Changes in soil organic matter composition after 130 years of afforestation in Jaun, Switzerland

Following the Kyoto Protocol, afforestation has been acknowledged as a promising strategy for soil organic matter (SOM) conservation and to mitigate anthropogenic CO₂ emissions (Huang et al., 2011). However, the effect of carbon sequestration in soils depends on ecosystem properties, the former land use and on type of trees planted. Some studies showed a decline in SOM (Hiltbrunner et al., 2013) while others reported an increase in SOM 30 to 40 years after afforestation of former pastures (Thuille and Schulze, 2006). Thus, there is a need for well-designed and site-specific long-term experiments on a decadal scale to investigate changes in SOM dynamics following afforestation to predict the behaviour of carbon sequestration under changing environmental conditions. One approach to trace the sources of SOM is the application of molecular proxies like n-alkanes or fatty acids. Though, focusing only on one compound class may lead to flawed conclusions due to missing information offered by other compound classes. One way to obtain a more solid conclusion on the SOM dynamic in soils is the combination of multiple compound classes (Li et al., 2018). The aim of this project is to identify possible sources of OM in soils in a subalpine afforestation sequence (40-130 years) with Norway spruce (Picea abies L.) on a former pasture in Jaun, Switzerland, by combining molecular proxies from several compound classes originating from various plant and microbial sources.

A higher (+70%) number of fine roots (<2mm) was observed under pasture soils compared to spruce soils of all forest stand ages. The lower root frequency and the changes in litter composition under spruce compared to pasture result in a decline in SOM quality. Hiltbrunner et al. (2013) observed a change in SOM quality following afforestation of former pasture as fine roots of grass have a lower lignin concentration (240 mg g^-1) compared to fine roots of spruce (310 mg g^-1). In our project we expect a decline in the SOC stocks, specifically in the younger (40 to 55yr) forest stands and a change in SOM quality following afforestation.

References

Hiltbrunner, D., Zimmermann, S., and Hagedorn, F. (2013). Afforestation with Norway spruce on a subalpine pasture alters carbon dynamics but only moderately affects soil carbon storage. Biogeochemistry, 115, 251-266.

Huang, Z., Davis, M. R., Condron, L. M., and Clinton, P. W. (2011). Soil carbon pools, plant biomarkers and mean carbon residence time after afforestation of grassland with three tree species. Soil Biology and Biochemistry, 43, 1341-1349.

Li, X., Anderson, B. J., Vogeler, I., and Schwendenmann, L. (2018). Long-chain n-alkane and n-fatty acid characteristics in plants and soil-potential to separate plant growth forms, primary and secondary grasslands? Science of the Total Environment, 645, 1567-1578.

Thuille, A., and Schulze, E. D. (2006). Carbon dynamics in successional and afforested spruce stands in Thuringia and the Alps. Global Change Biology, 12, 325-342