegetation are an important source of nitrous oxide, accounting for 60% of all naturally produced emissions. Other natural sources include the oceans (35%) and atmospheric chemical reactions (5%).

A 2019 study showed that emissions from thawing permafrost are 12 times higher than previously assumed.

The main components of anthropogenic emissions are fertilised agricultural soils and livestock manure (42%), runoff and leaching of fertilisers (25%), biomass burning (10%), fossil fuel combustion and industrial processes (10%), biological degradation of other nitrogen-containing atmospheric emissions (9%) and human sewage (5%). Agriculture enhances nitrous oxide production through soil cultivation, the use of nitrogen fertilisers and animal waste handling. These activities stimulate naturally occurring bacteria to produce more nitrous oxide. Nitrous oxide emissions from soil can be challenging to measure as they vary markedly over time and space, and the majority of a year's emissions may occur when conditions are favorable during "hot moments" and/or at favorable locations known as "hotspots".

Among industrial emissions, the production of nitric acid and adipic acid are the largest sources of nitrous oxide emissions. The adipic acid emissions specifically arise from the degradation of the nitrolic acid intermediate derived from nitration of cyclohexanone.

Biological processes

Natural processes that generate nitrous oxide may be classified as nitrification and denitrification. Specifically, they include:

• aerobic autotrophic nitrification, the stepwise oxidation of ammonia (NH
  ₃) to nitrite (NO⁻
  ₂) and to nitrate (NO⁻
  ₃)
• anaerobic heterotrophic denitrification, the stepwise reduction of NO⁻
  ₃ to NO⁻
  ₂, nitric oxide (NO), N
  ₂O and ultimately N
  ₂, where facultative anaerobe bacteria use NO⁻
  ₃ as an electron acceptor in the respiration of organic material in the condition of insufficient oxygen (O
  ₂)
• nitrifier denitrification, which is carried out by autotrophic NH
  ₃-oxidising bacteria and the pathway whereby ammonia (NH
  ₃) is oxidised to nitrite (NO⁻
  ₂), followed by the reduction of NO⁻
  ₂ to nitric oxide (NO), N
  ₂O and molecular nitrogen (N
  ₂)
• heterotrophic nitrification
• aerobic denitrification by the same heterotrophic nitrifiers
• fungal denitrification
• non-biological chemodenitrification

These processes are affected by soil chemical and physical properties such as the availability of mineral nitrogen and organic matter, acidity and soil type, as well as climate-related factors such as soil temperature and water content.

The emission of the gas to the atmosphere is limited greatly by its consumption inside the cells, by a process catalysed by the enzyme nitrous oxide reductase.

Environmental impact

Greenhouse effect

Trends in the atmospheric abundance of long-lived greenhouse gases

Nitrous oxide has significant global warming potential as a greenhouse gas. On a per-molecule basis, considered over a 100-year period, nitrous oxide has 265 times the atmospheric heat-trapping ability of carbon dioxide (CO
₂). However, because of its low concentration (less than 1/1,000 of that of CO
₂), its contribution to the greenhouse effect is less than one third that of carbon dioxide, and also less than water vapour and methane. On the other hand, since 38% or more of the N
₂O entering the atmosphere is the result of human activity, control of nitrous oxide is considered part of efforts to curb greenhouse gas emissions.

A 2008 study by Nobel Laureate Paul Crutzen suggests that the amount of nitrous oxide release attributable to agricultural nitrate fertilisers has been seriously underestimated, most of which presumably, would come under soil and oceanic release in the Environmental Protection Agency data.

Nitrous oxide is released into the atmosphere through agriculture, when farmers add nitrogen-based fertilizers onto the fields, through the breakdown of animal manure. Approximately 79 percent of all nitrous oxide released in the United States came from nitrogen fertilization. Nitrous oxide is also released as a by-product of burning fossil fuel, though the amount released depends on which fuel was used. It is also emitted through the manufacture of nitric acid, which is used in the synthesis of nitrogen fertilizers. The production of adipic acid, a precursor to nylon and other synthetic clothing fibres, also releases nitrous oxide. The total amount of nitrous oxide released that is of human origins is about 40 percent.

