Winter cover crop legacy effects on litter decomposition act through litter quality and microbial community changes

In agriculture, winter cover crop (WCC) residues are incorporated into the soil to improve soil quality, as gradual litter decomposition can improve fertility. Decomposition rate is determined by litter quality, local soil abiotic and biotic properties. How these factors are interlinked and influenced by cropping history is, however, unclear. We grew WCC monocultures and mixtures in rotation with main crops Avena sativa (oat) and Cichorium endivia (endive) and tested how crop rotation influences WCC litter quality, abiotic and biotic soil conditions, and litter decomposition rates. To disentangle WCC litter quality effects from WCC soil legacy effects on decomposition, we tested how rotation history influences decomposition of standard substrates and explored the underlying mechanisms. In a common environment (e.g. winter fallow plots), WCC decomposition rate constants (k) correlated negatively with litter C, lignin and, surprisingly, N content, due to strong positive correlations among these traits. Plots with a history of fast-decomposing WCCs exhibited faster decomposition of their own litters as well as of the standard substrates filter paper and rooibos tea, as compared to winter fallow plots. WCC treatments differentially affected soil microbial biomass, as well as soil organic matter and mineral nitrogen content. WCC-induced soil changes affected decomposition rates. Depending on the main crop rotation treatment, legacy effects were attributed to biomass input of WCCs and their litter quality or changes in microbial biomass. Synthesis and applications. These results demonstrate that decomposition in cropping systems is influenced directly through crop residues, as well as through crop-induced changes in soil biotic properties. Rotation history influences decomposition, wherein productive winter cover crops (WCC) with low lignin content decompose fast and stimulate the turnover of both own and newly added residues via their knock-on effect on the soil microbial community. Thus, WCC have promise for sustainable carbon- and nutrient-cycling management through litter feedbacks.