Long-term effect of residue return and fertilization on microbial biomass and community composition of a clay loam soil

A field study was carried out to examine the response of microbial communities of a clay loam soil to long-term (30 years) effects of residue return and fertilization. The experimental design was a split-plot arrangement of treatments, consisting of three residue treatments (crop residues returned at rates of 0, 2500 and 5000 kg/ha) in combination with eight fertilization treatments (control, no fertilizer; N, mineral nitrogen (N) fertilizer; P, mineral phosphorus (P) fertilizer; K, mineral potassium (K) fertilizer; NP, mineral NP fertilizer; NK, mineral NK fertilizer; PK, mineral PK fertilizer; and NPK, mineral NPK fertilizer). Soil microbial communities were characterized by phospholipid fatty acid analysis. Results indicated that the more crop residues were returned, the lower ratio of fungi to bacteria was observed. However, soil microbial biomass was only found to be significantly higher in plots with residues returned at a rate of 5000 kg/ha but not 2500 kg/ha. This suggested there was a threshold for microbial biomass to increase under residue return for the clay loam soil studied. The fertilization effect on soil microbial biomass gradually decreased with increases in the amount of crop residues returned. A significant composition change was observed under N fertilization. Structural equation modelling indicated that soil microbial communities were influenced directly by residue return and indirectly by residue-induced change in ratio of carbon to N and fertilization-induced change in soil pH.