Understanding microbial dynamics is important in the development of new management strategies to reverse declining organic-matter content and fertility of agricultural soils. To determine the effects of crop rotation, crop residue management, and N fertilization, we measured changes in microbial biomass C and N and populations of several soil microbial groups in long-term (58-yr) plots under different winter wheat (Triticum aestivum L.) crop rotations. Wheat-fallow treatments included: wheat straw incorporated (5 Mg ha-1), no N fertilization; wheat straw incorporated, 90 kg N ha-1; wheat straw fall burned, no N fertilization; and wheat straw incorporated, 11 Mg barnyard manure ha-1. Annual-crop treatments were: continuous wheat, straw incorporated, 90 kg N ha-1; wheat-pea (Pisum sativum L.) rotation (25 yr), wheat and pea straw incorporated, 90 kg N ha-1 applied to wheat; and continuous grass pasture. Total soil and microbial biomass C and N contents were significantly greater in annual-crop than wheat-fallow rotations, except when manure was applied. Microbial biomass C in annual-crop and wheat-fallow rotations averaged 50 and 25%, respectively, of that in grass pasture. Residue management significantly influenced the level of microbial biomass C; for example, burning residues reduced microbial biomass to 57% of that in plots receiving barnyard manure. Microbial C represented 4.3, 2.8, and 2.2% and microbial N 5.3, 4.9, and 3.3% of total soil C and N under grass pasture, annual cropping, and wheat-fallow, respectively. Both microbial counts and microbial biomass were higher in early spring than other seasons. Annual cropping significantly reduced declines in soil organic matter and soil microbial biomass.
