Fungal necromass carbon contributes to organic carbon sequestration within soil macroaggregates under manure application combined with plastic film mulching

Purpose Microbial necromass carbon (C) is a major contributor to soil organic C (SOC) sequestration. Soil aggregates are key functional units in regulating SOC dynamics. Fertilization and plastic film mulching, as common agricultural management practices, affect soil aggregation and microbial activities. However, how microbial necromass C is accumulated and contributes to organic C sequestration within soil aggregates remains poorly understood, especially under fertilization combined with plastic film mulching. Methods We set the mulching and no-mulching sub-treatments under three long-term (28 years) fertilization treatments (no fertilization, NF; inorganic fertilization, IF; manure application, MF) and conducted a 900-day field mesocosm experiment. We quantified the proportion of microbial necromass C in organic C within soil aggregates using amino sugar biomarkers, and investigated the effects of fertilization combined with mulching on the dynamics of microbial necromass C within soil aggregates. Results Microbial necromass C accounted for 28.2-42.9% of organic C of macroaggregates (> 0.25 mm) and 40.4-55.8% of organic C of microaggregates (< 0.25 mm) on day 900. The proportion of fungal necromass C in organic C of soil aggregates was nearly two times more than that of bacterial necromass C within 360-900 days. Regardless of mulching or no-mulching, the MF treatment increased the microbial necromass C content within macroaggregates and microaggregates, on average, by 148.6% and 84.5% compared with the NF treatment during the entire incubation period, respectively. Mulching facilitated microbial necromass C accrual within macroaggregates under the NF and IF treatments only on day 360, but increased it under the MF treatment on both day 360 and day 900. Mulching increased the fungal necromass C content and its contribution to organic C within macroaggregates by an average of 17.4% and 11.2% under the MF treatment during the entire incubation period, respectively. Conclusions Manure application combined with mulching promoted organic C sequestration within macroaggregates via the accumulation of fungal necromass C.