Stable soil organic carbon is positively linked to microbial-derived compounds in four plantations of subtropical China

Abstract. Indigenous broadleaf plantations are increasingly being developed to substitute pure coniferous plantations to increase biodiversity and soil fertility in subtropical China. To assess how plantation types affect soil organic carbon (SOC) chemical composition, we used the solid-state ¹³C nuclear magnetic resonance spectroscopy with cross-polarization and magic-angle spinning (CPMAS-NMR) technique to analyze SOC and litter C chemical compositions in a coniferous (Pinus massoniana) and three broadleaf (Castanopsis hystrix, Michelia macclurei and Mytilaria laosensis) plantations in subtropical China. Soil microbial community composition and biomass were investigated with the phospholipid fatty acids (PLFAs) and chloroform fumigation-extraction methods, respectively. The SOC chemical composition varied with plantation type, with 34% of the SOC found in the alkyl C fraction in the P. massoniana plantation compared to < 28% in the broadleaf plantations. The amount of total PLFAs, bacterial and particularly the gram-positive bacterial population size, and microbial C / N ratio were correlated with the alkyl C content and alkyl C / O-alkyl C ratio. However, the soil alkyl C content was not correlated with the recalcitrance of leaf litter or fine roots. We thus suggest that the stable SOC composition could be attributable to the contribution of microbial-derived C compounds, rather than leaf litter or fine root quality or a direct flux of C from recalcitrant litter materials to more stable SOC pools.