Soluble Carbon and Nitrogen Pools of Prairie and Cultivated Soils: Seasonal Variation

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Abstract
Recent evidence indicates that long‐term cultivation of prairie soils reduces the ambient concentrations of soluble amino N and soluble sugars. It is not known whether cultivation has affected how these constituents fluctuate with season. This study was performed to determine if cultivation has affected how soluble sugars, soluble C, and soluble amino N fluctuate with season and whether this fluctuation corresponds with seasonal changes in microbial biomass and extractable N. Soils were sampled bimonthly and analyzed for 0.5 M K2SO4 soluble sugars (anthrone‐reactive carbon [ARC]), soluble organic C, microbial biomass, and 2 M KCl extractable amino N, NH4, and NO3. Cultivation has clearly reduced the levels of microbial biomass C, soluble ARC, and soluble amino N in these soils. Microbial biomass C was greatest in late February after thaw. Soluble ARC generally was greater in prairie soils than in cultivated soils but did not increase in concert with microbial biomass. Soluble amino N declined when microbial biomass and N mineralization increased in February. The ratio of soluble ARC to NO3‐N indicates that there is generally insufficient C available, as soluble sugars, to allow for immobilization of ambient NO3 in cultivated soils throughout the course of a year. The high ratio of soluble C to biomass C in cultivated soils may also be evidence of a reduction in organic matter quality as higher levels of soluble C appear to support lower levels of microbial biomass. Cultivation has reduced concentrations of soluble sugars and amino N and has affected how these constituents change with season. However, the relation of soluble sugars and amino N to microbial biomass is not clear.

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