Soil structural indicators as predictors of biological activity under various soil management practices

Soil structure is a key feature in controlling the turnover of organic matter in soils. The spatial arrangement of solids and pores in agricultural topsoil can be actively influenced by management practices, such as tillage and cropping systems, which in turn can affect the resident microbial communities and their activities. However, carbon mineralisation and microbial activity are usually measured in sieved samples, which provides information on gross potentials under optimal conditions. Under these conditions, the spatial heterogeneities that are specific to different management practices are reduced or totally removed. In this study, we combined X-ray computer tomography (X-ray CT) and isothermal calorimetry to investigate the effect of soil structure on heat dissipation, as an indicator of biological activity. Samples were collected from the topsoil of a long-term field experiment (12 years) that included four different land uses: conventional vs. reduced tillage, each with either maize or winter wheat as the main crop in the rotation. We compared the response of undisturbed soil cores (3 cm in height, 2.7 cm in diameter) to the addition of water and glucose in specific pore sizes, ranging in radii of 15 to 75 µm or 3 to 75 µm. The pore structure and indicators of particulate organic material were quantified using X-ray CT with a voxel resolution of 15 µm. This allowed us to distinguish between the effects of crop rotation and tillage regime on biological activity, soil structure and the feedback between the two. Heat dissipation correlated significantly with X-ray CT derived porosity, pore surface density and soil matrix grey value, all of which were affected by both tillage regime and crop rotation. Heat dissipation in maize plots after glucose addition to the pore size range with radii of 3 to 75 µm was greater than in the winter wheat systems, but not when added to the pore size range with radii of 15 to 75 µm. The study showed that structural indicators can explain up to 81 % and 95 % of the variance in total heat dissipation after glucose and water addition, respectively, but only 60 % of the heat dynamics, here defined as the time taken for 50 % of total heat to be dissipated. The results emphasise the importance of soil structure in regulating microbial decomposition of soil organic matter and warrants further investigations.