Soil moisture determines dead wood effects on soil organic matter in temperate beech forests

Dead wood has important functions in forest ecosystems. It is a biodiversity hot spot, serves as a storage of water and stores 8% (73 +- 6 Pg) of the world’s forest carbon (Pan et al., 2011). The fate of this carbon (C) is still highly debated particularly concerning its influence on soil organic matter (SOM) and its contribution to the forest soil’s C sink. The aim of this research is to investigate how downed beech dead wood affects the stable soil C pool of temperate beech forests, and how this depends on soil moisture.

The research was conducted in a near natural beech forest near Leipzig, Germany (Dübener Heide) and is part of the BENEATH-Project. We sampled three sites representing a soil moisture gradient, i.e. dry, intermediate (i.e. moist) and wet conditions. Undisturbed soil cores were taken from these sites in three depth (0-10 cm, 10-20 cm and 20-30 cm) beneath dead wood at an advanced stage of decay. Reference soils were sampled at a distance of about 2 m. Soil moisture and soil temperature are constantly monitored. We applied a physical fractionation scheme to identify the effects of dead wood on differently stable SOM fractions. The samples were separated in the free light fraction (F-LF), the occluded light fraction (O-LF) and the heavy fraction (HF) via density fractionation using sodium polytungstate solution (ρ =1,6 g cm-³). For each fraction, the organic C and N contents were determined.

Our results indicate a positive influence of dead wood on SOC stocks in the dry and wet regions of our soil moisture gradient. In the intermediate region of the soil moisture gradient, dead wood has no or even a negative effect on SOC stocks. Changes in the SOC content under dead wood compared to the reference soil occurred manly in the F-LF and HF fraction at 0 cm and 10 cm depth. The observed pattern of dead woods effect on SOC along the moisture gradient is suggested to be a result of the relationship between soil moisture and microbial activity. According to the literature, we assume that the microbial activity should be highest in the intermediate moist soil and to some extent inhibited under either wet or dry conditions. In this case, it is not the input but the rate of decomposition that changes with soil moisture, resulting in a different net increase in SOC. To test our hypothesis, we attempt to estimate the theoretical time of effective microbial decomposition per year based on soil moisture and soil temperature data for our three sites. Correlation analysis will be used to test this indicator of microbial activity for the effect of dead wood on SOC.

Our results should sharpen the picture of the dead wood’s role for long-term C stabilization in forest soils and how this process is affected by differences in the soil moisture status. They will give implications for climate mitigating forest management.