Nitrous oxide from incorporated crop residues and green manures
Baggs, Elizabeth Mary
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A series of field and laboratory experiments were undertaken to examine the effects of incorporation of plant material on emissions of N₂0 from agricultural soils. The overall aim was to increase understanding of that part of the agricultural N cycle, associated with the release of N after incorporation of crop residues and green manures into soil, and subsequent N₂0 emissions to the atmosphere. N20 emissions from growing crops and following addition of various residues and green manures to soil were measured and compared. The effects of crop type, fertiliser application, cultivation techniques, soil type, and climatic conditions, and also of the addition of high C substrate in the form of paper waste, on these emissions were investigated.Emissions of N₂0 were increased after cultivation of soil, attributed to increased accessibility of organic matter to soil microbes, and improved gaseous diffusion. Emissions were higher following incorporation of plant material than emissions from bare soil. Generally, fluxes were increased within a few hours or days after cultivation and/or incorporation, but the effect was short-lived. Most of the N₂0 was emitted during the first 2 weeks. The magnitude and timing of N₂0 released within this period was highly dependent on temperature and rainfall following incorporation, and the cultivation technique employed.The C:N ratio of the incorporated plant material had a considerable effect on rates of decomposition, and on subsequent N₂0 production during nitrification and denitrification. Higher emissions were typically measured after incorporation of material with a low C:N ratio, such as legumes, than when material with higher ratios, such as cereal straw, was involved. When material with a high C:N ratio was added, N was immobilised. Nevertheless, the presence of high C:N paper waste increased N₂0 emissions from incorporated vegetable crop residues. This was attributed to the creation of more anaerobic sites in the soil.Emissions of N₂0 increased within a few days of applying mineral N fertiliser to spring-sown cereal crops. Again, these fluxes were short-lived. Use of 15N-labelling in this experiment showed that approximately 50% of crop N at harvest was derived from applied fertiliser. In other experiments, the presence of a growing crop, particularly a legume, increased emissions, compared with those measured from bare soil.