Emissions of N 2 O were measured from different agricultural systems in SE Scotland. N 2 O emissions increased temporarily after fertilization of arable crops, cultivation of bare soil, ploughing up of grassland and incorporation of arable and horticultural crop residues, but the effect was short-lived. Most of the emission occurred during the first two weeks, returning to`background' levels after 30^40 days. The highest flux was from N-rich lettuce residues, 1100 g N 2 O-N ha À1 being emitted over the first 14 days after incorporation by rotary tillage.The magnitude and pattern of emissions was strongly influenced by rainfall, soil mineral N, cultivation technique and C : N ratio of the residue. Comparatively large emissions were measured after incorporation of material with low C : N ratios. Management practices are recommended that would increase N-use efficiency and reduce N 2 O emissions from agricultural soils.

SUMMARY The fate of both faecal Escherichia coli and E. coli O157 in slurry following application to arable and grass plots on a clay loam soil was studied. Slurry (5% dry matter) containing 53 × 104 ml−1 E. coli and 30 E. coli O157 100 ml−1 was spread in early March. Initially, almost all E. coli were retained in the upper layers of the soil. Escherichia coli numbers steadily declined to less than 1% of those applied by day 29, and E. coli O157 were only detected in the soil and on the grass for the first week after application. There was some transport of bacteria to deeper layers of the soil, but this was approximately 2% of the total; transport to drains over the same period was mainly associated with rainfall events and amounted to approximately 7% of applied E. coli. However, there were indications that periods of heavy rainfall could cause significant losses of E. coli by both leaching and run‐off. Experimental studies showed that E. coli O157 on grass, which was subsequently ensiled in conditions allowing aerobic spoilage, could multiply to numbers exceeding 106 g−1 in the silage.

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A method for labelling of suspended solids in sewage effluent is used to trace the distribution of deposited solids in three soils following leaching with effluent. This distribution is related to the effect of sewage effluent application on the soil hydraulic conductivity. A sand, a sandy loam, and a loessial silt loam were studied. The reduction in relative hydraulic conductivity was greatest for the silt loam. This was due to accumulation of solids at the soil surface. In the sand little deposition of solids in the soil and little change in hydraulic conductivity occurred. Results indicate that with proper management sewage effluent can he used for irrigation without causing problems due to lack of soil permeability.

Two complementary approaches to modelling soil anoxia and denitrification are compared. The first postulates a structurally and biologically heterogeneous soil matrix (physical aggregates) and Michaelis-Menten kinetics; the second a random distribution of cylindrical air-filled pores, a uniform metabolic activity and zero-order reaction kinetics. Simple functional approximations to both models are developed, allowing their differences to be explored. At water contents corresponding to aggregate saturation the anoxic fractions and denitrification rates predicted by the aggregate model exceed those predicted by the simple-structure model. In drier soils the predictions of the latter model typically exceed those of the former by between one and three orders of magnitude, the discrepancy lessening as the oxygen reaction potential and the mean radius of the aggregates increase. The aggregate model is much more sensitive to air-filled porosity, and shows a decreased denitrification efficiency when nitrate concentrations are low. It is likely to predict sharper rainfall-induced denitrification events, and a smaller background activity, than the distributed-pore treatment. Whichever of these or other process-based treatments of denitrification is adopted, simple approximations like those presented here greatly facilitate inclusion in largervolume systems models.