Soil organic carbon and pH govern the divergent trajectories of organic and inorganic sulfur under long-term fertilization

Abstract

Globally, declining atmospheric sulfur (S) deposition has increased the risk of S deficiency in intensive agricultural systems, yet the long-term changes in soil S fractions and their regulatory mechanisms remain poorly understood. Here, we used three 30-year (1990–2019) field experiments spanning China's temperate-to-subtropical climate gradient to examine changes in soil organic S (SOS) and inorganic S (SIS) fractions under contrasting fertilization regimes. Despite consistently positive apparent S budgets, topsoil SIS declined by 30–112 mg kg⁻¹ under most site-treatment combinations, whereas SOS remained stable or increased, indicating a decoupling between apparent S inputs and SIS retention. This divergence reflected the different controls on organic S stabilization and inorganic S retention. Linear mixed-effects models and partial least squares path analysis showed that soil organic carbon (SOC) and pH were the major regulatory factors: SOC was significantly positively correlated with SOS and its fractions, whereas pH governed SIS retention through pH-dependent sulfate adsorption and calcium-associated sulfate stabilization mechanisms. Integrated organic-mineral fertilization, namely NPKM (nitrogen, phosphorus, and potassium fertilizers plus manure), emerged as a promising strategy, as it simultaneously maintained crop yields, sustained SOS accumulation, kept soil available S above the reference deficiency threshold across sites, and reduced acidification pressure compared with mineral fertilization alone. These findings demonstrate that sustainable soil S management should move beyond input replacement and instead regulate soil-internal cycling processes, particularly SOC accumulation and pH buffering, to sustain soil S fertility under declining atmospheric S deposition.