Mineralization-immobilization of sulfur in a soil during decomposition of crop residues varying by their biochemical quality and S content

Sulfur (S) is an essential element for plants and animals. Because elements combine in soil organic matter, the C and S cycles are interdependent. The recycling and decomposition of plant residues can be an important source of available sulfate in soils, compared to the soil available S, which requires prediction for adequate sulfur management in agricultural soils. The aims of our study was to investigate (i) the effect of soil sulfate availability on crop residue decomposition and associated S mineralization and (ii) the relationships between plant residue quality (both plant biochemistry and S content) and S and C mineralization. Therefore in a first soil incubation experiment we examined the effect of initial soil sulfate content on decomposition by adding sulfate at two different concentrations in a presence of a single rate of rapeseed straw addition in a loamy soil. Then in a second experiment, we used four other residues of different biochemical compositions and S contents (wheat straw, tall fescue, mustard and beech leaves) to assess the relationship between S mineralization and plant residue composition. Net C and S mineralization were measured continuously during a 175-day incubation at 20°C and gross S mineralization and immobilization were quantified using 35S soil labeling applied as pulses during incubation. The data obtained allowed to quantify S fluxes during plant residues decomposition, to discuss the relationships between kinetics of gross and net S mineralization, C dynamics, residues quality and their decomposition, and between net S mineralization and the C/S of plant residues.