Plastic film mulching combined with manure fertilizer application promotes microbial necromass carbon accumulation within soil macroaggregates

Plastic film mulching is a common agricultural management to increase crop yield in the dry and cold regions. The improved soil hydrothermal environment under mulching conditions could change soil microbial activities and soil aggregation, thereby affecting soil organic carbon (C) sequestration. However, it remains not clear that how mulching regulates microbial necromass C accumulation and distribution within soil aggregates, especially under different fertilizer applications. We analyzed the contents of fungal and bacterial necromass C (taking amino sugar as biomarkers) and their contributions to organic C within soil aggregates under mulching combined with different fertilization treatments (no fertilization, CK; inorganic fertilizer application, IF; and manure fertilizer application, MF) in a 900-day in-situ field experiment. On day 360, the contents of fungal and bacterial necromass C within macroaggregates were 25% and 12% higher in the mulching combined with IF treatment, and were 20% and 32% higher in the mulching combined with MF treatment relative to the corresponding no-mulching treatments, respectively. On day 900, the mulching combined with CK and IF treatments decreased microbial necromass C content within soil aggregates, while the mulching combined with MF treatment promoted microbial and fungal necromass C accumulation within macroaggregates (>0.25 mm), compared with the corresponding no-mulching treatments. Mulching increased the fungal/bacterial necromass C ratio within macroaggregates on day 900, but decreased this ratio within microaggregates during the whole incubation period compared with the corresponding no-mulching treatments. Moreover, microbial necromass C occupied 28%–43% and 40%–56% of organic C within macroaggregates and microaggregates on day 900, respectively. Overall, mulching combined with the application of manure fertilizer greatly promoted microbial necromass C accumulation, and thus increased organic C sequestration within macroaggregates.