One possible explanation for the difference between their results and our results could be the process of aggregate formation over the period of the experiment and the type of soil. When residues are incorporated into soil, there may be rapid immobilization of the N during the early stages of decomposition. The mineral N may form complexes with the binding agents, preventing them from forming aggregates [11]. At a later stage, there will be a steady increase in mineral N thereafter, indicating that mineralization is greater than immobilization. Furthermore, the initial soil and exogenous N availability for the residue, expressed as the proportion (residue þ soil) N/total residue dry matter, varied from 1.6% to 4.5%. These values are above the threshold value (1.1%e 1.2% of residue dry matter) proposed by Recous et al. [14] for the overall N concentration limiting C decomposition. This is supported by the fact that increasing the soil N content had no effect on residue C mineralization [15].The addition of mineral N increased the microbial biomass for all treatments.