Surface and subsurface microbial biomass, community structure and metabolic activity as a function of soil depth and season

Microbial biomass, size and community structure along with an estimate of microbial activity and soil chemical parameters were determined at three depths in two soils (e.g. sandy loam Ultic Hapludalf and silt loam Mollic Hapludalf) replicated three times under one winter and summer season. Microbial biomass and community structure were estimated from phospholipid-PO₄ content and fatty acid methyl ester (FAME) measurements. Microbial activity and assimilative capacity were estimated using a ³H-acetate incorporation into phospholipids and by incubating the soil samples at the average winter and summer temperatures, 3 and 20 °C, respectively. We found that the size of the microbial biomass in both the surface and the subsurface soils was not significantly affected by the seasonal variation but activity increased by as much as 83% at the summer temperatures in the surface soil. We demonstrated using FAME analysis that for both soils seasonal changes in the subsurface microbial community occurred. These findings suggest that winter conditions will shift the population activity level in both the surface and subsurface systems and the biochemical structure of the community in the subsurface. In all cases, the inorganic chemical properties of the soil, as a function of season, remained constant. The greatly increased activity of microbial population at the higher temperature will favor the capacity of the system to utilize nutrients or organic materials that may enter soil. During low temperature seasons the capacity of either surface or subsurface soils to assimilate materials is generally diminished but the reduction reflects changes in metabolism and not a reduced biomass size.