Divergent pathways of straw versus biochar integration with manure to enhance SOC sequestration in coastal saline soils

The capacity of soil organic carbon (SOC) storage in agroecosystems can be effectively increased through the application of organic amendments. However, information on the pathways through which different organic amendments regulate SOC sequestration in degraded coastal saline land is limited. Thus, a two-year field experiment was established to assess SOC, microbial necromass C (MNC) contents, enzyme kinetics, pore structure, and microbial co-occurrence networks under different amendment applications (compound fertilizer, CK; compound fertilizer plus manure, OM; compound fertilizer plus manure plus straw, OM+ST; and compound fertilizer plus manure plus biochar, OM+BC). Compared with CK and OM, OM+ST and OM+BC increased SOC content and reduced soil salinity. However, their functional divergence clearly emerged. OM+ST primarily increased the activity of C-cycling enzymes, labile C pools (i.e., dissolved organic C and easily oxidizable organic C), and the accumulation of microbial necromass C. In contrast, OM+BC promoted macroaggregate formation and aggregate stability by transforming particles in the size range of 0.053–2 mm and optimizing macropores, ultimately increasing SOC storage capacity. Microbial network reconstruction revealed that the OM+ST treatment was associated with reduced abundance of Actinobacteria-dominated network modules, whereas OM+BC treatment increased the relative abundance of Ascomycota-dominated modules associated with SOC storage. Overall, our findings provide valuable insights into the promotion of SOC accumulation through integrated organic amendment strategies in coastal saline-alkali agroecosystems.