Background: Enhancing soil organic carbon (SOC) sequestration in degraded lands is critical for climate mitigation and sustainable agriculture. While straw amendment combined with microbial inoculants holds great promise, the underlying mechanisms governing its impact on soil microbiome and carbon cycling genes remain poorly understood. Results: Here, we employed metagenomic sequencing to analyze responses in soil carbon (C) cycling genes, microbial community structure, and functional profiles across three degradation levels (severely, moderately, and non-degraded) of cinnamon soils under straw application alone or in combination with microbial inoculants. Results showed that both straw and straw-microbial inoculants treatments significantly improved soil properties, with improvements in available nitrogen and microbial biomass carbon (severe degradation), SOC (moderate degradation), and available nutrients (non-degradation). The combined application notably reshaped microbial communities by enhancing bacterial alpha diversity while reducing fungal diversity, and strengthened the relationship of relevant key soil C genes in severely degraded soils. Soil pH exhibited significant positive correlations with soil C cycling genes. Key bacterial genera ( Sphingomonas , Bradyrhizobium ) showed strong associations with ABC transporters and glycoside hydrolases, and fungal genus ( Chaetomium ) linked to pyruvate and purine metabolism. Importantly, we observed degradation-level specificity: straw addition significantly increased the abundance of the amylase gene K01214 (encoding α-amylase for starch hydrolysis) in severely degraded soils, whereas the straw-inoculant combination enriched the chitinase gene K01207 (encoding chitinase for chitin hydrolysis) in moderately degraded soils. Conclusions: Accordingly, we propose targeted application of straw with a customized chitinolytic-cellulolytic synthetic microbial community (1–5% of straw mass) to restore carbon cycling functions in degraded soils, while adopting optimized agronomic management to preserve microbiome stability in non-degraded soils. Our findings provide novel insights into microbial-mediated carbon cycling and a foundation for targeted soil restoration.

Soil film mulching can improve crop productivity, yet the relative importance of hydrothermal, nutrient, and microbial pathways remains unclear. In this spring wheat–summer maize rotation field experiment conducted in the Yellow River Irrigation District, we evaluated the effects of a soil film formed from carboxymethyl cellulose ammonium (CMC-NH₄) solution on crop yield, soil properties, and microbial communities. The results showed that soil film mulching enhanced soil temperature and moisture retention, with greater CMC-NH₄ application rates intensifying these hydrothermal improvements. Soil moisture played a more important role than soil temperature in promoting yield in both crop seasons. Bacterial community structure shifted markedly, with α diversity initially increasing before declining and β diversity showing a decreasing trend over time. PLS-PM further revealed contrasting pathways between the two crops, with hydrothermal conditions dominating spring wheat yield and hydrothermal and chemical pathways contributing comparably to summer maize yield. These findings indicate that soil film mulching enhances crop yield, with the dominant pathway differing between spring wheat and summer maize. This suggests that film application should be managed according to crop season and local water and nutrient conditions, offering a practical option for sustainable crop production in irrigated regions of the Yellow River Irrigation District.