Keywords: Nitrogen cycle, Nitrate, Nitrous oxide, 15N, Fertiliser, Model.
Productive agriculture inevitably leads to some leakage of nutrients to the environment, especially nitrate and phosphate to water and nitrogen (N) -containing gases to the atmosphere. It is necessary to manage agricultural systems such that production and environmental goals are integrated. Some environmental goals conflict with others: several of the measures to decrease nitrate movement to water are likely to increase emissions of nitrous oxide (N2O) which is a greenhouse gas and contributes to ozone depletion. Agriculture is estimated to contribute over 30% to total UK N2O emissions.
Under UK conditions experiments in which 15N-labelled fertilisers are applied to crops in spring show losses of fertiliser N during the growing season of between 5 and 35% of the N applied but commonly about 20%. Loss increases with increasing rainfall in the 3 week period following application. Gaseous pathways, mainly denitrification, predominate and usually account for at least two thirds of the loss but some direct leaching of nitrate from fertiliser can occur if spring rainfall is unusually large.
More accurate matching of fertiliser N to crop requirement will decrease the risk of under- or over-fertilisation and of leaving excessive residual nitrate in soil after harvest. In a maritime climate residual nitrate in soil in autumn is prone to leaching during winter. In arable agriculture at least 90% of the nitrate in soil during this period is usually derived from the mineralisation of soil organic N. Mineralisation is greater where manures have been applied or grass has recently been ploughed.
Dynamic models of N cycle processes are being developed as the basis for decision support systems for N fertiliser advice. Crop and soil diagnostics are also being sought to fine-tune advice based on models and as the basis for a new generation of models containing pools that can be measured. Models can also be used to plan less leaky crop rotations at the farm scale and, if appropriately modified, to simulate N losses from different agricultural scenarios at the catchment or national scale.
Maintaining crop cover during winter is generally an effective means of decreasing nitrate loss to water. The establishment of wetland buffer zones adjacent to rivers and the manipulation of drainage regimes also offer some promise. Varying fertiliser applications within a field to better match expected crop growth, as influenced by variations in soil properties, should decrease wastage of N and decrease residual soil nitrate in areas of poorer crop growth.
D.S. Powlson, Soil Science Department, IACR-Rothamsted, UK
42 pages, 3 figures, 109 refs.