How can climate change modify the carbon stock of forest soils?

Organic matter, the most complex and heterogeneous component of soil. SOM is a very relevant indicator for soil quality, as it can change the behavior and direction of many properties, soil functions, transformation processes. Less water reduces the amount of biomass produced, resulting in lower production and less plant residue in the soil. Under drier conditions, organic matter decomposes faster due to dominant aerobic processes, thereby reducing soil organic matter content. As the temperature rises, the rate of degradation processes and the intensity of soil respiration increases, which may further increase the reduction of soil carbon stock. Our forests are under high pressure due to climate change, especially in the Carpathian Basin. Therefore, beech and sessile oak are expected to replace with Turkey oak and the afforestation may lead to a change in carbon storage of forests.

To create a database and estimate the changes, we measured the carbon stock of soil in three different regions in Hungary, where the research sites formed on loess bedrock, on 150 and 250 m a.s.l., 650-710 mm precipitation sum with 10-10.4 °C annual temperature.

We took a 1.1 m soil column with soil borer and divided it into 11 samples in each column. Physical (texture, bulk density, water holding capacity) and chemical (pH, CaCO₃) soil properties and SOM were determined based on the methods of the Hungarian Standard in the soil laboratory.

During the evaluation, the amount of SOC was the highest in the topsoil layers. In summary, we found a larger amount (104 C t/ha) of SOC in the soil of stands, where sessile oak were the main stand-forming tree species. The amount of carbon was lower where turkey oak was dominant in sessile oak stands (70 C t/ha on average).

To conclude, the SOC order in case of the stand-forming tree species: sessile oak (/hornbeam) > beech > Turkey oak. We detected that different forest utilization and tree species have an effect on the forest carbon as the litter as well (amount, composition). Our measurements are not representative of the whole stand, but the homogenous loess bedrock demonstrates the impact of different mixture forests on carbon stock. After all, vegetation depends on site conditions (e.g. moisture) and not vice versa. The effects of future climatic changes on soil carbon storage are difficult to predict. In the future, it would be important to expand the use of continuous forest cover farming modes.