Microbial Biomass, Activity, and Community Structure in Subsurface Soils

ABSTRACT Microbial biomass, community structure and activity were determined in the subsurface horizons of four contrasting soil types common to Alabama. Biomass and community structure were determined by analyzing the fatty acids of the extractable phospholipids. Activity was estimated by measuring the rate of enzymatic hydrolysis of fluorescein diacetate (FDA). In all four soils, biomass and activity declined with increasing depth; however, the magnitude and pattern of this decline varied as a function of soil type. Biomass concentrations in the lower Ap horizon ranged from 8.2 to 18.0 nmoles phospholipid/g dry wt soil. In the deepest subsoil horizons, the range was 0.0085 to 0.059 nmoles phospholipid/g dry wt soil. The rate of FDA hydrolysis was highly correlated with biomass (r=0.90). Polyenoic fatty acids, which are present only in cukaryotic microorganisms, were found in all horizons; however, their relative abundance differed as a function of soil type. Stepwise regression analysis revealed that soil characteristics could explain 89 and 82% of the variation in biomass and activity, respectively. The results indicated that the vertical distribution of microorganisms in a soil profile differs greatly as a function of soil type. Hence, soil type may be an important determinant as to whether potential ground‐water pollutants are biodegraded as they pass through the unsaturated zone of a soil profile.