Lignin biochemistry and soil N determine crop residue decomposition and soil priming

Residue lignin content and biochemistry are important properties influencing residue decomposition dynamics and native soil C loss through priming. The relative contribution of high lignin residues to soil organic matter (SOM) may be less than previously believed, be more sensitive to soil N status, and may be more sensitive to increased temperature. We examined the role of residue biochemistry, temperature, and soil N on the decomposition dynamics of five crop residues varying in lignin content and composition (corn, sorghum, soybean, sunflower and wheat). We used natural abundance δ13CO2 to quantify residue decomposition and soil priming from a soil previously cropped to wheat-fallow or to corn-millet-wheat at 20 and 30 °C in a laboratory incubation. High lignin residues decomposed more completely than low lignin residues, supporting a new model of SOM formation suggesting high lignin residues have a lower efficiency for stabilizing SOM due to inefficient microbial processing. However, residues with lower residue respiration had greater soil C respiration (soil priming). Residue SG lignin was positively related to residue C respired and H-lignin positively related to soil C respired in all soils and temperatures, resulting in no net lignin chemistry effect on the combined total C respired. Effects of lignin on residue decomposition were most apparent in treatments with lower soil N contents indicating N limitation. Measuring both residue and soil respiration and considering soil N status is important to accurately assess the effects of residue biochemistry on soil organic carbon.