Dynamics of microbial biomass carbon and nitrogen and extractable nitrate in long-term rotation studies at Indian Head

Non-Peer ReviewedMineralization studies in the laboratory indicate that the net turnover of nitrogen is greatly affected by crop rotation. Recently, the field applicability of such incubation - intermittent leaching experiments has been questioned. Therefore, field sampling was undertaken during the summer of 1988 to determine the influence of rotation history on microbial biomass C and N, and levels of extractable NO3-N. Four long term rotation plots (Ag. Canada, Indian Head) were sampled in the fallow phase. These rotations consisted of: (1) Fallow-Wheat-Wheat, (2) Fallow-Wheat- Wheat (fertilized, straw retained), (3) Fallow-Wheat-Wheat (fertilized, straw removed), and (4) Fallow-Wheat-Wheat-Hay-Hay-Hay. Biomass C and N, as measured by a chloroform-direct extraction technique, was found to be significantly higher in the soils from the hay rotation. The proportion of organic C present as biomass was, on average, 29 % higher than in the three year rotations without hay. Similarly, nitrate levels were found to be significantly affected by rotation history and correlate strongly with the size of the microbial biomass. The prediction of microbial biomass based on carbon added as crop residue was poor since the intrinsic assumption that all carbon is equally available for decomposition does not hold for all residues. However, the levels of biomass C and N were closely related to the N content of the residues returned. This is to be expected since the N-rich "metabolic" fraction is readily decomposed and incorporated into the microbial biomass. These relationships are clearly illustrated using a conceptual model of N turnover