Impact of crop residue location on carbon and nitrogen distribution in soil and in water-stable aggregates

One of the main effects of soil tillage is that it determines the distribution of crop residues in the soil profile. Little information is available on how this affects the distribution of carbon (C) and nitrogen (N) in the soil and more specifically in soil aggregates, as in general the contribution of other effects of tillage (i.e. mechanical disturbance) cannot be distinguished. A laboratory experiment was conducted with repacked soil columns to improve our understanding of the mechanisms of soil aggregate stabilization and storage of organic C in relation to residue location. Double-labelled oilseed rape residues (C-13 and N-15) were incorporated in the 0-10 cm soil layer or left on the soil surface. The soil columns were incubated for 33 weeks at 20 degrees C. Artificial rain events followed by evaporation imposed several dry-wet cycles on the soil-residue system. Spatial separation of surface-placed residues from the soil slowed down the decomposition rate compared with incorporated residues. This resulted, in the short term, in a large fraction of residue fragments at the soil surface (equivalent to 52% of carbon applied). In contrast, initial crop residue location did not significantly affect incorporation of residue-C in soil aggregates (only 10-12% of carbon applied), as new C entered the aggregate fractions mainly as soluble C. However, concentrations of residue-derived C and N were greater and aggregation increased near the soil surface with mulch, which in the longer term can modify physical and biological properties of the upper cm of the soil.