Soil Biochemical and Microbial Responses to Different Soil Use and Management: Implication for Assessment of Soil Quality and Sustainability

Differences in land use and land management significantly alter the physicochemical and microbial properties of soil, especially in the long term. These properties in turn determine the land’s ability to sustain plant growth and support agricultural productivity. Studies of soil properties, mainly microbial and enzymatic ones, have mainly focused on surface horizons, although substantial biological activity and transformation of soil organic matter are also known to occur in deeper horizons. An important role in shaping the biological activity of soil, including in its deeper horizons, is attributed to plant cover, and more precisely to variations in root mass and structure. That is why it is essential to assess the influence of differences in land use and plant species diversity by determining how plants with contrasting root system morphologies affect soil enzymes and other properties at different depths of the soil profile. We therefore aimed to study the differences in a set of soil properties among various soil depths (horizons) in eight soil profiles sampled from land under four different soil uses and management practices. The four land use systems represented were: arable lands (A), orchard (O), hop plantations (H), and grasslands (G). The potential hydrolase activities involved in the cycling of C, N and P were determined, as well as fluorescein diacetate hydrolysis (FDAH). In addition, the activity of selected oxidoreductases and the content of microbial biomass C and N were determined. We also evaluated the content of total and dissolved forms of C and N, pH in CaCl2, CEC, available K and P, and clay content. The agricultural land uses differed in terms of their influence on both the microbial biomass content and enzyme activities. Considering the mean values for all five depths, six of the studied enzymes exhibited the highest activity in the G profiles, while five other enzymes were most active in the A profiles. A similar pattern of changes in the potential enzymatic activity in relation to the land use system was also noted for the top horizons of the studied profiles. Regardless of the cultivated plant species, soil variables exhibited a significant decline with increasing depth, and the most pronounced changes occurred between the surface and the second soil horizon (I–II) in the A and G profiles. The enzymatic activity throughout the O and H profiles was less variable, with relatively high activity in deep soil compared to the topsoils. The activity of two extracellular oxidases (phenol oxidase and peroxidase) differed in the patterns in their activity throughout the studied profiles as compared to the other enzymes and did not decrease progressively with depth. Sometimes, their activity was higher in deep horizons than in the surface ones. The soil C and N content had a stronger influence on the studied enzyme activities than other soil properties. This was confirmed by the significant and positive correlations calculated between various forms of C and N and enzyme activity. The highest correlation coefficients were noted between TOC and the FDAH rate and between TN and the FDAH rate (r = 0.818 and 0.888). No significant correlation coefficients were found between enzymatic activity and either pH in CaCl2 or clay content. Because enzyme activities respond variously to different land use systems, they serve as suitable indicators for soil health assessment. Analysis of the variation in microbial and enzymatic properties throughout the soil profiles fills gaps in the existing knowledge about the biogeochemical processes and nutrient cycling that affect soil fertility and the health of agroecosystems.