Soil Carbon Accumulation Under Afforestation Is Driven by Contrasting Responses of Particulate and Mineral‐Associated Organic Carbon

Abstract Afforestation is widely believed to sequester carbon (C) in soil. However, the effect of afforestation on soil organic C (SOC) accumulation is still debated due to the contrasting features of particulate and mineral‐associated organic C (POC and MAOC). We conducted a field investigation of 144 paired sampling sites by comparing afforested and non‐afforested lands to investigate the POC and MAOC dynamics after afforestation across the Danjiangkou basin in subtropical China, where forests are dominated by Platycladus orientalis, Quercus variabilis and Pinus massoniana. The average contents of SOC, POC, and MAOC were significantly increased by afforestation; however, POC and MAOC responded differently to afforestation type. All afforestation types promoted the POC content, and MAOC also showed positive responses to afforestation except that afforestation with P. massoniana from shrubland significantly reduced the MAOC content. With increasing SOC content, the POC grew at a faster rate than MAOC at high SOC levels. Afforestation hindered the growth rate of POC, while it promoted the growth rate of MAOC as SOC accrued, which potentially obscured the distinct patterns of C accumulation triggered by afforestation. The variation partitioning suggests that, under afforestation, microbial traits had a higher contribution to both POC and MAOM variations compared with non‐afforested land. These results suggest that the robust buildup of microbial biomass due to increased plant C input following afforestation could contribute to soil C accumulation by promoting microbial necromass.