Changes in carbon-degrading enzyme activities and microbial biomass content – The effect of soil depth and soil-forming processes

Although the potential enzymatic activity often significantly decreases with depth, some studies have shown relatively high enzymatic activity in the deeper soil layers, sometimes the same as or even higher than that determined in the topsoil. Due to the importance of the C-cycling enzyme activity in the transformation of organic matter throughout the soil profiles and because the factors that influence their activity in the deeper layers are not yet well understood, we aimed to determine how the soil enzymatic activity of C-transforming and microbial biomass content changed as a function of soil depth and pedogenic processes such as the lessivage and gleyic process. Therefore, four 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. The examined profiles were developed on the same parent material but differed in their pedogenic processes. Two groups of the C-cycle enzymes were expected to behave differently throughout the soil profiles; enzymes that act on the large organic complexes (endo-cellulase, exo-cellulase, endo-xylanase, α-amylase) (1st group) and enzymes that target the easily decomposable compounds (exo-xylanase, α- and β-glucosidase, invertase) (2nd group). Additionally, the carbon and nitrogen of soil microbial biomass (MBC, MBN) and some basic physicochemical properties (e.g. total organic carbon - TOC, dissolved organic carbon - DOC, pH, clay content, bulk density) were determined. The enzymatic activity expressed on a soil mass basis was generally highest in the surface horizons (Ap) and decreased with depth. The 1st group of enzymes showed a significant reduction in activity starting from the subsurface horizons, while the smallest decrease in activity throughout the soil profiles was noted for the 2nd group of enzymes. The specific enzymatic activity, expressed per unit of soil TOC and MBC, was more variable in response to profile depth than the enzymatic activity expressed per soil mass, and this did not allow for clear trends to be determined. Concerning the lessivage process, most of the studied enzymes were more active in the Bt horizons (Cutanic Luvisol) as compared to the E horizons (Haplic Luvisol). No clear effect of the gleyic process was found in the case of the 1st group of enzymes, while the activity of the 2nd group of enzymes was significantly higher in the horizons with reducing conditions as compared to the layers with oxidative conditions. Our findings showed that the studied enzymes were primarily determined by the organic C content and enzymatic substrate quality and availability not only in the Ap horizons, but also in the deeper horizons, while the influence of the factors associated with the soil-forming processes was less pronounced.