Humic Acid with Manure Promotes Fungal Necromass Carbon Accumulation in Saline-Alkali Soils.

Abstract

Microbial necromass carbon is increasingly recognized as a vital precursor for stable soil organic carbon (SOC). However, how diverse organic amendments regulate its accumulation and the underlying mechanisms in saline-alkali soils remains poorly understood. Based on a 4-year field experiment and an additional 13C-labeling incubation, we investigated the variation in fungal necromass carbon (FNC) and bacterial necromass carbon (BNC) under four organic amendments (organic manure (OM), organic manure plus crop straw (OM+CS), organic manure plus humic acid (OM+HA), and an unamended control). OM stimulated nitrogen mineralization through leucine aminopeptidase activity, while structural C sources in crop straw amendments (OM+CS) enhanced cellulolytic enzyme activities and biomass accumulation. In contrast, OM+HA upregulated N-acetyl-glucosaminidase at 0-20 cm and alkaline phosphatase activity at 20-40 cm, concurrently enriching K-strategist fungi (e.g., Basidiomycota) through deterministic assembly. This was accompanied by more complex, highly modular fungal networks. Glucose-derived C was preferentially retained in the FNC pool after 150 days, with 13C-labeling confirming FNC contributes 28.8%-30.4% of 13C-SOC versus merely 4.6%-4.7% for BNC. While OM+CS promoted necromass via biomass-enzyme co-enhancement, OM+HA synergizes enzyme-driven FNC/BNC co-transformation with K-strategist fungal enrichment and increased 13C-BNC efficiency by 19.0% versus OM. These findings highlight OM+HA as a promising strategy for promoting microbial necromass C accumulation in degraded saline-alkali ecosystems.