The divergent accumulation mechanisms of microbial necromass C in paddy soil under different long-term fertilization regimes

To investigate the stabilization mechanism of microbial necromass carbon (C) in rice paddy soil, the accumulation of microbial necromass and its contribution to SOC in bulk soil and in various fractions were addressed under a 40-year fertilization trial. The fertilization regimes included no fertilizer (Control), nitrogen, phosphorus and potassium inorganic fertilizer (NPK), and inorganic fertilizer plus manure (NPKM). Coarse particulate organic matter (cPOM), fine inter-microaggregate POM (fPOM), intra-microaggregate POM (iPOM), non-occluded silt plus clay fraction (s + c_f), and silt plus clay occluded within microaggregates (s + c_m) fractions were isolated. The results indicated that fungal necromass were the main component of microbial necromass across the fractions; however, the bacterial role was enhanced in mineral fractions (s + c_f and s + c_m) compared to POM fractions (cPOM, fPOM, and iPOM). The iPOM and mineral fractions stored more than 90 % of microbial necromass C due to the physical protection (organo-mineral associations and occlusion within microaggregates). Compared to Control, the significantly increased content of microbial necromass C and its contribution to SOC in bulk soil were attributed to the enhanced content of fungal necromass in iPOM fraction under NPK treatment. The highest content of microbial necromass C was found under NPKM treatment; moreover, the increased content of microbial necromass C in iPOM and s + c_m fractions made up to 82.80 % of that in bulk soil compared to Control, while a saturation behavior was found in s + c_f fraction. The contribution of microbial necromass to SOC was not altered under NPKM treatment compared to Control. However, a reduced fungal/bacterial necromass C ratio was found in bulk soil and iPOM fraction under NPKM treatment compared to Control, mainly due to the supply of higher quality substrates and the increased soil pH. As a whole, this study revealed the diverse responses of microbial necromass C accumulation and its contribution to SOC under different fertilization regimes in paddy soil.