Soil and residue management effects on cropping conditions and nitrous oxide ̄uxes under controlled traf®c in Scotland 2. Nitrous oxide, soil N status and weather

Nitrogen from fertilisers and crop residues can be lost as nitrous oxide (N2O), a greenhouse gas that causes an increase in global warming and also depletes stratospheric ozone. Nitrous oxide emissions, soil chemical status, temperature and N2O concentration in the soil atmosphere were measured in a ®eld experiment on soil compaction in loam and sandy loam (cambisols) soils in south-east Scotland. The overall objective was to discover how the intensity and distribution of soil compaction by tractor wheels or by roller just before sowing in ̄uenced crop performance, soil conditions and production and emissions of N2O under controlled traf®c conditions. Compaction treatments were zero, light compaction by roller (up to 1 Mg per metre of length) and heavy compaction by loaded tractor (up to 4.2 Mg). In this paper we report the effects on production and emissions of N2O and relate them to soil and crop conditions. Nitrous oxide ̄uxes were substantial only when the soil water content was high (>27 g per 100 g). Fertiliser application stimulated emissions in the spring whereas crop residues stimulated emissions in autumn and winter. Heavy compaction increased N2O emissions after fertiliser application or residue incorporation more than light or zero compaction. The bulk densities of the heavily and lightly compacted soils were up to 89% and 82% of the theoretical (Proctor) maxima. Higher soil cone resistances, temperatures and nitrogen availability and lower gas diffusivities and air-®lled porosities combined to make the heavily compacted soil more anaerobic and likely to denitrify than the zero or lightly compacted soil. Compaction suf®cient to increase N2O emissions signi®cantly corresponded with adverse soil conditions for winter barley (Hordeum vulgare L.) growth. Soil tillage, which ensures that soil compaction is no greater than in our light treatment and is con®ned to near the soil surface, may help to mitigate both surface ̄uxes of N2O and losses to the subsoil. # 1999 Elsevier Science B.V. All rights reserved.