Humus Forms and Soil Microbiological Parameters in a Mountain Forest: Upscaling to the Slope Scale

Abstract

Humus forms are the morphological results of organic matter decay and distribution in the topsoil, and thus important indicators for decomposer activities in forest ecosystems. The first aim was to examine if humus forms are suitable indicators of microbiological properties of the topsoil in a high mountain forest (Val di Rabbi, Trentino, Italian Alps). The second aim was to predict microbiological parameters based on the topsoil pH value on two slopes of the study area (ca. 1200-2200 m a.s.l.). We investigated humus forms and determined pH values and microbiological parameters (enzymatic activities, carbon/nitrogen (C/N) ratio and the ratio of bacterial/archaeal abundance) of the uppermost mineral horizon. The results reveal significant correlations between pH value and microbiological parameters (except for bacterial/archaeal abundance), which enable upscaling to the landscape scale using linear models. Based on a random forest with kriging of model residuals, predictive maps of humus form, pH value and microbiological parameters show that decomposition processes in our study area correspond with the topography. As compared to locations on south-facing slopes or close to the valley bottom, locations on north-facing slopes or close to the upper treeline exhibit Moder (scarcely Mull or Amphimull), more acidic topsoil (around pH 4), a lower activity of leucine-aminopeptidase, a lower ratio of alkaline/acid phosphomonoesterase activity and a higher soil C/N ratio (above 20). Our results suggest a high potential of humus forms to indicate soil microbiological properties in a high mountain forest. Together with the pH values of the topsoil, humus forms proved to be a useful tool as a basis for predictive maps of leucine-aminopeptidase activity, ratio of alkaline/acid phosphomonoesterase activity and C/N ratio of the mineral topsoil.

Modeling Spatial Patterns of Humus Forms in Montane and Subalpine Forests: Implications of Local Variability for Upscaling

“…We analyzed humus forms at three different spatial scales on the basis of three sets of investigation sites ( Figure 1). Humus forms were classified as described by Hellwig et al [3] according to Ad-hoc-AG Boden [34] and Graefe [35]. Similar to other international humus form classifications (e.g., [36][37][38]), this approach relies on the characterization of organic layers above the mineral soil and of the biogenic soil structure in the mineral soil (details are given in Reference [3]).…”

“…At the slope scale, we built upon the model results by Hellwig et al [3] referring to the two main characteristics of humus forms: (a) Presence of organic layers above the mineral soil and (b) biogenic soil structure in the mineral soil. We reclassified these results to predict the occurrence of humus form classes according to Table 1.…”

“…Humus forms are a distinctive morphological indicator of soil organic matter decomposition, since they integrate both the sequence of organic horizons (OL = litter, OF = fragmented residues, OH = humified residues) above the mineral soil and the soil structure in the mineral soil as a result of soil biological activities [1][2][3]. Hence, the spatial arrangement of humus forms corresponds to spatial patterns of soil ecological processes.…”

“…Hence, the spatial arrangement of humus forms corresponds to spatial patterns of soil ecological processes. In montane and subalpine forests, this relationship has been recently investigated for microannelid assemblages, microbial communities, and chemical properties of the topsoil [3][4][5][6].…”

“…Therefore, geostatistical approaches are not that suitable to model the variability of soil properties. Recent studies have demonstrated that an investigation of the soil-landscape interrelationship is more promising for modeling and predicting patterns related to soil organic matter decomposition in montane and subalpine forests [3,6,27,28]. These studies focused either on the slope or on the landscape scale.…”