How do wheat straw and sulfate application drive redox state and microbial communities to impact sulfur transformation in waterlogged paddy soils?

Sulfur (S) is essential for rice growth and plays a pivotal role in soil pollution remediation. Wheat straw (W) amendment and sulfate (SO₄^2-) fertilization are common agricultural practices in rice cultivation, yet their combined effects on S transformation, including organic sulfur (OS), available sulfate (AS), and reduced inorganic sulfur (RIS), in waterlogged paddy soils remain poorly understood. We conducted a 162-day incubation experiment with different W levels (0 %, 0.1 %, 0.5 %, and 1.0 %, w/w) and S rates (0 and 30 mg·sulfur·kg⁻¹, as SO₄²⁻) in flooded soil. The results demonstrated that both W and S applications alone enhanced RIS formation, and their co-application exhibited synergistic effects. Compared to the control, the co-application of W and S increased the proportion of RIS in total S by 76 % (90 %), 70 % (54 %), and 94 % (65 %) with 0.1 %, 0.5 %, and 1.0 % W at the early (middle) stages of incubation, respectively. The increase of RIS was attributed to the enhanced reduction of iron oxides and SO₄^2-, mediated by reducing bacteria (especially Fe reducing bacteria) under low pe + pH. Besides, W addition increased AS levels during the early stage irrespective of S application, especially at 1 % W, due to organic sulfate mineralization. In addition, the influence of W and S applications on S transformation diminished over time. These findings suggest that the co-application of W and S under waterlogged soil could optimize sulfate bioavailability and RIS formation, which could meet the demand of rice sulfur nutrition and contribute to soil pollution remediation.