Interacting effects of water and nitrogen addition on soil-plant sulfur dynamics in a semi-arid grassland

How soil sulfur (S) transformation processes and S availability respond to nitrogen (N) addition and precipitation increment are still not clearly understood in grassland ecosystems. Thus, we examined the inorganic S fractions, organic S and available S in the soil. We also measured the S concentrations in the above-ground tissues of six plant species under N and water addition in a semi-arid steppe. Nitrogen addition promoted soil acidification and the transformation of inorganic S fractions as demonstrated by the dissolution of insoluble S into soluble S and adsorbed S, therefore increasing soil S availability. This resulted in significant increases in plant S concentrations, except Leymus chinensis, under N addition. Water addition accelerated the transformation of soil S fractions through increasing the abundances of S cycling genes and enzyme-encoding sulfate reduction genes. Specifically, water addition increased the ratio of soluble S to adsorbed S. Additionally, higher soil organic S and soil total S concentrations under water addition treatment could also be due to increased S return from above-ground plant biomass, sulfate input from irrigation water, and S transported from deep soil layers by plant roots. However, water addition decreased the tissue S concentrations of Stipa krylovii and Leymus chinensis, possibly due to the dilution effect caused by enhanced biomass production. Nitrogen and water addition synergistically accelerated soil S cycling and transformations, which could mitigate ecosystem S deficiency and enlarge soil S pools in semiarid grasslands.