The mixture of cover crop residues induced a synergistic effect on microbial communities and an additive effect on soil organic matter priming

Applying cover crop residues to increase soil organic matter (SOM) is a widely used strategy to sustainably intensify agricultural systems.  However, fresh residue inputs create “hot spots” of microbial activity during decomposition which could also “prime” the decomposition of native SOM, resulting in accelerated SOM depletion and greenhouse gas emissions. Microbes exert control over SOM decomposition and stabilisation as a consequence of their carbon use efficiency (CUE), the balance between microbial catabolism and anabolism. The CUE during residue decomposition and the extent to which native SOM decomposition is primed by residue addition may depend on residue biochemical quality.  Given that cover crops may be grown in monoculture, or in species mixes with the aim of providing multiple benefits to agricultural ecosystem services, it is important to understand whether applying cover crop residues as a mixture results in a different CUE and soil carbon stock, than would be expected by observations made on the application of individual residues. We used ¹³C labelled cover crop residues (buckwheat, clover, radish, and sunflower) to track the fate of cover crop residue-derived carbon and SOM derived carbon in treatments comprising a quaternary mixture of the residues and the average effect of the four individual residues (non-mixture) one day after residue incorporation in a laboratory microcosm experiment. The soil microbial community composition was measured by phospholipid-derived fatty acids (PLFA) fingerprint. Our results indicate that, despite all treatments receiving the same amount of plant-added carbon (1 mg C g^-1 soil), the total microbial biomass (¹²C + ¹³C) in the treatment receiving the residue mixture was significantly greater, by 3.69 µg C g^-1, than the average microbial biomass observed in the four treatments receiving individual components of the mixture. The microbial biomass in the quaternary mixture, compared to the average of the individual residue treatments, that can be attributed directly to the plant matter applied, was also significantly greater by 3.61 µg C g^-1. However, there was no evidence that the mixture resulted in any more priming of native SOM than average priming observed in the individual residue treatments. The soil microbial community structure measured by analysis of similarities (ANOSM) was significantly different in the soil receiving the residue mixture, compared to the average structure of the four communities in soils receiving individual residues. Differences in the biomass of fungi and Gram-positive bacteria were responsible for the observed synergistic effect of cover crop residue mixtures on total microbial biomass and plant-derived microbial biomass; especially biomarkers 16:0, 18:1ω9, 18:2ω6 and 18:3ω3. Our study demonstrates that applying a mixture of cover crop residues initially increases soil microbial biomass to a greater extent than would be expected from applying individual components of the mixture and that this increase may occur either due to faster decomposition of the cover crop residues or greater CUE, but not due to greater priming of native SOM decomposition. Therefore, applying cover crop residue mixtures could be an effective method to increase soil microbial biomass, and ultimately soil carbon stocks in arable soils.