Priming effects on soil organic carbon decomposition induced by high C:N crop inputs

Abstract

Carbon budgets in soils can be computed by a simple equation in which inputs are accounted by the humification of residues added to the soil, and outputs are estimated by the respiration of soil organic carbon (C_S). It is generally accepted that physical protection in soil aggregates and chemical recalcitrance limit the rate of C_S decomposition. We propose that soybean-based crop sequences will have lower steady state C_S stocks than those based on corn, because of lower C inputs and enhanced soil respiration of C_S due the so-called priming effect. To address this hypothesis, we studied C_S turnover from δ¹³C natural abundance changes in no-till corn and soybean crops. Soybean and corn crops were sown with a complete randomized design with three replicates on an old pasture with an intermediate soil δ¹³C signal of -16.34. Soils were sampled at the beginning and after two years of continuous no till cropping. Aboveground and belowground C inputs were measured each year for corn and soybean crops. Soil and plant samples were analyzed for C, nitrogen and ¹³C/¹²C ratio. Corn C inputs where significantly higher than soybean (10.7 and 7.0 Mg ha^-1), but no significant changes in C_S where observed among treatments (68 Mg ha^-1 in the first 0.3 m). However, the Cs turnover and humification rates under corn (0.015 and 0.18 y^-1) were higher than under soybean (0.006 and 0.11 y^-1). Contrary to our predictions high C:N corn residues enhanced C_S decomposition, but also the stabilization of fresh residues into C_S, suggesting that soil microbes mine the soil organic matter while feeding on high C:N crop residues. Possible explanations for such result are discussed.