Soils are the main sources of the greenhouse gas nitrous oxide (N2O). The N2O emission at the soil surface is the result of production and consumption processes. So far, research has concentrated on net N2O production. However, in the literature, there are numerous reports of net negative fluxes of N2O, (i.e. fluxes from the atmosphere to the soil). Such fluxes are frequent and substantial and cannot simply be dismissed as experimental noise.

Net N2O consumption has been measured under various conditions from the tropics to temperate areas, in natural and agricultural systems. Low mineral N and large moisture contents have sometimes been found to favour N2O consumption. This fits in with denitrification as the responsible process, reducing N2O to N2. However, it has also been reported that nitrifiers consume N2O in nitrifier denitrification. A contribution of various processes could explain the wide range of conditions found to allow N2O consumption, ranging from low to high temperatures, wet to dry soils, and fertilized to unfertilized plots. Generally, conditions interfering with N2O diffusion in the soil seem to enhance N2O consumption. However, the factors regulating N2O consumption are not yet well understood and merit further study.

Frequent literature reports of net N2O consumption suggest that a soil sink could help account for the current imbalance in estimated global budgets of N2O. Therefore, a systematic investigation into N2O consumption is necessary. This should concentrate on the organisms, reactions, and environmental factors involved.

Soils, a sink for N2O? A review

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Atmospheric methane and global change

Fertiliser nitrogen use in Australia has increased from 35 Gg N in 1961 to 972 Gg N in 2002, and most of the nitrogen is used for growing cereals. However, the nitrogen is not used efficiently, and wheat plants, for example, assimilated only 41% of the nitrogen applied. This review confirms that the efficiency of fertiliser nitrogen can be improved through management practices which increase the crop's ability to compete with loss processes. However, the results of the review suggest that management practices alone will not prevent all losses (e.g. by denitrification), and it may be necessary to use enhanced efficiency fertilisers, such as controlled release products, and urease and nitrification inhibitors, to obtain a marked improvement in efficiency. Some of these products (e.g. nitrification inhibitors) when used in Australian agriculture have increased yield or reduced nitrogen loss in irrigated wheat, maize and cotton, and flooded rice, but most of the information concerning the use of enhanced efficiency fertilisers to reduce nitrogen loss to the environment has come from other countries. The potential role of enhanced efficiency fertilisers to increase yield in the various agricultural industries and prevent contamination of the environment in Australia is discussed.

Prospects of improving efficiency of fertiliser nitrogen in Australian agriculture: a review of enhanced efficiency fertilisers

Seasonal changes of CO2, CH4 and N2O fluxes in relation to land-use change in tropical peatlands located in coastal area of South Kalimantan

Methane (CH4) emissions from rice fields were determined using automated measurement systems in China, India, Indonesia, Thailand, and the Philippines Mitigation options were assessed separately for different baseline practices of irrigated rice, rainfed, and deepwater rice irrigated rice is the largest source of CH4 and also offers the most options to modify crop management for reducing these emissions Optimizing irrigation patterns by additional drainage periods in the field or an early timing of midseason drainage accounted for 70%–80% of CH4 emissions of the respective baseline practice In baseline practices with high organic amendments, use of compost (5863%), biogas residues (10–16%), and direct wet seeding (16–22%) should be considered mitigation options In baseline practices using prilled urea as sole N source, use of ammonium sulfate could reduce CH4 emission by 1067% In all rice ecosystems, CH4 emissions can he reduced by fallow incorporation (11%) and mulching (11%) of rice straw as well as addition of phosphogypsum (9–73%) However, in rainfed and deepwater rice, mitigation options are very limited in both number and potential gains The assessment of these crop management options includes their total factor productivity and possible adverse effects Due to higher nitrous oxide (N,O) emissions, changes in water regime are only recommended for rice systems with high baseline emissions of CH4 Key objectives of future research are identifying and characterizing high-emitting rice systems, developing site-specific technology packages, ascertaining synergies with productivity, and accounting for N2O emissions

Characterization of methane emissions from rice fields in Asia. III. Mitigation options and future research needs

I discuss production, emission and oxidation of CH4 in rice paddy fields and N2O in fertilized soils. The quantity of CH4 emitted from rice paddy fields depends upon several important factors including soil factors, nutrient management, water regimes, cultivation practices and others. Important factors for N2O emitted from fertilized soils are soil water content, temperature, nitrate or ammonium concentration, available organic carbon for denitrification and pH. I provide an estimate of mitigation potential in agricultural systems based on this estimate and the management technology.

K. Minami

Papers

Atmospheric methane and nitrous oxide: sources, sinks and strategies for reducing agricultural emissions.