Impacts of crop residue management on soil organic matter stocks: A modelling study

With crop roots, crop residues make up most of organic matter inputs in cultivated soils. These inputs are necessary to maintain soil carbon stocks. While maintaining soil carbon stocks, and possibly increasing them, has become a critical issue at local and global scales, crop residues are also a potential source of bioenergy. Therefore the following question arises: is it possible to use crop residues as a bioenergy source without depleting soil carbon stocks? To address this question we need to better understand the relationship between crop residue inputs to a soil and its corresponding carbon stock at equilibrium. Current models predict a linear relationship but they overlook a possibly important source of non linearity, namely priming effects. Priming effects are interactions between the decomposition processes of distinct organic substrates, such as crop residues and humified substrates. Here I present the results stemming from an analysis using a new mechanistic model for soil organic matter dynamics, which predicts a priori the existence of priming effects. I show that, in spite of priming effects, first order kinetics can emerge for the decomposition of humified organic matter, only with a different apparent constant than that which characterizes those substrates when decomposed alone. Thus priming effects would decrease the slope of the linear relationship between soil carbon stocks and crop residue inputs. I also show that methanization of crop residues could offer a promising trade-off between bioenergy production and soil conservation, because it reduces the priming intensity while producing humified organic matter which can be brought back to soils. Finally, I also use the model at the agroecosystem level, and show that plant covers with low N:C ratios and high turnover rates could further enhance both residue production and soil carbon stocks.