Analysis of changes in forest soil carbon stocks due to different site conditions and the impact of climate change

One of the greatest challenges of the 21st century is combating global climate change and adapting to its effects. In addition to reducing CO₂ emissions, natural processes capable of absorbing CO₂ are of paramount importance. Forest ecosystems play a critical role in this regard as they are the largest terrestrial absorbers of CO₂ and organic carbon reservoirs. 

Our research revealed that in recent decades, systematic surveys of soil (and litter layer) organic carbon content have not been conducted in Hungary. Existing soil data are often incomplete (e.g., lack measurements of bulk density), were obtained using different analytical methods (e.g., determining soil humus content), and do not always include forest stand data. Data on litter layers are even more scarce. Consequently, our primary goal was to expand this dataset. To achieve this, we began assessing the effects of tree species, site conditions, and silvicultural interventions on the organic carbon stocks of soils and litter layers in Hungarian forest stands.

Soil samples were collected using a motorized auger to a depth of 1–1.1 meters. The undisturbed samples were divided into 10 cm sections rather than genetic horizons, and for each section, a 100 cm³ cylinder was used to extract undisturbed samples for bulk density determination. The remaining soil in each section was retained for further analysis. From a 5-meter radius around each sampling point, litter samples were randomly collected from three locations per stand. The litter included all decomposing leaves and small to medium-sized twigs above the raw soil surface. Each sample was placed in a separate bag, resulting in 25 samples per location.

This study focuses on results from beech (Fagus sylvatica) stands, as laboratory analyses are still ongoing. The collected soil samples were analyzed for bulk density, pH, lime content, acidity, particle size distribution, and organic carbon content. For the litter samples, in addition to dry mass, pH, the proportions of various fractions, and C/N content were determined.

In the approximately 40 examined beech stands, the average soil pH was 5.17 pH_H2O, with minimum and maximum values of 3.87 pH_H2O and 8.4 pH_H2O, respectively. Humus content in individual layers ranged from 0.16% to 15.65%. The average organic carbon stock of the 10 cm soil layers was 6.39 C t/ha, with a minimum of 1.46 C t/ha and a maximum of 34.03 C t/ha.

Based on the results, the carbon stocks under beech stands in Hungary, which are retreating due to climate change, are likely to disappear if the pace of climate change accelerates in the Carpathian Basin.

This article was made in the frame of the project TKP2021-NKTA-43 which has been implemented with the support provided by the Ministry of Innovation and Technology of Hungary (successor: Ministry of Culture and Innovation of Hungary) from the National Research, Development and Innovation Fund, financed under the TKP2021-NKTA funding scheme. Some of the tools used during the research were acquired within the framework of the "Investigation of the conditions for the cultivation of wood biomass - GINOP-2.3.3-15-2016-00039" project.