Objectives Microbial necromass carbon (MNC) is a major source of stable soil organic carbon (SOC), and its accumulation is regulated by soil nutrient contents and their stoichiometry. This study investigated the contents and stoichiometric characteristics of soil carbon (C), N and P in citrus orchards on red-soil hills and to clarify their influence on microbial necromass accumulation, thereby providing a scientific basis for enhancing the carbon-sequestration capacity of orchard soils.

Methods Twenty-one representative Jinggang pomelo orchards in Ji’an City, Jiangxi Province, were selected. SOC, total N (TN) and total P (TP) were measured, and the C∶N, C:P and N:P ratios were calculated. MNC was quantified by amino-sugar biomarkers.

Results The SOC (11.19 g/kg) and TN (1.04 g/kg) in the 21 orchards were low, whereas TP (0.84 g/kg) was comparatively high; SOC exhibited the greatest among-orchard variability. The average C∶N, C:P and N:P ratios were 10.9, 13.9 and 1.3, respectively, indicating overall N limitation. MNC accounted for 41.5% of SOC and was dominated by fungal residues. MNC and fungal necromass C were positively correlated with SOC, TN and TP, whereas bacterial necromass C correlated only with SOC and TN. The contribution of total, fungal and bacterial necromass C to SOC decreased significantly with increasing C∶N ratio; only the bacterial contribution increased with N:P ratio.

Conclusions The soils in red soil hilly orchards of Jiangxi Province have relatively low SOC and TN contents, but relatively high TP content. The spatial variability of SOC is large, while the C∶N, C:P and N:P ratios are relatively low, and the soil as a whole presents N limitation. Microbial necromass carbon accounts for as high as 41.48% of the soil organic carbon in the orchard, and it is dominated by fungal residual carbon. The accumulation of microbial necromass in citrus orchard soils is regulated by both soil nutrient contents and stoichiometric ratios. Nutrient contents (SOC, TN, TP) mainly determine the yield of microbial residues, while the C∶N and N:P ratios control the contribution of microbial residues to SOC. Therefore, it is necessary to optimize the soil C∶N and N:P stoichiometric ratios, alleviate microbial nutrient limitation, improve the efficiency of the microbial carbon pump, and further realize the soil carbon sequestration function and sustainable management.