Ozone layer depletion

Nitrous oxide has also been implicated in thinning the ozone layer. A 2009 study suggested that N
₂O emission was the single most important ozone-depleting emission and it was expected to remain the largest throughout the 21st century.

Legality

In the United States, possession of nitrous oxide is legal under federal law and is not subject to DEA purview. It is, however, regulated by the Food and Drug Administration under the Food Drug and Cosmetics Act; prosecution is possible under its "misbranding" clauses, prohibiting the sale or distribution of nitrous oxide for the purpose of human consumption. Many states have laws regulating the possession, sale and distribution of nitrous oxide. Such laws usually ban distribution to minors or limit the amount of nitrous oxide that may be sold without special license. For example, in the state of California, possession for recreational use is prohibited and qualifies as a misdemeanor.

In August 2015, the Council of the London Borough of Lambeth (UK) banned the use of the drug for recreational purposes, making offenders liable to an on-the-spot fine of up to £1,000.

In New Zealand, the Ministry of Health has warned that nitrous oxide is a prescription medicine, and its sale or possession without a prescription is an offense under the Medicines Act. This statement would seemingly prohibit all non-medicinal uses of nitrous oxide, although it is implied that only recreational use will be targeted legally.

In India, transfer of nitrous oxide from bulk cylinders to smaller, more transportable E-type, 1,590-litre-capacity tanks is legal when the intended use of the gas is for medical anaesthesia.

 

Climate-friendly gardening

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Climate-friendly_gardening 

Climate-friendly gardening is gardening in ways which reduce emissions of greenhouse gases from gardens and encourage the absorption of carbon dioxide by soils and plants in order to aid the reduction of global warming. To be a climate-friendly gardener means considering both what happens in a garden and the materials brought into it and the impact they have on land use and climate. It can also include garden features or activities in the garden that help to reduce greenhouse gas emissions elsewhere.

Orchard garden showing orchard trees, herbaceous perennials and ground-cover plants, at Hergest Croft Gardens, Herefordshire, Britain.

Land use and greenhouse gases

Most of the excess greenhouse gases causing climate change have come from burning fossil fuel. But a special report from the Intergovernmental Panel on Climate Change (IPCC) estimated that in the last 150 years fossil fuels and cement production were responsible for only about two-thirds of climate change: the other third has been caused by human land use.

The three main greenhouse gases produced by unsustainable land use are carbon dioxide, methane, and nitrous oxide. Black carbon or soot can also be caused by unsustainable land use, and, although not a gas, can behave like greenhouse gases and contribute to climate change.

Carbon dioxide

Carbon dioxide, CO
2, is a natural part of the carbon cycle, but human land uses often add more, especially from habitat destruction and the cultivation of soil. When woodlands, wetlands, and other natural habitats are turned into pasture, arable fields, buildings and roads, the carbon held in the soil and vegetation becomes extra carbon dioxide and methane to trap more heat in the atmosphere.

Gardeners may cause extra carbon dioxide to be added to the atmosphere in several ways:

• Using peat or potting compost containing peat;
• Buying garden furniture or other wooden products made from woodland which has been destroyed rather than taken as a renewable crop from sustainably managed woodland;
• Digging soil and leaving it bare so that the carbon in soil organic matter is oxidised;
• Using power tools which burn fossil fuel or electricity generated by burning fossil fuel;
• Using patio heaters;
• Heating greenhouses by burning fossil fuel or electricity generated by burning fossil fuel;
• Burning garden prunings and weeds on a bonfire;
• Buying tools, pesticides, synthetic nitrogen fertilizers (over 2 kilograms of carbon dioxide equivalent is produced in the manufacture of each kilogram of ammonium nitrate), and other materials which have been manufactured using fossil fuel;
• Heating and treating swimming pools by burning fossil fuel or electricity generated by burning fossil fuel;
• Watering their gardens with tapwater, which has been treated and pumped by burning fossil fuel, with a greenhouse gas impact of about 1 kg CO₂e/m³ water.

Gardeners will also be responsible for extra carbon dioxide when they buy garden products which have been transported by vehicles powered by fossil fuel.

