Straw interlayer burial improved soil humic fractions via pore-microbe mediated life strategy hierarchy regulating...

Humus is a special form of organic matter in the soil and an increase in soil humus is of great relevance to maintaining soil carbon (C) stability and improving soil quality. While straw interlayer burial has demonstrated efficiency in saline soil amelioration through salt leaching and return suppression, its mechanistic impacts on soil organic C (SOC) and the humus C content, as well as associated drivers remains unclear. Therefore, a 4-year field experiment was conducted to clarify the impacts of varying straw interlayer (0, 6, 12, 18 Mg ha^-1) on SOC content, humic fractions (HA, humic acid; FA, fulvic acid; HU, humin), pore structure obtained by CT scanning, microbial life strategies across soil depths. After buried straw interlayers for 4 years, SL12 (12 Mg ha^-1) increased SOC content by 25-34 % at 40-55 cm compared to CK (0 Mg ha^-1). At upper soil (30-35 cm), increased >200 μm macro-porosity under SL12 and SL18 stimulated r-strategy bacteria (Proteobacteria, Bacteroidetes, and Firmicutes), enriching FA and HU contributions to SOC as compared with CK. Within the interlayer (40-45 cm), SL12 and SL18 elevated oligotrophic K-strategy bacteria (Acidobacteria, Chloroflexi, and Planctomcetes), driving lignocellulose conversion to humic precursors and subsequent HA polymerization (21 % higher HA/FA ratio in SL12 vs. CK). In deeper subsoil (50-55 cm), macro-porosity with >200 μm under SL12 and SL18 facilitated downward transport of dissolved organic C coupled with dual-pathway salinity suppression, suppressing mineralization to improve the contribution of HA and FA to SOC. This hierarchical pore-microbe interplay regulated C redistribution, maximal SOC accumulation at 40-55 cm while improving humus stability. Straw interlayers burial thus establishes an ecological engineering strategy for sustainable C management in degraded lands.