Crop residue decomposition, residual soil organic matter and nitrogen mineralization in arable soils with contrasting textures

To evaluate the significance of cropping, soil texture and soil structure for the decomposition of ¹⁴C- and ¹⁵N-labelled crop residues, a study was conducted in a sand and a clay soil under laboratory and field conditions. The distribution of residual ¹⁴C, residual ¹⁵N and microbial biomass ¹⁴C of different aggregate size classes and physical protection of soil organic matter as indicated by the rates of ¹⁴C and ¹⁵N mineralization after soil disaggregation were also studied in the same soils. Soil texture and soil structure were not determining factors in the decomposition of residual labelled soil organic matter, but residue type was important for N mineralization soon after incorporation. Recently formed labelled soil organic matter was less well physically protected than older soil organic matter and adsorption of soil organic matter on to silt and clay particles was the main mechanism of physical protection in sand soil. In clay soil the results were not conclusive as regards the main mechanism of protection of recently formed soil organic matter. In conclusion because soil structure broken up by soil disruption, and soil texture were found to have no effect on the rate of decomposition of recently formed organic matter in soil, we do not need to include these factors when the C and N mineralization from crop residues in arable soils has to be estimated.