Long-term land-use conversion effects on soil microstructure across a texture gradient

Soil microstructure regulates infiltration, aeration, root growth, and carbon cycling and is shaped by biotic and abiotic processes and management practices. Cropland-to-grassland conversion can modify soil microstructure through changes in rooting, continuous vegetation cover, and reduced mechanical disturbance. However, long-term land-use effects on three-dimensional pore architecture have rarely been evaluated across contrasting soil textures. We combined X-ray computed tomography (30-µm voxel size) and deep-learning segmentation (nnUNet) to quantify soil microstructure at nine paired grassland and cropland sites across Germany spanning 4–85% sand, soil quality ratings of 45–98, and grassland durations of 8–100 years. We analyzed 450 intact cores from two depths (10–20 and 45–60 cm), focusing on empty root channels, macroporosity, and total particulate organic matter (total POM; fresh roots, degraded roots, and other POM). We evaluated how grassland–cropland differences varied with texture (median particle diameter, D 50 ), site productivity potential (soil quality rating, SQR), grassland duration, and sampling season. Across sites, empty root-channel volume fraction (RC) was higher in grassland than cropland at both depths, whereas macroporosity responses were more variable and depended on texture and depth. Principal component analysis showed that variation in soil microstructural properties was more strongly associated with texture at both depths, while land-use effects were stronger in topsoil. Grassland–cropland contrasts in empty root channels and macroporosity decreased with increasing D 50 and were larger at sites with higher SQR. Grassland duration showed weaker associations with CT-derived structural properties than texture and site productivity potential. Structural equation modelling showed season-dependent POM responses in topsoil and both seasonal and duration-related patterns in subsoil. Overall, cropland-to-grassland conversion was associated with enhanced root-channel development, with structural differences depending more strongly associated with texture, site conditions and depth than on grassland duration. These findings highlight that the structural benefits of cropland-to-grassland conversion are strongly context dependent and should be evaluated in relation to site-specific soil conditions.