Medium-term transformations of organic N in a cultivated soil

We followed in situ the evolution of nitrogen recently incorporated into a soil under maize culture for 4 years. Each year, a different pair of plots treated by removal or return of maize crop residues received a single pulse of ¹⁵N-labelled fertilizer. Unlabelled fertilizer was otherwise supplied. In parallel, plots supplied with unlabelled fertilizer received a single pulse of ¹⁵N-labelled maize crop residues. Varying weather affected total and fertilizer-derived N in the crop and residual inorganic N in the topsoil, but it did not affect fertilizer-N immobilization and remineralization. There was no consistent effect of crop residue return on total soil N, immobilization of fertilizer N, or the decay kinetics of recently immobilized N. Recently incorporated organic N from crop residues and microbial immobilization of inorganic N displayed similar mid-term decay kinetics. Crop residue N and immobilized N enter a labile compartment with an average residence time of a few months. A proportion, estimated at 28%, enters a more stable compartment from which the mineralization was imperceptible in 4 years. Particle-size fractions >50 um, which receive most of the crop residue N, retained it for only a short time. The mid-term stabilization of N was mainly in soil fractions <50 um.