managing soil organic matter - Grains Research & Development ...

52MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEThe effect of nitrogen fertilisers on soil organiccarbon stocks should be assessed on thenet balance between increased carbon inputsfrom increased production versus increaseddecomposition if it occurs. In some cases, nitrogeninputs can actually slow down carbon loss becausemore carbon is stabilised. In other cases, nitrogenfertilisers can indirectly degrade soil organic carbonreserves because their addition stimulates a rangeof bacteria that feed on carbon for their growthand reproduction. For every tonne of fertilisernitrogen applied, bacteria consume about 30tonnes of carbon (based on a carbon to nitrogenratio of 30 to 1).Nitrous oxideNitrous oxide (N 2 O) emissions account for about10 per cent of global greenhouse gas emissions,with 90 per cent of these emissions derived fromagricultural practices (Smith et al. 2007). The mainsource of nitrous oxide emissions worldwide ismineral nitrogen fertilisation and its influence on thenitrogen cycle.As a consequence of its high global warmingpotential, nitrous oxide emissions from land can havea large bearing on the assessment of greenhousegases from cropping systems (Australian NationalGreenhouse Accounts 2011). Barton et al. (2010)report nitrous oxide emitted after the applicationof synthetic nitrogen fertilisers to land under graincropping systems to be 17 times lower than theIntergovernmental Panel on Climate Change (IPCC)default value of 1.0 per cent.Nitrous oxide (N 2 O) emissionsaccount for about 10 per cent ofglobal greenhouse gas emissions,with 90 per cent of theseemissions derived from agriculturalpractices (Smith et al. 2007).Nitrous oxide in soils is associated withmicrobial processes associated with nitrogentransformations. Denitrification occurs underanaerobic (waterlogged) conditions and involves thereduction of nitrate (NO 3 -) to nitrogen gas (N 2 ), withnitrous oxide as a by-product (de Klein and Eckard2008). Nitrification contributes to a lesser extent tonitrous oxide emissions by oxidising ammonium(NH 4 +) to nitrate (NO 3 -), with nitrous oxide as aby-product. Nitrification can be favoured at hightemperatures and inhibited at acid pH values(Mengel and Kirkby 1987).The influence of organic matter on nitrous oxideemissions is related to substrate availability. Currentevidence suggests this is most closely associatedwith the carbon to nitrogen ratio of the dissolved(soluble) organic matter fraction, which differsamong species and decomposition stages. A lowercarbon to nitrogen ratio provides more mineralisablenitrogen substrate for microbial nitrous oxideproduction and increases the bioavailability ofdissolved organic carbon.MethaneDespite a short residence time (about 10 years) inthe atmosphere, methane is the main hydrocarbonpresent in the atmosphere and due to its ability toabsorb infrared radiation has 20-30 times the globalwarming potential of carbon dioxide (Rodhe 1990).Nearly 16 per cent of Australia’s greenhouse gasemissions are associated with methane productionfrom agriculture. A large proportion (just over 67per cent) of this comes from methane producedby Australia’s cattle and sheep industries (NationalGreenhouse Gas Inventory 2010).Methane (CH 4 ) is a natural by-product of ruminantdigestion in animals, wetland rice paddy farmingand anaerobic decomposition of biological material.Its global warming potential is 21 times that ofcarbon dioxide. In soils, methane emissions are thenet result of two bacterial processes influenced byland use, management and soil type — methaneproduction and methane consumption. Methane isproduced by methanogens in anaerobic soils thatconstrain oxygen diffusion such as in water-loggedsoils and methane consumption in aerobic soilsby methane-oxidizing bacteria (Le Mer and Roger2012).CARBON OFFSETS FOR GREENHOUSEGAS EMISSIONS AND CARBONTRADINGGlobal markets for greenhouse gas emissionsCarbon trading markets have been introduced insome countries as a response to climate changeimperatives and typically greenhouse gas emissionsare limited by tradeable permits. Hence, a price fora set amount of emissions is determined by marketforces, which should promote changes in productiontowards a lower-emission producing industry. In a