Methane

Methane, CH₄, is a natural part of the carbon cycle, but human land uses often add more, especially from anaerobic soil, artificial wetlands such as rice fields, and from the guts of farm animals, especially ruminants such as cattle and sheep.

Gardeners may cause extra methane to be added to the atmosphere in several ways:

• Compacting soil so that it becomes anaerobic, for example by treading on soil when it is wet;
• Allowing compost heaps to become compacted and anaerobic;
• Creating homemade liquid feed by putting the leaves of plants such as comfrey under water, with the unintended consequence that the plants may release methane as they decay;
• Killing pernicious weeds by covering them with water, with the unintended consequence that the plants may release methane as they decay;
• Allowing ponds to become anaerobic, for example by adding unsuitable fish species which stir up sediment that then blocks light from and kills submerged oxygenating plants.

Nitrous oxide

Nitrous oxide, N₂O, is a natural part of the nitrogen cycle, but human land uses often add more.

Gardeners may cause extra nitrous oxide to be added to the atmosphere by:

• Using synthetic nitrogen fertilizer, for example "weed and feed" on lawns, especially if it is applied when plants are not actively growing, the soil is compacted, or when other factors are limiting so that the plants cannot make use of the nitrogen;
• Compacting the soil (for example by working in the garden when the soil is wet) which will increase the conversion of nitrates to nitrous oxide by soil bacteria;
• Burning garden waste on bonfires.

Black carbon

Black carbon is not a gas, but it acts like a greenhouse gas because it can be suspended in the atmosphere and absorb heat.

Gardeners may cause extra black carbon to be added to the atmosphere by burning garden prunings and weeds on bonfires, especially if the waste is wet and becomes black carbon in the form of soot. Gardeners will also be responsible for extra black carbon produced when they buy garden products which have been transported by vehicles powered by fossil fuel especially the diesel used in most lorries.

Gardening to reduce greenhouse gas emissions and absorb carbon dioxide

There are many ways in which climate-friendly gardeners may reduce their contribution to climate change and help their gardens absorb carbon dioxide from the atmosphere.

Climate-friendly gardeners can find good ideas in many other sustainable approaches:

• Agroforestry;
• Forest gardening;
• Orchards;
• Organic gardening;
• Permaculture;
• Rain garden;
• Vegan organic gardening;
• Water-wise gardening;
• Wildlife garden.

Protecting and enhancing carbon stores

Protecting carbon stores in land beyond gardens

Woodland and wetland in the New Forest, Hampshire

 

Woodland and trees in Herefordshire

 

Kitchen garden at Charles Darwin's home, Down House, Kent, showing greenhouse, waterbutt, box hedging and vegetable beds.

 

Alliums, lavender, box and other water-thrifty plants in the dry garden at Cambridge Botanic Garden

Climate-friendly gardening includes actions which protect carbon stores beyond gardens. The biggest carbon stores in land are in soil; the two habitat types with the biggest carbon stores per hectare are woods and wetlands; and woods absorb more carbon dioxide per hectare per year than most other habitats. Climate-friendly gardeners therefore aim to ensure that nothing they do will harm these habitats.

According to Morison and Morecroft (eds.)'s Plant Growth and Climate Change, the net primary productivity (the net amount of carbon absorbed each year) of various habitats is:

• Tropical forests: 12.5 tonnes of carbon per hectare per year;
• Temperate forests: 7.7 tonnes of carbon per hectare per year;
• Temperate grasslands: 3.7 tonnes of carbon per hectare per year;
• Croplands: 3.1 tonnes of carbon per hectare per year.

The Intergovernmental Panel on Climate Change's Special Report Land use, land-use change, and forestry lists the carbon contained in different global habitats as:

• Wetlands: 643 tonnes carbon per hectare in soil + 43 tonnes carbon per hectare in vegetation = total 686 tonnes carbon per hectare;
• Tropical forests: 123 tonnes carbon per hectare in soil + 120 tonnes carbon per hectare in vegetation = total 243 tonnes carbon per hectare;
• Temperate forests: 96 tonnes carbon per hectare in soil + 57 tonnes carbon per hectare in vegetation = total 153 tonnes carbon per hectare;
• Temperate grasslands: 164 tonnes carbon per hectare in soil + 7 tonnes carbon per hectare in vegetation = total 171 tonnes carbon per hectare;
• Croplands: 80 tonnes carbon per hectare in soil + 2 tonnes carbon per hectare in vegetation = total 82 tonnes carbon per hectare.

