Long-term effects of grass-clover leys on the structure of a silt loam soil in a cold climate

Grass/legume leys are commonly included in the crop rotation in mixed farming systems in cold or humid regions in order to sustain the supply of nutrients and maintain soil fertility. Leys are also known to sequester soil organic carbon and also improve the structural stability and mechanical properties of soil. However, few studies have investigated the long-term effects of ley rotations on the architectural properties of the structural pore space in soil. We investigated the effects of grass/clover leys on soil structure in the topsoil and upper subsoil of a silt loam in a long-term field trial established in 1956 at Offer in northern Sweden. This experiment includes four treatments with varying proportions of ley (1, 2, 3 or 5 years) in 6-year rotations. We used X-ray tomography to quantify topsoil structural pore space at a resolution of 65 μm in the first year of arable cropping following the ley break, a few weeks after sowing in spring. Earthworm populations were quantified by both hand-sorting and chemical extraction, while near-saturated infiltration was measured as a proxy for soil structure in the upper subsoil. In the topsoil, the treatments with a greater proportion of ley had larger organic carbon contents, smaller bulk densities and larger porosities. However, effects of crop rotation on the pore space were limited to pores smaller than 65 μm, as no treatment effects were found for the volume, size distribution, connectivity or complexity of the X-ray imaged pore space, even though the grass-clover leys promoted larger numbers and biomass of topsoil-dwelling earthworm species. Furthermore, no positive effects of grass/clover leys on organic carbon content and soil structure were found in the subsoil. The macropore infiltration capacity in upper subsoil was generally very small (overall median value of 3 mm h−1) indicating a lack of functional macroporosity below plough depth. Consistent with these results, no individuals of deep-burrowing earthworm species were found at the site, while previous observations showed only shallow rooting, both of which are attributed to the cold climate and poor subsoil drainage.