A Rapid Assay to Estimate Soil Microbial Biomass Potassium in Agricultural Soils
2010
Lorenz, Nicola | Verdell, Kenneth | Ramsier, Cliff | Dick, Richard P.
Various assays for microbial biomass C (C ₘᵢc) and other elements in the soil are available but there are none for microbial biomass K (K ₘᵢc). Our objective was to develop a rapid chloroform-fumigation extraction (CFE) assay to estimate Kₘᵢc The extractants ammonium acetate (CH₃COONH₄) and sodium hydrogen carbonate (NaHCO₃) were tested at three molarities on a Hoytville clay. A 1 mol L⁻¹ CH₃COONH₄ yielded the highest levels of extractable K. To calibrate the CFE efficiency for K ₘᵢc, bacteria and fungi isolated from soils were cultured, suspended, and mixed with three soils of different texture: a fine sand, a silt loam, or a clay. The known concentrations of K ₘᵢc in the inoculum served as a basis to calculate the microbial biomass K conversion factors (k EK) for the three soils tested. Soils from corn (Zea mays L.) fields in Ohio under no-till, minimum tillage, and plow tillage had K ₘᵢc values ranging from 68 to 179 mg K kg⁻¹ The K ₘᵢc value was highest in the silt loam under no-till and in the clay soil under minimum tillage. Soil exchangeable K (K ₑₓcₕ) and nonexchangeable K (K ₙₑₓcₕ) did not correlate with K ₘᵢc, but with total C. On average, K ₑₓcₕ was 2.7 and K ₙₑₓcₕ 8.3 times higher than K ₘᵢc The data presented in this study are based on an initial study, and more soils and ecosystems need to be evaluated before the method can be generally adopted for soil samples. Our study shows, however, that K ₘᵢc is a significant K pool in soils that has been overlooked in the past.
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