Spatial variability of soil microbial biomass measured by the fumigation extraction method, and KEC as affected by depth and manure application

Abstract

The horizontal and vertical variability of “total” soil microbial biomass measured by the fumigation-extraction method (FEM), and the extraction efficiency coefficient K EC, were investigated at the plot scale in an unfertilized soil and in a dairy manure-treated (100 t ha −1) soil. Soil cores (0–60 cm) were taken according to a balanced nested sampling design and were sectioned into 20-cm increments. Microbial biomass was measured by the fumigation-extraction method (FEM), and the K EC was determined by in situ 14C-labelling; total and soluble C were also measured on all samples. Manure application increased the variability of all measured properties at the 0–20 cm and 20–40 cm depths. The range was generally inferior to 0.1 m in the unfertilized plot and between 0.1 and 1 m in the manure-treated plot. Microbial biomass spatial variability was similar to that of total C, and was highly correlated with total C throughout the soil profile. At the plot scale, sampling distances of greater than 1 m were required to adequately measure biomass. K EC values did not correlate with organic carbon, soluble carbon, clay or silt content. K EC values for soils sampled from all depths fell within the range of 0.30 to 0.43, close to the commonly used consensus value of about 0.4. However, the K EC for subsurface soil samples was significantly lower and more variable than that for surface soils. The results of this study suggest that using K EC values for surface soils to calculate subsurface soil biomass using the FEM would result in a significant underestimate of the true subsurface soil biomass value.