The figures quoted above are global averages. More recent research in 2009 has found that the habitat with the world's highest known total carbon density - 1,867 tonnes of carbon per hectare - is temperate moist forest of Eucalyptus regnans in the Central Highlands of south-east Australia; and, in general, that temperate forests contain more carbon than either boreal forests or tropical forests.

Carbon stores in Britain

According to Milne and Brown's 1997 paper "Carbon in the vegetation and soils of Great Britain", Britain's vegetation and soil are estimated to contain 9952 million tonnes of carbon, of which almost all is in the soil, and most in Scottish peatland soil:

• Soils in Scotland: 6948 million tonnes carbon;
• Soils in England and Wales: 2890 million tonnes carbon;
• Vegetation in British woods and plantations (which cover only 11% of Britain's land area): 91 million tonnes carbon;
• Other vegetation: 23 million tonnes carbon.

A 2005 report suggested that British woodland soil may contain as much as 250 tonnes of carbon per hectare.

Many studies of soil carbon only study the carbon in the top 30 centimetres, but soil is often much deeper than that, especially below woodland. One 2009 study of the United Kingdom's carbon stores by Keith Dyson and others gives figures for soil carbon down to 100 cm below the habitats, including "Forestland", "Cropland" and "Grassland", covered by the Kyoto Protocol reporting requirements.

• Forestland soils: average figures in tonnes carbon per hectare are 160 (England), 428 (Scotland), 203 (Wales), and 366 (Northern Ireland).
• Grassland soils: average figures in tonnes carbon per hectare are 148 (England), 386 (Scotland), 171 (Wales), and 304 (Northern Ireland).
• Cropland soils: average figures in tonnes carbon per hectare are 110 (England), 159 (Scotland), 108 (Wales), and 222 (Northern Ireland).

Protecting carbon stores in wetland

Permeable paving of wood chip with birch-log edging at the Royal Horticultural Society garden at Wisley

 

A ground-cover and rain-garden plant - Symphytum grandiflorum, creeping comfrey (with Cotinus coggygria)

Climate-friendly gardeners choose peat-free composts because some of the planet's biggest carbon stores are in soil, and especially in the peatland soil of wetlands.

The Intergovernmental Panel on Climate Change's Special Report Land Use, Land-Use Change and Forestry gives a figure of 2011 gigatonnes of carbon for global carbon stocks in the top 1 metre of soils, much more than the carbon stores in the vegetation or the atmosphere.

Climate-friendly gardeners also avoid using tapwater not only because of the greenhouse gases emitted when fossil fuels are burnt to treat and pump water, but because if water is taken from wetlands then carbon stores are more likely to be oxidised to carbon dioxide.

A climate-friendly garden therefore does not contain large irrigated lawns, but instead includes water-butts to collect rainwater; water-thrifty plants which survive on rainwater and do not need watering after they are established; trees, shrubs and hedges to shelter gardens from the drying effects of sun and wind; and groundcover plants and organic mulch to protect the soil and keep it moist.

Climate-friendly gardeners will ensure that any paved surfaces in their gardens (which are kept to a minimum to increase carbon stores) are permeable, and may also make rain gardens, sunken areas into which rainwater from buildings and paving is directed, so that the rain can then be fed back into groundwater rather than going into storm drains. The plants in rain gardens must be able to grow in both dry and wet soils.

Protecting carbon stores in woodland

Wetlands may store the most carbon in their soils, but woods store more carbon in their living biomass than any other type of vegetation, and their soils store the most carbon after wetlands. Climate-friendly gardeners therefore ensure that any wooden products they buy, such as garden furniture, have been made of wood from sustainably managed woodland.

Protecting and increasing carbon stores in gardens

Juglans elaeopyren, an American walnut, at Cambridge Botanic Garden

After rocks containing carbonate compounds, soil is the biggest store of carbon on land. Carbon is found in soil organic matter, including living organisms (plant roots, fungi, animals, protists, bacteria), dead organisms, and humus. One study of the environmental benefits of gardens estimates that 86% of carbon stores in gardens is in the soil.

