A legume-grass cover crop builds mineral-associated organic matter across variable agricultural soils

Soil organic matter (SOM) fractions convey unique functions in agroecosystems, but the degree to which cover crops build multiple fractions of SOM and increase soil C storage is understudied. Particulate organic matter (POM) releases nutrients through microbial decomposition, whereas mineral-associated organic matter (MAOM) is associated with long-term C storage. We conducted a greenhouse experiment using ¹³C to trace the transfer of C from four cover crop treatments – cereal rye (Secale cereale), crimson clover (Trifolium incarnatum), a rye-clover mixture, and a no cover crop fallow – into these two SOM fractions in soils from 10 working farms that varied in texture, management history, and soil microbial communities. On average, MAOM C was 7.4% higher in the mixture treatment than in the fallow; however, this was not a significantly greater increase than in the cereal rye treatment. The amount of C transferred to MAOM and POM increased with cover crop biomass and soil C content, and was also moderated by fungal community composition. When compared to the rye treatment, the mixture provided a threefold greater transfer of C and a 2% greater transfer of nitrogen from POM to the more stable MAOM fraction, which is associated with long-term C sequestration. Overall, our results suggest that cover crop mixtures are a useful management strategy to increase agroecosystem multifunctionality. When grown in soils with high biological activity, mixtures can simultaneously stabilize C in soil while also increasing internal N cycling capacity of agroecosystems.