Impact of land-use on soil structure and soil ecological properties in a long-term field experiment

Land use is known to exert a dominant impact on a range of essential soil functions like water retention, carbon sequestration, matter cycling and plant growth. In addition, land use management is known to have a strong influence on soil structure, e.g. through tillage and compaction. While the difference in topsoil structure between grassland and agricultural soil is huge, differences among different farming or grassland management practices can be more subtle. At the same time, soil structure is known to be a suitable indicator for many soil functions. That is, differences in carbon content or plant-available field capacity between different land uses can often be explained by different structural properties.

This impact of land use on the relationship between soil structure and biological indicators for soil processes was explored in the Global Change Exploratory Facility, a well-established (>5 years) field experiment in Bad Lauchstädt, Germany, comprising five land use types (conventional farming, organic farming, intensive meadow, extensive meadow, extensive pasture). 15 intact topsoil cores were sampled from each land use type in spring 2020 and soil structure and microbial activity were measured using X-ray CT and respirometry, respectively. Microbial activity was estimated by basal respiration at field moisture and by substrate-induced respiration with glucose solution under wet conditions. The aims of this study were to (1) quantify the impact of land use on these structural and biological soil properties and (2) to assess in how far microbial activity can be predicted by the structural properties.

Surprisingly, image-derived macroporosity did not differ between farming and grassland plots mainly due to the huge variability among compacted and non-compacted samples in the farming plots. Other pore metrics like pore distance and pore connectivity followed the same trend, whereas mean pore size was larger in the grassland plots due to more large biopores. Basal respiration increased in the order farming < meadow < pasture, whereas the order was reversed for substrate-induced respiration. The predictability of basal respiration (R²=0.29) and substrate-induced respiration (R²=0.5) with explanatory variables based on pore metrics and bulk soil properties was rather low, with root mass and bulk density being the best predictors.