From substrate limitation to competition control: straw input regulates sulfate-mediated suppression of methanogenesis in paddy soils

Sulfate fertilization has been considered an effective method for mitigating CH₄ emissions from paddy soils. However, whether sulfate-mediated suppression of methanogenesis is controlled primarily by sulfate availability or by the availability of shared substrates during straw decomposition remains unclear. To address this, we added sulfate (50 or 200 mg kg⁻¹ SO₄²⁻) with and without rice straw (0.5%, w/w, dry soil basis) to an acidic paddy soil. Measurements included gas production, carbon substrate analyses, stable carbon isotopes, and metagenomic profiling. Without straw addition, methanogenesis remained low and was not significantly affected by sulfate amendment. In contrast, straw incorporation markedly increased acetate availability, stimulated methanogenic taxa such as Methanocella and Methanosarcina , and increased the abundance of genes involved in methanogenesis, indicating that straw inputs alleviated substrate limitation and enhanced methanogenic activity. Under these straw-amended conditions, high sulfate addition (200 mg kg⁻¹ SO₄²⁻) reduced CH₄ production by 23%, coinciding with rapid sulfate consumption, acetate depletion, reduced abundance of acetoclastic methanogenesis genes, and enrichment of dissimilatory sulfate reduction genes. Carbon isotope fractionation factors (αC < 1.05) indicated that CH₄ production remained dominated by acetoclastic methanogenesis in straw-amended treatments. Notably, non-canonical sulfate-reducing lineages, such as Nitrospirota and Acidobacteriota , dominated the dsrAB gene pool under sulfate-amended treatments. These findings demonstrate that straw incorporation shifts paddy soils from substrate limitation to competition-dominated microbial dynamics, thereby enabling stronger competition between sulfate reduction and acetoclastic methanogenesis for shared intermediates, particularly acetate. This work provides new insight into how substrate availability regulates microbial competition and CH₄ emissions in rice systems.