Climate‐sensitive ecosystem carbon dynamics along the soil chronosequence of the Damma glacier forefield, Switzerland

Abstract

We performed a detailed study on the carbon build-up over the 140-year-long chronosequence of the Damma glacier forefield, Switzerland, to gain insights into the organic carbon dynamics during the initial stage of soil formation and ecosystem development. We determined soil carbon and nitrogen contents and their stable isotopic compositions, as well as molecular-level composition of the bulk soils, and recalcitrance parameters of carbon in different fractions. The chronosequence was divided into three age groups, separated by small end moraines that resulted from two glacier re-advances. The net ecosystem carbon balance (NECB) showed an exponential increase over the last decades, with mean annual values that range from 100 g C m À2 yr À1 in the youngest part to over 300 g C m À2 yr À1 in a 60-80 years old part. However, over the entire 140-year chronosequence, the NECB is only 20 g C m À2 yr À1 , similar to results of other glacier forefield studies. The difference between the short-and long-term NECB appears to be caused by reductions in ecosystem carbon (EC) accumulation during periods with a colder climate. We propose that two complementary mechanisms have been responsible: 1) Reductions in net primary productivity down to 50% below the long-term mean, which we estimated using reconstructed effective temperature sums. 2) Disturbance of sites near the terminus of the re-advanced glacier front. Stabilization of soil organic matter appeared to play only a minor role in the coarse-grained forefield. We conclude that the forefield ecosystem, especially primary productivity, reacts rapidly to climate changes. The EC gained at warm periods is easily lost again in a cooling climate. Our conclusions may also be valid for other high mountain ecosystems and possibly arctic ecosystems.

Potential sources of microbial colonizers in an initial soil ecosystem after retreat of an alpine glacier

“…Our findings suggest that atmospheric deposition of microorganisms is not the dominant sources of microbial colonizers of recently deglaciated mineral debris for the period sampled. However, inputs of C, N (as DON, nitrate and ammonium) and other nutrients (for example, sulphate and phosphate) from atmospheric deposition represent important inputs to nutrient and C pools, which are known to be small in barren soils (Bernasconi et al, 2011;Smittenberg et al, 2012). Therefore, these inputs may facilitate the establishment of pioneer microbial communities in the mineral debris (Frey et al, 2010(Frey et al, , 2013Brunner et al, 2011).…”

“…They also represent ideal ecosystems to study natural establishment of microbial communities in terrestrial environments (Edwards et al, 2014). When glacier ice melts, entrapped rocks, sediments and fine debris are released and microbial pioneers present on these debris initiate soil formation by weathering rock minerals and incorporating organic matter (Frey et al, 2010;Smittenberg et al, 2012).…”

Nitrogen and phosphorus availability at early stages of soil development in the Damma glacier forefield, Switzerland; implications for establishment of N2-fixing plants

“…These photographs were taken in 2008 from 4 m above the centre of the site, using a digital camera (Nikon D70, 18-70 mm) mounted on a pole covering an area of 25 m 2 . Classification of vegetation and rocks was based on colour separation using Adobe Photoshop CS3 software (version 10.0.1, extended for Mac) (Smittenberg et al 2012). To estimate annual foliar biomass production, the aboveground biomasses of non-woody plants, and the foliage biomass of woody plants (Salix sp., Rhododendron ferrugineum, and Alnus viridis) were summed.…”

“…However, there were setbacks behind each end moraine, probably caused by disturbances such as burial of sites with new C and N-free sediment, (partial) erosion, and reduction of standing biomass during the readvance of the glacier (Smittenberg et al 2012) In situ resin N -which can be regarded as a measure of plant available N -showed a different pattern; it decreased from the pioneer stage of succession (first 16 years of soil development) to the intermediate stage of the successional gradient (60-80 years). The high availability of N in the pioneer stage implies that the input of N exceeds the uptake.…”

“…The higher concentration of N in close vicinity of rocks was mirrored in the N concentration of plants, which was higher in plants growing next to the rocks compared to those growing further away. Due to the deposition of glacial sediments, and accumulation of plant-originated organic matter, rocks are buried in the soil as the time proceeds (Smittenberg et al 2012) (Fig. 8).…”