Application of the ninhydrin-reactive N assay for microbial biomass in acid soils

Values for microbial biomass C and N, as assayed by a fumigation-incubation and a fumigation-extraction method were compared for a range of acid soils. Values for microbial biomass C obtained from CO2-evolution data using the fumigation-incubation method, were far less than those calculated from ninhydrin-reactive N obtained by the fumigation-extraction method, and in some cases were negative. For a given soil, discrepancies between the two methods of assay increased with soil acidity (pH range, 4-6), and were attributed to an over-correction in the fumigation-incubation assay for CO2 values of the unfumigated control soil. The extent of overcorrection seemed to be related to the failure of the fumigated, re-inoculated soils to convert organic C to CO2 at the same rates as in the unfumigated control soils, but not to the amounts per se of readily-decomposable substrate in the soils. By contrast, the amounts of NH4+ N released during oxidative decomposition of killed microbial cells in the fumigation-incubation method, compared closely with the amounts of NH4+ N plus amino acid N derived by autolysis and deamination in the fumigation-extraction method. The amounts of nitrogenous products released by the two methods, and hence calculated values for biomass N, were not significantly different. Biomass N (fumigation-incubation) = 1.05 X Biomass N (fumigation-extraction). The performance of the fumigation-extraction assay for biomass C was further evaluated by measuring biomass C in each of the acid soils, as soil pastes in water, and in buffers with pHs ranging between 4-6. The magnitude of the ninhydrin-reactive N flush from chloroform-fumigated soils was increased by soil water content and was affected by soil pH. For soil pastes at 100% water holding capacity, there was a linear relationship between estimated values of biomass C and assay pH. This relationship determined the extent of adjustment of assayed biomass C values to correct for the effects of different soil pHs arising from specific agronomic practices. Biomass C Adjustment Factor = e0.301(delta soil pH) Thus, a one unit increase in pH, over the pH range 4-6, gave a 35% increase in the estimated biomass C value. The effects of soil pH and water content on biomass values demonstrates the need for closer standardization of assay indicators, preferably with a wider pH range of soils extending to those of neutral to alkaline pH.