Tree roots exert greater influence on soil microbial necromass carbon than above-ground litter in subtropical natural and plantation forests

The stabilization of soil organic carbon (SOC) relies heavily on both the production of microbial necromass carbon (MNC) and its protection by clay minerals. Above- and below-ground carbon (C) sources differ not only in the C quality and quantity that drive MNC production but also in MNC-mineral interactions. Here we test the hypothesis that root litter and exudates play a more important role in the formation and stabilization of soil MNC than above-ground litterfall in a natural subtropical moist forest and a tree (Castanopsis carlesii) plantation of southeastern China. We employed a two-factorial design of field experiment consisting of four treatments: control (CT), litterfall exclusion (NL), root exclusion (NR), and no C input (NI). We found that NR treatment substantially reduced total soil MNC and both bacterial (BNC) and fungal (FNC) necromass C as well as fractions of MNC and FNC to total SOC in both natural and plantation forests, but did not alter ratios of FNC to BNC or BNC to total SOC. In comparison, NL treatment had no effect on the ratios of MNC, FNC or BNC to total SOC in both forests. Furthermore, NR treatment reduced both bacterial and fungal biomass, but NL treatment had no significant effect on bacterial or fungal biomass in the natural forests. Whereas the NR and NI treatments significantly reduced MNC and FNC in both forests, the NL treatment reduced MNC and FNC only in the natural forest and reduced BNC only in the plantation forest. Analysis of structural equation model revealed that the C treatments could alter the accumulation of MNC directly or indirectly through changing soil available substrates (i.e., DOC, NH4+) and microbial community structure. Our data suggests that plant roots exert a stronger influence on the production and stabilization of MNC than above-ground C source.