Depth-Related Changes in Soil P-Acquiring Enzyme Activities and Microbial Biomass—The Effect of Agricultural Land Use/Plant Cover and Pedogenic Processes

Although the phosphatase enzymes regulate phosphorus (P) turnover throughout the soil profile, at present, they are rarely studied and are less well understood in the deeper soil layers than in the surface horizons. Hence the changes in P-associated soil properties were assessed throughout five Phaeozem profiles for different agricultural land uses including alfalfa, winter wheat, grapevine, apple trees and hops. The acid phosphatase (AcP), alkaline phosphatase (AlP) and phosphodiesterase (PDE) activity was assayed, as were the microbial biomass carbon (MBC) and phosphorus (MBP) contents and also other properties (e.g., available phosphorus, total organic carbon, total nitrogen). We have also determined the mass, length and surface area of the plant roots. In general, the activities of the studied enzymes were highest in the topsoil in four out of the five profiles studied, which corresponded to the highest level of root mass. The older the plant, the greater the root mass and increased enzymatic activity in the deeper horizons of apple trees and hop profiles in comparison to the surface layers. The greatest enzymatic activity, MBC and MBP contents were found in the horizons with a TOC content >0.5% and decreased down the soil profiles similarly to the changes in TOC and TN contents. While the studied properties were determined to varying degrees by means of the organic C content and availability in all of the genetic horizons, the influence of the prevailing conditions and the factors related to soil depth and pedogenic processes were less pronounced. The clay content was related to a significant extent to all of the studied enzyme activities, but only in horizons with a TOC content <0.5%. Significantly higher phosphatase activity under aerobic as opposed to anaerobic conditions were determined in this study, while the opposite trend was found for the content of MBC and MBP as well as the ratio of MBC/MBP. Overall, we pointed out the complex effect of the soil depth, soil forming-processes and cultivated plants on soil P-associated enzyme activities and other properties throughout the soil profiles. This knowledge will allow better understanding of the state of enzymes and their contribution to the biogeochemical cycle of soil P, especially in subsoils, where the enzyme activities follow different patterns than those in the surface horizons.