Alterations of soil organic matter following 130 years of afforestation assessed by molecular markers

In alpine areas of the European Alps, many of the pastures are no longer economically profitable and are converted into forests (Bolli et al., 2007). Afforestation on former pastures affects soil organic matter (SOM) dynamics through alteration of quality and quantity of root and aboveground biomass litter input. Compared with pasture OM, forest OM is less decomposable and characterized by increased C:N ratio as well as increased lignin concentration (Hiltbrunner et al., 2013). Therefore, it could be expected that long-term afforestation on a centennial scale may have a severe impact on SOM dynamics, an aspect that remains so far unknown as most of the earlier studies focused on successions between 30 and 50 years (Vesterdal et al., 2002).

In the current study, we aimed to identify the major sources of SOM in a subalpine afforestation sequence (40-130 years) with Norway spruce (Picea abies L.) on a former pasture in Jaun, Switzerland. Therefore, we combined plant- and microorganism-derived molecular proxies from several compound classes such as free-extractable fatty acids and phospholipid fatty acids.

We observed a decline in soil organic carbon (SOC) stock (9.6 ± 1.1 kg m^-2) after 55 years and a recovering of the SOC stock 130 years (12.7 ± 0.9 kg m^-2) after afforestation. Overall, there is no alteration of the SOC stock in the mineral soil following afforestation of former pasture (13.3 ± 0.9kg m^-2) after 130 years. But if we consider the additional SOC stock accumulated in the organic horizons (between 0.8 and 2 kg m^-2), the total SOC stock slightly increased, although OM in organic horizons is less stabilized than mineral-bound OM. An increase of the C:N ratio in the O_i-horizon with increasing forest age (40yr: 36.9 ± 2.6; 55yr: 40.9 ± 4.1; 130yr: 42.4 ± 6.6) reflects the alteration in litter quality towards poorly decomposable compounds in older forests. In addition, preliminary results show an increase in the abundance of Gram⁺ (+3%) and Gram^- bacteria (+6%), especially in the young (40yr) forest. Thus, the bacterial community seems to proliferate in the early succession before the fungal-dominated community takes over. Thus, the change in SOM source and quality following afforestation may not result in considerable stock changes, but results in better stability of SOM in the mineral soil.

References

Bolli, J. C., Rigling, A., Bugmann, H. (2007). The influence of changes in climate and land-use on regeneration dynamics of Norway spruce at the treeline in the Swiss Alps. Silva Fennica, 41, 55.

Hiltbrunner, D., Zimmermann, S., 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.

Vesterdal, L., Ritter, E., Gundersen, P. (2002). Change in soil organic carbon following afforestation of former arable land. Forest Ecology and Management, 169, 137-147.