Changes in soil organic carbon in response to climate manipulation under Cassiope tetragona in Zackenberg, Greenland

Abstract

It is projected that climate change will be more pronounced in the Arctic than further south. Climate change will affect the quantity and/or quality of soil organic carbon (SOC) which is directly related to carbon dioxide fluxes. Thus, it is crucial to understand how soil will respond to climate change in the Arctic where large amount of SOC is accumulated due to low temperature. In this study, we aimed to understand the characteristics of SOC and the effects of climate manipulation on SOC under the high arctic heath. We hypothesized that change in temperature and in growing season length would affect microbial activities and thus the amount and chemical composition of SOC. This study was conducted in the long-term climate manipulation plots established in Zackenberg, Greenland in 2004 by Prof. Michelsen and coworkers. There are one control (C) and four treatments plots: warming (T), shading (S), short growing (SG), and long growing (LG). Soil sampling was conducted in the Cassiope tetragona site in 2011. Three 15 cm depth soil cores (5 cm diameter) were taken in one plot, and each soil core was divided as the litter layer, 0-5, 5-10, and 10-15 cm depths, and then three cores in the same depth interval were pooled together. SOC was measured by a combustion method (950 ºC). Density fractionation (sodium polytungstate, 1.55 g cm-3) separated SOC as free light fraction (FLF), occluded light fraction (OLF), and heavy fraction (HF). 13C NMR was conducted for one FLF sample per each treatment. Preliminary analyses showed that thawing depth and litter layer thickness were not significantly different among treatments. Soil temperature in the T treatment was 1.2-1.5 °C higher than in the other treatments. Bulk density, moisture content, and soil pH did not vary among treatments. There were no statistical differences in SOC, and SOC in the 0-5 cm depth was in a range of 3.5 to 5.2 %. Total inorganic carbon was rarely or never detected across all treatments. Density fractionation showed that there were differences in FLF, with a lower amount of FLF in the T and SG treatments than that in control. 13C NMR showed that in the T treatment, the ratios of the O/N-alkyl C (labile) and that of the alkyl C (recalcitrant) groups were relatively lower and higher than the others, respectively. Warming did not alter the amount of SOC, however, it might cause the change in SOC quality by decreasing a labile portion of SOC