Humus horizon development during natural forest succession process in the Polish Carpathians

Abstract

As a result of socio-economic changes and land abandonment, the main ecological driver of the Carpathian landscape is the progression of the natural forest succession process. Thus, aspects of this process have become worthy of attention, especially in the context of carbon sequestration and the management of protected areas. Soil processes, especially within the topsoil, are some of the most susceptible to change, due to the accumulation of organic matter during such land-use transformations. The purposes of this study were to investigate the differences in topsoil development using the A Horizon Development Index (ADI) and to study the composition of humic substances and advanced organic matter humification in different land-use areas in selected Carpathian national parks, i.e. Bieszczady, Magura and Pieniny National Parks in southern Poland. Additionally, a goal of this study was to compare the ADI and the spectroscopic coefficients of humic substances as indicators of the degree of humus horizon shaping as well as advanced organic matter humification. In total, ten transects were selected, each consisting of three different land-use areas: semi-natural meadow, successional forest and old-growth forest. Soil colour was determined in fresh and air-dried samples using the Munsell colour chart. In air-dried soil samples pH, soil texture, total organic carbon and total nitrogen were measured. Humic substances were extracted and further characterized by UV-VIS spectroscopy. The ADI confirmed the influence of natural forest succession on soil colour darkening and the development of the uppermost soil layer. Spectroscopic analyses of humic substances showed two different patterns depending on soil depth. In the 0–10 cm layer, natural forest succession reduced the rate of the humification process and decreased the degree of maturity of fulvic acids; in the 10–20 cm layer, it led to an increase in the rate of the humification process and a decrease in the content of humic and fulvic acids at the beginning of the transformation. The comparison of two different indicators of soil development — the ADI and the spectroscopic coefficients of humic substances (Q4/6, Q2/4, Q2/3, Δ log K) — indicated that these indexes are based on different features of soil and cannot be used interchangeably.