Legume cover crops enhance soil organic carbon via microbial necromass in orchard alleyways

Cover crops may enhance soil organic carbon (SOC) sequestration, but the links among SOC fractions, microbial necromass carbon (C), and SOC sequestration potential in legume and non-legume cover-cropped orchards are unclear. We leveraged data from seven orchards with varying climatic and edaphic properties in China to assess the effects of legume and non-legume cover crops on SOC and microbial necromass C in bulk soil, particulate organic matter (POM), and mineral-associated organic matter (MAOM). Legume cover crops led to a significantly greater increase in SOC than non-legumes (23 % vs. 2 %), compared with no cover crops. The MAOM-C with legumes and non-legumes were similar but were both significantly greater than the control plots without cover crops. Legume cover crops positively influenced bacterial and fungal necromass C in all SOC fractions, while non-legume cover crops only affected fungal necromass C in bulk soil and MAOM. The effect of cover crops on the microbial necromass C to SOC ratio was negligible. Fungal necromass contributed most of the microbial necromass C to SOC accumulation, particularly in MAOM. Linear regression showed SOC, POM-C, and MAOM-C accumulated with increasing microbial necromass C. Microbial necromass C was the key factor explaining the variation in POM-C and MAOM-C compared to other climatic and edaphic factors. Partial least squares path modeling showed a causal relationship among cover crops, climate, soil substrates, soil properties, microbial necromass, and SOC and SOC fractions. Our study suggests that changes in microbial necromass C and composition may explain most of the increase in SOC in cover-cropped orchards and that MAOM-C is the principal sink of microbial necromass. This information is valuable for improving our understanding of the potential impact of microbial necromass C on enhancing SOC sequestration potential in orchard soils under cover crop management.