Crop Rotation and Residue Management Effects on Soil Carbon and Microbial Dynamics

Abstract

Abstract 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.