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 th ere have been few integrated studies of the quantities released or the mech anisms involved. No-tillage may increase emissions of nitrous oxide (N2O) a nd the fixation of carbon by decreasing carbon dioxide (CO2) emissions. Til lage may also decrease the oxidation rate of atmospheric methane (CH4) in a erobic soil. These effects are partly due to compaction and to the lack of both soil disturbance and residue incorporation. Our objective was to inves tigate how tillage practices, soil conditions and weather interact to influ ence greenhouse gas emissions. Here we present early measurements of N2O an d CO2 emission and CH4 oxidation in two field experiments in Scotland under a cool moist climate, one involving soil compaction plus residue incorpora tion and the other involving no-tillage and two depths of mouldboard plough ing 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 regul arly, at short intervals and over long periods, a novel automatic gas sampl ing system which allows subsequent automated determination of both N2O and CO2 fluxes was used. Both N2O and CO2 fluxes were episodic and strongly dep endent on rainfall. Peak N2O emissions were mainly associated with heavy ra infalls after fertilisation, particularly with no-tilled and compact soils. In the tillage experiment, N2O fluxes and treatment differences were great er under spring barley (Hordeum vulgare L.) (up to 600 g N ha(-1) per day) than under winter barley. CO2 emissions in the few weeks after sowing were not strongly influenced by tillage and diurnal variations were related to s oil temperature. However, periods of low or zero CO2 fluxes and very high N 2O fluxes under no-tillage were associated with reduced gas diffusivity and air-filled porosity, both caused by heavy rainfall. Early results show tha t CH4 oxidation rates may best be preserved by no-tillage. The quality of t he loam/clay-loams and the climate in these experiments makes ploughing, pr eferably to 300 mm depth, and the control of compaction necessary to minimi se soil N2O and CO2 losses. The gas exchange response of different soil typ es to tillage, particularly methane oxidation rate which is affected by lon g-term soil structural damage, is a potentially useful aspect of soil quali ty when taken in conjunction with other qualities. (C) 1999 Elsevier Scienc e B.V. All rights reserved.