Evaluating soil structure and biological activity in soil cores under different management systems

Soil structure is a key feature in controlling microbial access to organic matter in soils. The spatial arrangement of solids and pores in agricultural soils is shaped by the used tillage and crop system. However, spatial heterogeneities make it difficult to determine relationships between soil biology and soil structure, and often homogenized, sieved soils are used to evaluate organic matter turnover in soils. In this study, we used heat dissipation as an indicator for biological activity in soils taken from two different tillage systems (conventional vs. reduced tillage) and two different cropping systems (crop rotations with either maize or winter wheat as main crop) running for 12 years. In order to evaluate the impact of soil structure, we investigated the response of both repacked and undisturbed soil cores (3 cm in height, 2.7 cm in diameter) to water and glucose addition. Pore structure indicators and particulate organic matter content were quantified by X-ray computer tomography at a resolution of 15 µm.

We will show that calorimetry is a suitable tool to monitor the biodegradation of C sources in undisturbed soil cores and that both tillage system and crop rotation effect biological activity in soil. In summary, soil under maize cultivation dissipated more heat compared to the wheat crop rotation. In both, repacked and undisturbed samples, conventional tillage promoted heat dissipation in response to water addition, likely due to the annual incorporation of labile organic matter. However, structural and organic matter indicators could only explain the variance in heat dissipation to some extent. Thus, the usage of undisturbed soil cores provides new challenges to evaluate the link between soil structure and microbial activity due to increased variability.