Abstract. Nitrogen losses via nitrate leaching, ammonia volatilization and nitrous oxide emissions were measured from contrasting outdoor pig farming systems in a two year field study. Four 1-ha paddocks representing three outdoor pig management systems and an arable control were established on a sandy loam soil in Berkshire, UK. The pig management systems represented: (i) current commercial practice (CCP) -25 dry sows ha 71 on arable stubble; (ii)`improved' management practice (IMP) -18 dry sows ha 71 on stubble undersown with grass, and (iii)`best' management practice (BMP) 12 dry sows ha 71 on established grass. Nitrogen (N) inputs in the feed were measured and N offtakes in the pig meat estimated to calculate a nitrogen balance for each system.In the first winter, mean nitrate-N concentrations in drainage water from the CCP, IMP, BMP and arable paddocks were 28, 25, 8 and 10 mg NO 3 l 71 , respectively. On the BMP system, leaching losses were limited by the grass cover, but this was destroyed by the pigs before the start of the second drainage season. In the second winter, mean concentrations increased to 111, 106 and 105 mg NO 3 -N l 71 from the CCP, IMP and BMP systems, respectively, compared to only 32 mg NO 3 -N l 71 on the arable paddock. Ammonia (NH 3 ) volatilization measurements indicated that losses from outdoor dry sows were in the region of 11 g NH 3 -N sow 71 day 71. Urine patches were identified as the major source of nitrous oxide (N 2 O) emissions, with N 2 O-N losses estimated at less than 1% of the total N excreted. The nitrogen balance calculations indicated that N inputs to all the outdoor pig systems greatly exceeded N offtakes plus N losses, with estimated N surpluses on the CCP, IMP and BMP systems after 2 years of stocking at 576, 398 and 264 kg N ha 71, respectively, compared with 27 kg N ha 71 on the arable control.These large N surpluses are likely to exacerbate nitrate leaching losses in following seasons and make a contribution to the N requirement of future crops.

The recovery of nitrogen ‘retained’ through cover crop uptake, delayed ploughing and immobilization by straw was assessed in a spring cropping rotation on a chalk loam in Eastern England (1989–96). The effect of annual cover cropping on yield of the subsequent spring crops and on the soil N balance was also investigated. The recovery of retained N was in part dependent upon cover crop management. Late August-sown cover crops which were incorporated in February/March tended to reduce spring crop yields and crop N offtake. Adverse effects on soil N supply, seedbed conditions and soil water reserves were not in evidence and so an allelopathic effect from the decomposition of the rye cover crop, previously reported by others, may be responsible for the reduction in yield of spring crops. When the cover crops were drilled later and their early destruction was followed by a short fallow period, spring crop yields and N offtake were increased. The soil N balance indicated that over the course of the experiment there was a positive N input to the system due to continuous cover cropping. This input may be held as immobilized organic N, in which case it could be made available to subsequent crops over a number of years or lost via other routes. Nitrate concentrations in drainage water increased with the number of years under cover cropping.

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