Wild strawberries in flower below a British hedge.

The first priorities for climate-friendly gardeners are, therefore, to:

• Protect the soil's existing carbon stores;
• Increase the soil's carbon stores.

To protect the soil, climate-friendly gardens:

• Are based on plants rather than buildings and paving;
• Have soil that is kept at a relatively stable temperature by shelter from trees, shrubs and/or hedges;
• Have soil that is always kept covered and therefore moist and at a relatively stable temperature by groundcover plants, fast-growing green manures (which can be used as an intercrop in kitchen gardens of annual vegetables) and/or organic mulches.

Mulch of woodchips protecting soil at the Royal Horticultural Society garden at Wisley in Surrey.

Climate-friendly gardeners avoid things which may harm soil. They do not tread on the soil when it is wet, because it is then most vulnerable to compaction. They dig as little is possible, and only when the soil is moist rather than wet, because cultivation increases the oxidation of soil organic matter and produces carbon dioxide.

To increase soil carbon stores, climate-friendly gardeners ensure that their gardens create optimal conditions for vigorous healthy growth of plants, and other garden organisms above and below ground, and reduce the impact of any limiting factors.

In general, the more biomass that the plants can create each year, the more carbon will be added to the soil. However, only some biomass each year becomes long-term soil carbon or humus. In Soil Carbon and Organic Farming, a 2009 report for the Soil Association, Gundula Azeez discusses several factors which increase how much biomass is turned into humus. These include good soil structure, soil organisms such as fine root hairs, microorganisms, mycorrhizas and earthworms which increase soil aggregation, residues from plants (such as trees and shrubs) which have a high content of resistant chemicals such as lignin, and plant residues with a carbon to nitrogen ratio lower than about 32:1.

Nitrogen-fixing nodules on Wisteria roots (hazelnut for scale)

Climate-friendly gardens therefore include:

• Hedges for shelter from wind;
• A light canopy of late-leafing deciduous trees to let in enough sunlight for growth but not so much that the garden becomes too hot and dry (this is one of the principles behind many agroforestry systems, such as Paulownia's use in China partly because it is late-leafing and its canopy is sparse so that crops below it get shelter but also enough light);
• Groundcover plants and organic mulches (such as woodchips over compost made from kitchen and garden "waste") to keep soil moist and at relatively stable temperatures;
• Nitrogen-fixing plants, because soil nitrogen may be a limiting factor (but climate-friendly gardeners avoid synthetic nitrogen fertilizers, because these may cause mycorrhizal associations to break down);
• Many layers of plants, including woody plants such as trees and shrubs, other perennials, groundcover plants, deep-rooted plants, all chosen according to 'right plant, right place', so that they are suited to their growing conditions and will grow well;
• A wide diversity of disease-resistant, vigorous plants for resilience and to make the most of all available ecological niches;
• Plants to feed and shelter wildlife, to increase total biomass, and to ensure biological control of pests and diseases.
• Compost made from garden and kitchen "waste".

Lawns, like other grasslands, can build up good levels of soil carbon, but they will grow more vigorously and store more carbon if besides grasses they also contain nitrogen-fixing plants such as clover, and if they are cut using a mulching mower which returns finely-chopped mowings to the lawn. More carbon, however, may be stored by other perennial plants such as trees and shrubs. They also do not need to be maintained using power tools.

Climate-friendly gardeners will also aim to increase biodiversity not only for the sake of the wildlife itself, but so that the garden ecosystem is resilient and more likely to store as much carbon as possible as long as possible. They will therefore avoid pesticides, and increase the diversity of the habitats within their gardens.

Reducing greenhouse gas emissions

Climate-friendly gardeners can directly reduce the greenhouse gas emissions from their own gardens, but can also use their gardens to indirectly reduce greenhouse gas emissions elsewhere.

Using gardens to reduce greenhouse gas emissions

Climate-friendly gardeners can use their gardens in ways which reduce greenhouse gases elsewhere, for example by using the sun and wind to dry washing on washing lines in the garden instead of using electricity generated by fossil fuel to dry washing in tumble dryers.

