Assessment of microbial carbon pool in chernozem and sod-podzolic soil of different types of agricultural land use by luminescence microscopy method

Microbial biomass carbon (Cmic) is a biologically active pool of soil organic matter and a sensitive indicator that responds to changes in land use much earlier than total organic carbon content and other pools within it. The luminescent microscopy method allows assessing not only the total content of Cmic but also the structure of microbial biomass: the number of spores and the length of fungal mycelium, as well as the number of bacterial cells and actinomycete hyphae. The aim of the study was to determine the structure of the microbial carbon pool in the profiles of chernozem and sod-podzolic soils depending on the type of agricultural use (hayfield, abandoned land, arable land, pasture) using the luminescent microscopy method. The microbial carbon pool in the surface layer of sod-podzolic soil ranged from 260 to 450 μg/g soil depending on the type of land use, in chernozem – 450 μg/g for abandoned land and 190 μg/g for arable land. Down to the depth of 30 cm, Cmic in chernozem under abandoned land was 1.5–3 times higher compared to arable land. In deeper horizons, differences between various land use types became less pronounced but persisted for a number of microbiological indicators. The fungal component, predominantly represented by mycelium, completely dominated (97–99%) the microbial carbon pool in both soils. The length of fungal mycelium in sod-podzolic soil and chernozem decreased by an order of magnitude with depth from hundreds of m/g to tens of m/g soil. Most indicators of the structure of the microbial carbon pool (fungal abundance, mycelium length of fungi and actinomycetes, prokaryotic biomass) were higher in chernozem under abandoned land compared to arable land; however, these patterns were not observed for sod-podzolic soil. Prokaryotic biomass and share, as well as mycelium length, were noted as the only indicators of the microbial carbon pool structure determined by the luminescent microscopy method that were associated with different types of land use for both considered soils. Very high fungi/bacteria ratios are likely due to the underestimation of bacterial cell biomass by the method used. Despite the identified limitations of the method, luminescent microscopy can be used as a supplement to more accurate methods for assessing the microbial carbon pool, allowing the determination of the size and structure of fungal biomass in soils of different types of agricultural land use.