Humic fractions as indicators of soil organic matter responses to clear‐cutting in mountain and lowland conditions of southwestern Poland

Abstract

AbstractClear‐cutting means forest removal (stems only) and is the most common type of forest harvesting but undoubtedly has a negative impact on the C budget in soils. This work aimed to describe responses of soil organic matter (SOM) in the mixed coniferous forest following the removal of trees under temperate climate conditions in lowland and mountain regions in southwestern Poland. Investigated soils are described as Brunic Arenosols derived from sand with an average content of total organic carbon (TOC) in the Oa‐horizons of 244 g·kg−1 in the lowland and Dystric Cambisols derived from gneiss; the average content of TOC in the Oa‐horizons in the mountain regions was 309 g.kg‐1. Analyzed soils were very acidic in the both regions (pH 2.7–3.5 in the topsoil). In the experiment, two replicated soil profiles were located on each site, both taken after clear‐cutting and without any harvesting. Humic fraction was isolated from Oa‐ and A‐horizons; differences between investigated objects were checked with a significance p < 0.05, (n = 16). Using the instrumental analysis, like electron paramagnetic resonance (EPR), proton nuclear magnetic resonance (1H NMR), and Fourier transformed infrared (FTIR) spectroscopy applied to isolated humic fractions, it has been found that clear‐cutting alters C cycling and accelerates decomposition in the forest floor, leading to loss of humic fractions in the investigated soils. In the mountain forests, the more labile, low‐molecular‐weight fulvic fraction decreased as the effect of harvesting practice by 30% in Oa‐horizon and by 54% in A‐horizon. The analysis of stable radicals by EPR showed a decrease in radical concentration observed for humic acids (HA) from mountain clear‐cut areas compared to the undisturbed forest. Results presented in this article have proved there is less aliphatic character of HA molecules from the lowlands, compared to the mountain forest as the effect of clear‐cutting. Changes in plant cover observed as a consequence of deforestation lead to the decrease in concentration of humic fractions with different solubility properties.