Soil structure and greenhouse gas emissions: a synthesis of 20 years of experimentation

Soil structure affects microbial activity and thus influences greenhouse gas production and exchange in soil. Structure is variable and increasingly vulnerable to compaction and erosion damage as agriculture intensifies and climate changes. Few studies have specifically related the impact of structure and its variability to greenhouse gas (GHG) emissions over a wide range of soils and management treatments. The objective of this study was to draw from research in Scotland, Japan and New Zealand, which examined how soil structures affected by wheel compaction, animal trampling, tillage 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), methane (CH4) and nitric oxide (NO) are included. Gas emissions were measured by using static chambers in the field or incubated intact cores. Poor structure, measured as small relative gas diffusivities and air permeabilities, restricted aeration, resulting in N2O emission or consumption dependent on mineral nitrogen contents. Structural damage (identifiable using the Visual Evaluation of Soil Structure) was especially important near the soil surface where microsites of microbial activity were exposed and aeration was impaired. Moist, well-aerated soils favoured CH4 oxidation and CO2 exchange. N2O emissions were not necessarily increased in anaerobic soils because of possible N2O consumption and microbial adaptation. Soil matric potential, volumetric water content, relative diffusivity, air permeability and water-filled pore space are relevant indicators for N2O and CH4 flux and aeration status. As pore continuity and size are so relevant, pore-scale models are likely to have an increasing role in understanding mechanisms of GHG production, transport and release.