Interpretation of Microbial Soil Indicators as a Function of Crop Yield and Organic Carbon

An interpretative framework for microbial biomass C (MBC), basal respiration, and the activity of soil enzymes cellulase, β-glucosidase, arylsulfatase, and acid phosphatase was developed for the clayey Oxisols of the Brazilian Cerrado. Soil samples (0–10-cm depth) were collected from 24 treatments from three long-term experiments and analyzed to determine their microbial attributes and soil organic C (SOC). These treatments presented a large range of Mehlich-extractable P and cumulative corn (Zea mays L.) and soybean Glycine max (L.) Merr. yields. The critical levels for the microbial indicators were defined based on criteria similar to those used in soil nutrient calibration tests. The microbial indicators were interpreted as a function of the relative cumulative yields (RCYs) of corn and soybean and the SOC using linear regression models. Adequacy classes for each microbial indicator as a function of the RCY and SOC were established based on the following criteria: ≤40%: low; 41 to 80%: moderate; and >80%: adequate. The critical levels equivalent to 80% of the RCY for MBC, basal respiration, cellulase, β-glucosidase, acid phosphatase, and arylsulfatase were: 375 mg C kg−1, 90 mg CO2–C kg−1, 105 mg glucose kg−1 d−1, 115 mg p-nitrophenol kg−1 h−1, 1160 mg p-nitrophenol kg−1 h−1, and 90 mg p-nitrophenol kg−1 h−1, respectively. Similar critical levels were obtained when SOC was used as the interpretation criterion. The interpretation tables provided in this study establish, for the first time, reference values for the soil microbial indicators based on crop yields and constitute a first approximation. Their applicability to other conditions must be evaluated.