Abstract
The rates of N cycling and soil enzyme activities involved in the transformation of soil N-related nutrients are rarely measured in soils below a 30 cm depth, even though substantial amounts of nitrogen are also stored in deep soils. The aim of this study was to determine how soil microbial and enzymatic properties changed as a function of depth across soil profiles that were developed on the same parent material but differed in terms of soil-forming processes. Two soil profiles were excavated in fields with lucerne and two under winter wheat. We assessed the N-cycling enzymes, the microbial utilization of the N-substrates, the microbial biomass carbon and nitrogen (MBC, MBN) content, and the related physicochemical properties. The most beneficial enzymatic (on a soil mass) and microbial properties, as well as nitrogen substrate utilization, were found in the Ap horizons and decreased with depth to varying degrees. The specific enzymatic activity (per unit of soil TOC and MBC), was more variable in response to the depth of the profile, but did not exhibit clear trends. The potential enzyme activities in the subsurface layers were also affected by factors that are associated with the pedogenic processes (e.g., the lessivage process, clay content). Only nitrate reductase activity was significantly higher in the horizons with potential reducing conditions compared to oxidative horizons, while the opposite trend was found for N-acetyl-β-D-glucosaminidase (NAG) activity. The cultivated plants had a significant impact on the degree of enzymatic activity and N-substrate utilization. The lessivage process significantly reduced microbial biomass and enzymatic activity (except for NAG activity). In general, nitrogen substrate utilization decreased with increasing soil depth and was greater in lucerne than the winter wheat profiles. Mollic Stagnic Gleysols (MSG) and Cambic Stagnic Phaeozems (CSP) horizons also have higher nitrogen substrate utilization than Luvisol profiles.
Highlights
Nitrogen (N) is the primary limiting nutrient in many terrestrial ecosystems, and, as a consequence, the content and transformations of soil N compounds have been widely studied [1]
We found that the specific enzyme activity was more variable in response to the depth of a profile when compared to the enzymatic activity which was expressed per soil unit, and this prevented clear trends from being
We found that the studied soil profiles demonstrated the highest potential enzymatic activity and nitrogen substrate utilization in surface soil horizons, which decreased with depth
Summary
Nitrogen (N) is the primary limiting nutrient in many terrestrial ecosystems, and, as a consequence, the content and transformations of soil N compounds have been widely studied [1]. Especially when the substantial thicknesses of subsurface profiles are taken into consideration, even low rates of microbial activity per unit soil mass at depth could contribute meaningfully to ecosystem-scale soil N cycling [3,4]. Despite this fact, most of the studies concerning soil N content and transformation involving different land uses have been focused on carbon-rich surface mineral layers (usually occurring to a depth of 30 cm), where plant root density and soil organic matter (SOM) concentrations typically reach the highest values [5]. In Chinese orchards, approximately 67% of the total N surplus for the last 28 years has accumulated in the soil profile in the form of nitrate [11]
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