SPATIAL VARIABILITY OF SOIL MICROBIAL BIOMASS MEASURED BY THE FUMIGATION EXTRACTION METHOD, AND K-EC AS AFFECTED BY DEPTH AND MANURE APPLICATION

The horizontal and vertical variability of ''total'' soil microbial bi omass measured by the fumigation-extraction method (FEM), and the extr action efficiency coefficient K-EC, were investigated at the plot scal e 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 bi omass was measured by the fumigation-extraction method (FEM), and the K-EC was determined by in situ C-14-labelling; total and soluble C wer e also measured on all samples. Manure application increased the varia bility of all measured properties at the 0-20cm and 20-40 cm depths. T he 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 spat ial variability was similar to that of total C, and was highly correla ted with total C throughout the soil profile. At the plot scale, sampl ing distances of greater than 1 m were required to adequately measure biomass. K-EC values did not correlate with organic carbon, soluble ca rbon, clay or silt content. K-EC values for soils sampled from all dep ths 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 sa mples was significantly lower and more variable than that for surface soils. The results of this study suggest that using K-EC values for su rface soils to calculate subsurface soil biomass using the FEM would r esult in a significant underestimate of the true subsurface soil bioma ss value. Crown copyright (C) 1998 Published by Elsevier Science Ltd. All rights reserved.