Abstract
Afforestation, defined as the establishment of trees on land that was not forested before is recommended by the Kyoto Protocol 1997 as a promising strategy to increase soil organic carbon storage through its potential of a long-term sequestration of soil organic matter. Soil organic matter plays an essential role in the global carbon cycle as it holds twice the amount of carbon than the atmosphere and living biomass combined. Thereby, even small changes regarding soil organic matter stabilization can have a substantial effect on the global carbon cycle. Nevertheless, the reported outcomes of afforestation in terms of its potential of a long-term soil organic matter sequestration are controversial. The intended increase in soil organic carbon mainly depends on the context, e.g. on former land-use, tree species planted, age plantation, climate conditions etc. Several studies reported increased soil organic carbon stocks following afforestation on former agricultural land. Afforestation of former pastures or grasslands, however, resulted often in decreased soil organic carbon stocks. Nevertheless, afforestation of grasslands or pastures is the most common land-use conversion in Europe, specifically in alpine regions. Alpine soils are characterized by a higher proportion of labile carbon compounds compared to soils in temperate ecosystems, which makes alpine ecosystems more sensitive to environmental changes. The conversion of pastures to forests in alpine or subalpine regions thereby might have a substantial effect on the soil organic carbon dynamics and thus on soil organic matter stabilization. However, the alteration in the proportion of aboveground biomass- and root-derived organic matter as well as the alteration in its incorporation into the soil following afforestation is not yet fully understood. In addition, the effects of these afforestation-induced alterations on the stabilization or loss of soil organic matter still remains an open question. To date, most of the studies reporting changes in the soil organic matter dynamics following afforestation are either limited to young forest successions between 30 and 50 years or are restricted to upper soil horizons (0 - 10cm). Unfortunately, this lack of consensus results in uncertainties about the afforestation induced alterations in the soil organic matter dynamics in deeper soil horizons and in a longer period of time. To address this knowledge gap, I investigated the alteration in soil organic carbon stocks as well as in the soil organic matter composition following afforestation on a subalpine pasture in the Swiss Alps. To address the potential effects of afforestation in the long-term, I investigated a Norway spruce (Picea abies L.) afforestation sequence with stand ages between 40 and 130 years. Specifically, I used a multi-proxy molecular marker approach to determine the alteration in potential sources as well as the decomposition of soil organic matter, with an additional emphasis on soil microbiology. Afforestation with Norway spruce on a subalpine pasture in this specific study site did not result in any significant changes in the soil organic carbon stocks (40-year-old forest: 11.6 ± 1.1 kg m−2; 55-yearold forest: 8.3 ± 0.6 kg m−2; 130-year-old forest: 11.0 ± 0.3 kg m−2) after 130 years of afforestation. If the organic horizons are considered, there is a non-significant increase in soil organic carbon of +8% and of +7% in the 40-year-old and in the 130-year-old forests, respectively. Nevertheless, there was a clear shift in the potential source of soil organic matter from grass leaves to spruce needles with increasing forest stand age. Surprisingly, root-derived organic matter seems to play a minor role in the pasture soil as well as in forest soils of all stand ages as one of the potential sources of soil organic matter. The alteration in the quality of litter input following afforestation directly affected the soil microbial community. With increasing forest age there was an increased abundance of Gram+ bacteria as well as arbuscular mycorrhizal fungi. The observed shift in the soil microbial community structure following afforestation demonstrated the adaptation of the soil microbial community to the changes in the vegetation cover. Furthermore, the difference in the soil microbial community structure implies a use of different carbon substrates of the microorganisms between the pasture and forests which can have substantial effects on soil organic matter stabilization. In conclusion, within this study, afforestation of a subalpine pasture in the Swiss Alps did not result in any long-term carbon sequestration. This thereby shows a potential limitation of afforestation measures for increasing soil organic carbon sequestration in this study site.The application of a multi-proxy marker approach proved on the one hand, that the combination of multiple molecular compounds is a useful tool to identify alterations of potential sources of soil organic matter as a direct consequence of afforestation. On the other hand, the suitability of this approach further extends to determine the alteration in the decomposition of soil organic matter through the changes in the soil microbial community structure as a direct effect of the altered soil organic matter source. Although the presented results within this PhD-project are case specific, they contribute to a better understanding of the alteration in the soil organic matter dynamics as a direct consequence of afforestation. In light of soil organic carbon and soil organic matter processes, despite the small scale of the study site, the reported results within this PhD thesis can make a fundamental contribution to further meta-analysis studies on afforestation and their implication on long-term carbon sequestration.
Speckert, Tatjana Carina