Overcoming salinity barriers: granular straw promotes carbon cycling in saline soils by reshaping microbial community assembly

Saline soils, characterized by elevated salinity, depleted soil organic carbon (SOC), and degraded structure, constrain global agricultural productivity. While straw incorporation is known to promote SOC turnover, its efficacy in saline soils is limited by salt-impaired microbial decomposition. We evaluated how straw form regulates microbial-driven SOC turnover across salinity gradients (light-S1, moderate-S2, severe-S3) and temperatures (10 °C-T1, 20 °C-T2, 30 °C-T3) through a 360-day incubation. Treatments included conventional chopped wheat straw (CW), granular wheat straw (GW), wheat straw biochar (BW), and no-straw control (CK). Straw form significantly altered fungal α-diversity ( p < 0.05) but not bacterial. Fungal diversity was enhanced by BW and T3, while S2 exhibited maximal fungal richness. Ascomycota dominated fungal communities (65.0–97.7%), with GW increasing their relative abundance. Higher temperatures reduced Ascomycota dominance (T1 > T2 > T3, p < 0.05). Multivariate analyses confirmed that straw form, temperature, and salinity collectively drove distinct clustering of microbial communities. Under the same environmental temperature and soil salinity, the soil fungal communities of each treatment were significantly separated according to the different forms of straw application. Among them, the distribution of soil fungal communities in the GW treatment differed significantly from other treatments. Co-occurrence networks revealed GW enhanced microbial network complexity and stability. The Mantel test analysis showed that straw form to SOC turnover via microbial restructuring, demonstrating stronger fungal-physicochemical correlations than bacterial. Critically, GW altered microbial community structure, as evidenced by the distinct separation of fungal communities and increased complexity of co-occurrence networks, and promoted SOC turnover. These findings establish granular straw as an effective practice to overcome the decomposition barriers inherent to conventional straw in saline soils, thereby promoting nutrient cycling and supporting sustainable land management.