Comparison of microbiological methods for evaluating quality and fertility of soil

Routine soil testing procedures that are rapid and accurate are needed to evaluate C and N mineralization in agricultural soils in order to determine soil quality and fertility. Laboratory methods were compared for their usefulness in determining soil microbial biomass and potential activity in a Weswood silty clay loam (fine, mixed, thermic Fluventic Ustochrept) subjected to long-term tillage, crop sequence, and N-fertilizer management practices. The methods included basal soil respiration, net N mineralization during a 10-day incubation, soil microbial biomass C with the chloroform fumigation-incubation technique with and without subtracting a control value, soil microbial biomass N with the chloroform fumigation-incubation technique, substrate-induced respiration, and arginine ammonification. All methods were highly correlated with each other and, therefore, appear to adequately reflect soil microbial biomass and potential activity under laboratory conditions. The longer incubation times used with the basal soil respiration, N mineralization, and microbial biomass C and N assays resulted in higher correlations and lower variation among replications compared to the shorter incubation times used with substrate-induced respiration and arginine ammonification. The relatively rapid procedural time (3 h) required for the latter two assays could make these methods more attractive for routine soil testing, although multiple assays on the same sample may be necessary because these methods are less precise than the incubation methods that require 10 days.