Soil Structure and Greenhouse Gas Emissions

Keywords: nitrous oxide, methane, carbon dioxide, mitigation, tillage, trampling, compaction, porosity.

Soil structure affects microbial activity and thus influences greenhouse gas (GHG) production and exchange in soil. Structure is variable and increasingly vulnerable to compaction and erosion damage as agriculture intensifies and the climate changes. The objective of this paper was to draw from research mainly in Scotland that examined how soil structures affected by wheel compaction, animal trampling, tillage, fertiliser type and land-use change influence GHG emissions in order to help identify key controlling properties. Nitrous oxide (N₂O) is the main focus, though carbon dioxide (CO₂) and methane (CH₄) are included. Gas emissions were measured by using static chambers in the field or incubated intact cores. Poor structure and impeded drainage, measured as low porosities and pore continuities, restricted aeration such that N₂O was emitted or consumed dependent on mineral nitrogen (N) levels. Structural damage (identifiable using the Visual Evaluation of Soil Structure (VESS)) was especially important near the soil surface where microsites of microbial activity were exposed and aeration was impaired. Damage to structure was caused by compaction by tractors and field machinery and by trampling by cattle. N₂O emissions were not necessarily increased in anaerobic soils due to possible N₂O consumption and microbial adaptation. Moist, well-aerated soils favoured CH₄ oxidation and CO₂ exchange. A range of management interventions to obtain minimum risk of GHG emission is identified. Key management aspects are good choice and timing of fertiliser and manure applications, avoiding compaction and animal trampling of wet soils, localisation of machinery wheelings and increasing structural stability by applying amendments of organic matter.

Bruce Ball, Paul Hargreaves and Joanna Cloy, SRUC (Scotland’s Rural College), West Mains Road, Edinburgh EH9 3JG, UK.

35 pages, 12 figures, 7 tables, 77 references.