The legacy effect of Cover Crops on soil structure amelioration

Soil structure is highly sensitive to intensive management practices, making its amelioration a critical component of sustainable agriculture. Cover crops (CCs) are employed as part of an integrated approach to improve soil quality by stimulating plant- and microbial-mediated aggregate formation. This study investigated the legacy effect of CC applications on soil aggregation in a long-term field experiment in Germany, measuring organic carbon (OC) distribution and aggregate stability (Mean Weight Diameter, MWD) after three crop rotations in the second main crop (winter wheat) following the CC treatments. Four single CC species (mustard, phacelia, clover, and oat) and a fallow treatment were compared to a four-species mixture (Mix4) and a highly diverse 12-species mixture (Mix12). The results showed that all CC treatments significantly increased the MWD compared to the fallow, with increases ranging between 10% and 19%. Crucially, the average MWD increase was significantly higher for CC mixtures (16%) than for single CC species (12%). The highly diverse Mix12 resulted in the highest overall MWD improvement (19%). Structural Equation Modelling (SEM) demonstrated that CC type significantly impacted aggregate OC distribution. While most OC was stored in the smallest macroaggregate fraction (< 1 mm), single CCs tended to increase OC storage in this small fraction, whereas CC mixtures were more likely to increase OC accumulation in the largest macroaggregate fractions (8–16 mm), which correlate positively with MWD. Layers below the maximum cultivation depth (> 15 cm) showed the strongest positive response to CC treatments, with the biodiverse mixtures showing additional benefits in the 20–30 cm depth. These differences suggest that combining diverse functional root traits, varied litter qualities, and rhizodeposits enhance plant-microbial interactions as key mediators of aggregate formation are more effectively in mixtures than in single species. The consecutive integration of CCs provides a positive legacy effect for structure amelioration in arable soils. Increasing functional diversity in CC mixtures is recommended as a strategy to further enhance the benefits of CCs for sustainable soil management.