The main causes of the variation in the soil aggregate proportion and stability along gradient of natural forests in the outer Western Carpathians, Czech Republic

Abstract

In soils, the proportion and stability of aggregates tends to differ with local relief and/or plant cover. In this study, we assess the dependence of forest soil aggregation on physical, physicochemical and (bio)chemical properties in a) topsoil and subsurface horizons, and b) between reference soil groups, altitudes and natural vegetation, in the Outer Western Carpathians (Czech Republic). Relationships between soil properties were modelled using multiple logistic regression, while impacts of habitat divisions were assessed through discriminant analysis. Overall, soil properties impacted aggregate stability indices more (R2 0.40-0.57) than aggregate proportion (R2 0.12-0.58) with diameter < 3 mm. Aggregate stability in topsoil was mainly influenced by clay content, total organic carbon, microbial biomass carbon, base saturation and activity of the enzymes acid phosphomonoesterase and urease. In deeper horizons, the influence of biogenic activity was reduced, with aggregation controlled mainly by sorption processes such as pH, base saturation and catalase activity. Bulk density was most affected by biogenic components in topsoil, which indirectly influenced porosity, total organic carbon and acid phosphomonoesterase activity. Water-holding capacity and porosity, closely linked to aggregate structure, better predicted macroaggregation than microaggregation. Thus, in natural forests, soil aggregation is most correlated with distribution of soil properties. Aggregate proportion differed markedly between broadleaved and coniferous forests, while aggregate stability differed most between marginal forest-steppe conditions and temperate forests from floodplains to mountains. Importantly, the results strongly support the main hypothesis that vegetation (forest community) has a stronger influence on soil aggregation than natural factors such as soil type or altitude. In contrast, presence of natural forest accounted for 59.9-71.9 % of soil aggregate proportion (SAP) in topsoil or subsurface horizons, and 54.0-75.0 % of aggregate stability (AS), Altitude affected 63.3-85.4 % of SAP and 53.9-64.6 % of AS, and soil group 57.3-81.3 % of SAP and 51.8-71.3 % of AS. These patterns reflect strong ecosystem gradients, with mixed and broadleaved forests supporting deeper, more stable aggregation than coniferous or marginal forest-steppe. Our findings highlight the fact that soil microbial activity and enzymatic functions, enhanced by species-rich vegetation, are the drivers of carbon sequestration and soil structure resilience, and that mixed forests promote aggregation across horizons, increasing carbon retention and ecosystem adaptability under changing habitat conditions.