Plant residue biochemistry regulates soil carbon cycling and carbon sequestration

Substrate composition is one of the most important factors influencing the decomposition of plant residues in soils. The interaction of organic residue biochemistry with residue decomposition rates, soil aggregation and soil humus composition was determined in a laboratory experiment. Addition of seven different organic residues (2% w/w alfalfa, oat, canola, clover, soybean, corn and prairie grasses) to a Webster soil resulted in a rapid, transient increase in aggregate mean weight diameters (MWD) when incubated for 9 d with residues with low phenolic acid content (alfalfa, canola and clover) and was inversely correlated with soil carbohydrate content (r=−0.63). More pronounced improvement in aggregate size was noted upon increased incubation to 84 d with organic residues higher in phenolic acid content (corn, prairie grasses, oat and soybean) and was related to soil phenolic acid (r=0.65) and soil carbohydrate content (r=0.70). Total plant residue phenolic acid content was related to MWD measured after incubation for 84 d by a quadratic response and plateau function (r=0.96) and the MWD quadratically increased with an increase in vanillin-vanillic acid concentrations in the plant residues (r=0.997). Soil organic C after 84 d was related to the MWD (r=0.82) and the residueʹs vanillin-vanillic acid content (r=0.86), suggesting that C remaining in the soil following decomposition maybe related to the specific phenolic acid content. The results suggest that transient aggregate stability initiated by microbial decomposition of the carbohydrate and amino acid content of the residue, is then strengthened by the interaction with phenolic acids such as vanillin or vanillic acid released by microbial decomposition from residues structural components.