Chemical, biochemical and microbial diversity through a Pachic Humudept profile in a temperate upland grassland

There is great interest in understanding the factors that drive soil microbial activity and community composition in upland grassland ecosystems. We investigated the role of vertical gradients of chemical properties and various soluble C and N pools on soil microbial community structure by using a combination of chemical and biochemical methods coupled with PCR-DGGE community fingerprinting and PLFA (phospholipid fatty acid) profiling. Soil samples were collected at increasing depth from a temperate upland grassland. Soil organic matter-related pools (TOC, TN) and functionally related active pools (microbial biomass C and N, K2SO4-extractable C and N, extractable organic N) markedly decreased with soil depth and were positively related to each other. The microbial community deep in the soil profile appeared neither C- nor N-limited. Conversely, DGGE community fingerprinting of Bacteria, α-Proteobacteria, β-Proteobacteria and Actinobacteria revealed that each soil horizon was conducive to the dominance of particular ribotypes thus confirming links between soluble nutrients gradients and community structure. Soil microbial biomass C assessed by PLFA content decreased with soil depth less steeply than when estimated by the chloroform fumigation-extraction (CFE) method. This suggests that chloroform fumigation efficiency in lysing microbial cells varied with soil depth with the varying amount and distribution of total and soluble C. The PLFA biomarker for fungal biomass markedly decreased throughout the three upper horizons, whereas that for arbuscular mycorrhizae strongly decreased in the deeper AB horizon only. Taken together with the increase across soil profile of total saturated-to-total monounsaturated fatty acids ratio, the PLFA data suggest that a compositional shift from fungal to bacterial dominance has taken place throughout the grassland layers.