UNDERSTANDING THE SOIL-NITROGEN CYCLE

A quantitative knowledge of nitrogen cycle processes is required to design strategies for decreasing leakage of N from agriculture to the wider environment. However, it is remarkably difficult to make reliable measurements of many of the key processes under realistic field conditions. In impermeable soils hydrologically separated plots provide an invaluable method of measuring leaching and runoff. Estimates of nitrate leaching using porous ceramic cups agree well with lysimeter measurements on sandy soil but are suspect on more structured soils. Estimates of N2O flux from soil are subject to great spatial heterogeneity; developing long path-length measuring techniques may overcome this problem. N-15 labelling is valuable for assessing fertilizer N loss, forms of N left in soil and the fate of N from crop residues. The combination of experimental and modelling approaches can provide insights that are otherwise unattainable, including a basis for more precise advice on N fertilization. Mineralization of soil organic matter and crop or animal residues provides much of the nitrate leached during winter under the climatic conditions of north-west Europe, because mineralization is poorly synchronized with crop N uptake. Maintenance of crop cover during winter can greatly decrease leaching but the long-term effects on the N cycle of winter cover crops or incorporating cereal straw are not yet clear.