Comparison of microbial numbers and enzymatic activities in surface soils and subsoils using various techniques

Knowledge of microbial numbers and activity in subsoils is essential for understanding the transformation and downward movement of natural and synthetic organics. Soil cores were taken from two soil profiles (surface textures: silty clay loam and loamy sand), and samples extracted from the 0-30 cm (surface), 1.0-1.3 m (mid) and 2.7-3.0 m deep; clay) and 3.9-4.2 m (deep: sand) layers. A variety of soil biotic (microbial numbers, microbial biomass, enzyme activities) and abiotic properties (pH, organic C, texture, CEC) were measured. Bacterial numbers decreased with depth as indicated by viable counts and by calculations based upon biomass carbon and extracted DNA. Direct microscopic counts were the most sensitive method of enumeration and gave bacterial numbers between 37 and 442 X greater than colony formin g units and those calculated from DNA extracted from soil. DNA extracted from soil ranged from 1.23 (sand surface) and 1.34 (clay surface) mug g(-1) d wt soil to 0.02 (sand deep) and 0.01 (clay deep) mug g(-1) d wt soil. Bacterial numbers, estimated from biomass-C measurements, were comparable to direct counts. Large numbers of bacteria were recorded in the subsoils (direct counts: 5.6 X 10(8) sand, 4.5 X 10(8) clay) even though this was equivalent to only 4.7 and 1.7% of those in the surface soils. Fungi were isolated from surface and mid-depth layers of both soils but were absent from the deep soil samples. Enzymatic activities (arylsulphatase, beta-glucosidase, phosphomonoesterases, urease, dehydrogenase, FDA hydrolysis), assayed with or without buffers, also decreased with depth. The exception was urease activity in the clay soil where no difference was seen between mid and deep in non-buffered assays but a 2.9-fold greater activity was exhibited in the mid than in the surface soil when buffered, Strong positive correlations (R > 0.95) were observed between all enzyme activities (except with urease activity in clay soil and non-buffered phosphatase activity in sand soil) and between all methods of estimating bacterial abundance. Strong positive correlations (R > 0.90) were also found between bacterial abundance and enzyme activities and between enzyme activities and organic matter content. (C) 2002 Elsevier Science Ltd. All rights reserved