Enzymatic activity and functional diversity of soil microorganisms along the soil profile – A matter of soil depth and soil-forming processes

Most studies concerning soil microorganisms and enzymes are focused on the upper horizons of the soil profile even though they transform soil organic matter at every depth of the soil profile. The aim of this study was to determine how the soil microbial functional diversity and enzymatic activity changed as a function of depth across the soil profiles which developed on the same parent material (glacial till) but differed in their soil-forming processes thereby resulting in the formation of different reference soil groups (RSG) (Luvisols, Gleysols and Stagnosols). Four soil profiles were excavated at different sites that are located in two mesoregions of the South Baltic Lake District (central Poland) in fields with alfalfa (Medicago sativa L.) in the fourth year of its cultivation. Using the soil samples collected in June 2019, we assessed the oxidoreductase activity (DHA – dehydrogenases, POX – phenol oxidase, PER – peroxidase), the fluorescein diacetate hydrolysis (FDAH) rate, the microbial biomass carbon and nitrogen (MBC, MBN) content, the number of microbial groups and the related physico-chemical properties. The community-level physiological profiles of the soil microorganisms were evaluated using the Biolog EcoPlate™ method. In general, the highest level of enzymatic activity per soil mass unit was found in the Ap horizons, whereas the dehydrogenases activity and FDAH decreased gradually with depth, while the phenol oxidase and peroxidase activity revealed no consistent trends between the sub-surface horizons. The specific enzymatic activity, expressed per unit of soil TOC and MBC, was more variable in response to profile depth than the enzymatic activity, which was expressed per soil mass, and this did not allow for clear trends to be determined. Also, the specific activity was different with regard to the intracellular and extracellular enzymes. The DHA/MBC ratio was markedly higher in the Ap horizons (all profiles) and generally decreased with depth. The phenol oxidase and peroxidase activities, expressed in the terms of the MBC content, did not exhibit any clear trends in the studied profiles, however, they reached their highest levels in the deepest layers in all of them. The abundance of all of the microbial groups was generally highest in the Ap horizon and decreased sharply with depth. A greater abundance of microbial groups was found in the gleyic horizons of the Gleysols and Stagnosols as compared to the upper and lower horizons. The highest average well color development (AWCD) under aerobic conditions was observed in the Ap genetic horizons (with the exception of the Haplic Luvisol profile), while the highest values of AWCD in anaerobic conditions were found in the illuvial and gleyic horizons. In contrast to the AWCD, the Shannon diversity index (H) and Shannon evenness index (S) (equal usage of compounds) indices were more similar in the compared horizons. Our findings suggest that the studied microbial properties in the Ap horizons were primarily determined by the TOC content, while their activity in the sub-surface layers was also affected by factors associated with soil-forming processes.