From farmland

Walnut, Juglans regia, with ripening walnuts

Food is a major contributor to climate change. In the United Kingdom, according to Tara Garnett of the Food Climate Research Network, food contributes 19% of the country's greenhouse gas emissions.

Soil is the biggest store of carbon on land. It is therefore important to protect the soil organic matter in farmland. Farm animals, however, especially free-range pigs, may cause erosion, and cultivation of the soil increases the oxidation of soil organic matter into carbon dioxide. Other sources of greenhouse gases from farmland include: compaction caused by farm machinery or overgrazing by farm animals can make soil anaerobic and produce methane; farm animals produce methane; and nitrogen fertilizers can be converted to nitrous oxide.

Most farmland consists of fields growing annual arable crops which are eaten directly by people or fed to farm animals, and grassland used as pasture, hay or silage to feed farm animals. Some perennial food plants are also grown, such as fruits and nuts in orchards, and watercress grown in water.

Although all cultivation of the soil in arable fields produces carbon dioxide, some arable crops cause more damage to soil than others. Root crops such as potatoes and sugar-beet, and crops which are harvested not just once a year but over a long period such as green vegetables and salads, are considered "high risk" in catchment-sensitive farming.

Climate-friendly gardeners therefore grow at least some of their food, and may choose food crops which therefore help to keep carbon in farmland soils if they grow such high-risk crops in small vegetable plots in their gardens, where it is easier to protect the soil than in large fields under commercial pressures. Climate-friendly gardeners may grow and eat plants such as sweet cicely which sweeten food, and so reduce the land area needed for sugar-beet. They may also choose to grow perennial food plants to not only reduce their indirect greenhouse gas emissions from farmland, but also to increase carbon stores in their own gardens.

Grassland contains more carbon per hectare than arable fields, but farm animals, especially ruminants such as cattle or sheep, produce large amounts of methane, directly and from manure heaps and slurry. Slurry and manure may also produce nitrous oxide. Gardeners who want to reduce their greenhouse gas emissions can help themselves to eat less meat and dairy produce by growing nut trees which are a good source of tasty, protein-rich food, including walnuts which are an excellent source of the omega-3 fatty acid alpha-linolenic acid.

Researchers and farmers are investigating and improving ways of farming which are more sustainable, such as agroforestry, forest farming, wildlife-friendly farming, soil management, catchment-sensitive farming (or water-friendly farming). For example, the organisation Farming Futures assists farmers in the United Kingdom to reduce their farms' greenhouse gas emissions.

Farmers are aware that consumers are increasingly asking for "green credentials". Gardeners who understand climate-friendly practices can advocate their use by farmers.

From industry

Nitrogen-fixing and edible - Elaeagnus umbellatus at the Agroforestry Research Trust forest garden in Devon

Climate-friendly gardeners aim to reduce their consumption in general. In particular, they try to avoid or reduce their consumption of tapwater because of the greenhouse gases emitted when fossil fuels are burnt to supply the energy needed to treat and pump it to them. Instead, gardeners can garden using only rainwater.

Greenhouse gases are produced in the manufacture of many materials and products used by gardeners. For example, it takes a lot of energy to produce synthetic fertilizers, especially nitrogen fertilizers. Ammonium nitrate, for example, has an embodied energy of 67000 kilojoules/kilogramme, so climate-friendly gardeners will choose alternative ways of ensuring the soil in their gardens has optimal levels of nitrogen by alternative means such as nitrogen-fixing plants.

Climate-friendly gardeners will also aim to follow "cradle-to-cradle design" and "circular economy" principles: when they choose to buy or make something, it should be possible to take it apart again and recycle or compost every part, so that there is no waste, only raw materials to be made into something else. This will reduce the greenhouse gases otherwise produced when extracting raw materials.

From transport

Gardeners can reduce not only their food miles by growing some of their own food, but also their "gardening miles" by reducing the amount of plants and other materials they import, obtaining them as locally as possible and with as little packaging as possible. This might include ordering plants by mail order from a specialist nursery if the plants are sent out bare-root, reducing transport demand and the use of peat-based composts; or growing plants from seed, which will also increase genetic diversity and therefore resilience; or growing plants vegetatively from cuttings or offsets from other local gardeners; or buying reclaimed materials from salvage firms.

