Effects of freeze-thaw cycles on soil structure under different tillage and plant cover management practices

Soil structure plays a central role in many soil processes that are environmentally relevant. Intermittent freezing of the soil over winter is an important abiotic disturbance in temperate climates and its effects on soil structure depend on the soil’s preexistent structural strength and cohesion. Management choices such as tillage and plant cover after harvest strongly influence soil structure, and therefore the soil’s response to freeze-thaw. We examined the effects of 5 freeze-thaw cycles (FT) on the µCT-detectable structure of intact topsoil cores (Ø=100 mm, h=80 mm) from a long-term rotation and tillage experiment in Denmark. The cores were divided among two tillage treatments and two plant cover treatments, corresponding to a gradient of structural strength: CT-B<CT-V<NT-B<NT-V (CT=conventional tillage, NT=no-till, B=bare fallow and V=winter rye volunteers). We classified the µCT-detectable macropore volume in four size ranges (Full Range, <300 µm, 300–1020 µm and >1020 µm) and analyzed the macroporosity (Vt), mean macropore diameter (dm) and mean Euclidian distance to the nearest macropore (EDm). Additionally, we analyzed the effects of tillage and plant cover on several µCT-derived geometric parameters in Full Range. Overall, NT-B and NT-V resulted in lower macroporosity than in CT-B and CT-V. Similarly, we found fewer, less branched macropores with shorter mean branch length in NT compared to CT for both plant cover treatments. However, we propose that µCT-derived geometric parameters might be confounded by the overlapping influence of relatively few, complex and voluminous coarse macropores and the more abundant, less complex very fine macropores. Freeze-thaw, in turn, caused crumbling of soil around coarse macropores, reducing Vt and dm in Full Range and reducing Vt in the > 1020 µm range. Additionally, FT caused significant increases in Vt and reductions in dm and EDm in the < 300 µm range, indicating creation of new very fine macropores and expansion of previously indiscernible macropores. Overall, the effects of FT were reduced in NT (for equal plant cover treatments) and V (for equal tillage treatments), indicating greater resilience against FT in both cases.