Field N2O, CO2 and CH4 fluxes in relation to tillage, compaction and soil quality in Scotland

Tillage practices and weather affect the release of greenhouse gases but there have been few integrated studies of the quantities released or the mechanisms involved. No-tillage may increase emissions of nitrous oxide (N(2)O) and the fixation of carbon by decreasing carbon dioxide (CO(2)) emissions. Tillage may also decrease the oxidation rate of atmospheric methane (CH(4)) in aerobic soil. These effects are partly due to compaction and to the lack of both soil disturbance and residue incorporation. Our objective was to investigate how tillage practices, soil conditions and weather interact to influence greenhouse gas emissions. Here we present early measurements of N(2)O and CO(2) emission and CH(4) oxidation in two field experiments in Scotland under a cool moist climate, one involving soil compaction plus residue incorporation and the other involving no-tillage and two depths of mouldboard ploughing of a former grass sward, The experiments were located 10-15 km south of Edinburgh on a cambisol and a gleysol. In order to monitor emissions regularly, at short intervals and over long periods, a novel automatic gas sampling system which allows subsequent automated determination of both N(2)O and CO(2) fluxes was used. Both N(2)O and CO(2) fluxes were episodic and strongly dependent on rainfall. Peak N(2)O emissions were mainly associated with heavy rainfalls after fertilisation, particularly with no-tilled and compact soils. In the tillage experiment, N(2)O fluxes and treatment differences were greater under spring barley (Hordeum vulgare L.) (up to 600 g N ha(-1) per day) than under winter barley. CO(2) emissions in the few weeks after sowing were not strongly influenced by tillage and diurnal variations were related to soil temperature. However, periods of low or zero CO(2) fluxes and very high N(2)O fluxes under no-tillage were associated with reduced gas diffusivity and air-filled porosity, both caused by heavy rainfall. Early results show that CH(4) oxidation rates may best be preserved by no-tillage. The quality of the loam/clay-loams and the climate in these experiments makes ploughing, preferably to 300 mm depth, and the control of compaction necessary to minimise soil N(2)O and CO(2) losses. The gas exchange response of different soil types to tillage, particularly methane oxidation rate which is affected by long-term soil structural damage, is a potentially useful aspect of soil quality when taken in conjunction with other qualities. (C) 1999 Elsevier Science B.V. All rights reserved.