From houses

Climbers as insulation - Boston ivy, Parthenocissus tricuspidata, Boston ivy, in autumn

Climate-friendly gardeners can use their gardens in ways which reduce greenhouse gas emissions from homes by:

• Using sunlight and wind to dry washing on washing lines instead of fossil fuel-generated electricity to run tumble dryers;
• Planting deciduous climbers on houses and planting deciduous trees at suitable distances from the house to provide shade during the summer, reducing the consumption of electricity for air conditioning, but also such that at cooler times of year, sunlight can reach and warm a house, reducing heating costs and consumption;
• Planting hedges, trees, shrubs and climbers to shelter houses from wind, reducing heating costs and consumption during the winter (as long as any planting does not create a wind-tunnel effect).

Climate-friendly gardeners may also choose to reduce their own personal greenhouse gas emissions by growing and eating carminative plants such as fennel and garlic which reduce intestinal gases such as methane.

Reducing greenhouse gas emissions from gardens

Slow-growing yew, Taxus baccata, as hedge at Charles Darwin's home, Down House, Kent

 

Nitrogen-fixing red and white clover (Trifolium) as lawn plants

 

Leaf cage, compost heap and wormery at the Royal Horticultural Society garden at Wisley

There are some patent sources of greenhouse gas emissions in gardens and some more latent.

Power tools which are powered by diesel or petrol, or electricity generated by burning other fossil fuels, emit carbon dioxide. Climate-friendly gardeners may therefore choose to use hand tools rather than power tools, or power tools powered by renewable electricity, or design their gardens to reduce or remove a need to use power tools. For example, they may choose dense, slow-growing species for hedges so that the hedges only need to be cut once a year.

Lawns need[?] to be cut by lawn mowers and, in drier parts of the world, are often irrigated by tapwater. Climate-friendly gardeners will therefore do what they can to reduce this consumption by:

• Replacing part of or all lawns with other perennial planting such as trees and shrubs with less ecologically demanding maintenance requirements;
• Cut some or all lawns only once or twice a year, i.e. convert them into meadows;
• Make lawn shapes simple so that they may be cut quickly;
• Increase the cutting height of mower blades;
• Use a mulching mower to return organic matter to the soil;
• Sow clover to increase vigour (without the need for synthetic fertilisers) and resilience in dry periods;
• Cut lawns with electric mowers using electricity from renewable energy;
• Cut lawns with hand tools such as push mowers or scythes.

Greenhouses can be used to grow crops which might otherwise be imported from warmer climates, but if they are heated by fossil fuel then they may cause more greenhouse gas emissions than they save. Climate-friendly gardeners will therefore use their greenhouses carefully by:

• Choosing only annual plants which will only be in the greenhouse during warmer months, or perennial plants which do not need any extra heat during winter;
• Using water tanks as heat stores and compost heaps as heat sources inside greenhouses so that they stay frost-free in winter.

Climate-friendly gardeners will not put woody prunings on bonfires, which will emit carbon dioxide and black carbon, but instead burn them indoors in a wood-burning stove and therefore cut emissions from fossil fuel, or cut them up to use as mulch and increase soil carbon stores, or add the smaller prunings to compost heaps to keep them aerated, reducing methane emissions. To reduce the risk of fire, they will also choose fire-resistant plants from habitats which are not prone to wildfires and which do not catch fire easily, rather than fire-adapted plants from fire-prone habitats which are flammable and adapted to encourage fires and then gain a competitive advantage over less resistant species.

Climate-friendly gardeners may use deep-rooted plants such as comfrey to bring nutrients closer to the surface topsoil, but will do so without making the leaves into a liquid feed, because the rotting leaves in the anaerobic conditions under water may emit methane.

Nitrogen fertilizers may be oxidised to nitrous oxide, especially if fertilizer is applied in excess, or when plants are not actively growing. Climate-friendly gardeners may choose instead to use nitrogen-fixing plants which will add nitrogen to the soil without increasing nitrous oxide emissions.