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

Abstract

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.