Continuous manuring combined with chemical fertilizer affects soil microbial residues in a Mollisol

In this study, the influence of 10 years' continuous application of organic manure at various rates combined with chemical fertilizer on microbial residues was evaluated in a highly fertile temperate soil. The presence and origin of microbial residues were indicated by amino sugar analysis. The treatments were: (1) CK, unfertilized control; (2) OM0, only chemical fertilizer, no manure added; (3) OM1, organic manure added at 7.5 Mg ha(-1) year(-1) plus chemical fertilizer; (4) OM2, organic manure added at 15 Mg ha(-1) year(-1) plus chemical fertilizer; and (5) OM3, organic manure added at 22.5 Mg ha(-1) year(-1) plus chemical fertilizer. Fertilization significantly increased the total amino sugar concentrations, especially in the plots with higher manure addition rates (OM2 and OM3 plots, P < 0.05). This suggests a positive effect of organic manure combined with chemical fertilizer on the accumulation of microbial residues in soil. However, the highest manure rate (OM3) did not lead to further increase in the total amino sugar pool as compared with the moderate manure rate (OM2). This suggests manure addition saturates in its effect on microbial residue build-up. The different patterns of individual amino sugars suggest a change in the quality of microbial-derived soil organic matter after 10 years.