Abstract: Microbial residues are major contributors to soil organic carbon (SOC). However, their responses to mixed plantation forests remain unclear. In this study, we examined the accumulation of microbial residues in soil at depths of 0-20 cm in  Cunninghamia lanceolata-Michelia macclurei mixed plantation and C. lanceolata pure plantation by analyzing amino sugars and using the ratio of Gram-positive to Gram-negative bacteria to determine the conversion coefficient of bacterial residues. The results showed that bacterial residues at depths of 0-10 cm were 25.3% lower in the mixed plantation than that in the monoculture, leading to SOC being 11% lower in the mixed plantation. The lower accumulation of bacterial residues in the mixed plantation was mainly due to the lower accumulation in macroaggregates. However, the accumulation of total microbial residues and their contribution to SOC did not differ between the mixed plantation and monoculture. These results indicate that the M. macclurei and C. lanceolata mixed plantations after 35 years would reduce bacterial residues as well as soil carbon sequestration, which would be detrimental to the long-term maintenance of soil fertility.

Key words: amino sugar, microbial residue, Chinese fir, soil carbon, mixed plantation