Non-uniform Soil Respiration of Soils from an Afforestation Sequence in a Laboratory Incubation Experiment

Carbon cycling in alpine soils is prone to changes with temperature increase, for instance because of reduced frost periods (Zierl & Bugmann, 2007). Afforestation throughout the last decades and in future with warming climate and land-use change will influence carbon dynamics. To investigate the climate-driven response of carbon cycling in alpine soils, we conducted a jar incubation experiment under controlled conditions using ¹³C-labelled plant material and traced the decomposition of the organic material under different increasing temperature regimes.

Approximately 20 kg of soil samples were collected from the uppermost 10 cm of a 130-year old tree stand and a pasture site from a sub-alpine afforestation sequence in Jaun, Switzerland. The samples were sieved to 2 mm, roots and stones were removed. 50 g of the soil material was incubated in 2 l glass jars.

To investigate the degradation of the organic material, dried and cut shoots of ¹³C labelled plant material (Lolium perenne L.) were added to the soil samples. Additionally, samples without added plant material were incubated as a control group. The incubation was conducted at three different temperature regimes: 12.5°C (average growing season temperature, weather station by WSL-SLF, 2021), 16.5°C (+ 4°C) and 20.5°C (+ 8°C). Destructive sampling was conducted after 0, 2, 4, 8, and 26 weeks. NaOH traps were exchanged every 3-4 days in the beginning and every 3 weeks during later stages of the experiment to trace the respiration of CO₂ and the ¹³C label.

The measured basal respiration shows a temperature dependence. The values are highest at 20.5°C and subsequently decreased to 16.5°C and 12.5°C with the lowest basal respiration. Surprisingly, the basal respiration of the forest soil is always higher than that of the pasture soil of the same incubation temperature. This partially contradicts previous findings (Nazaries et al., 2015) and might be related to the more resilient microbial community in the forest compared to the pasture soil.

Litter-induced respiration increased sharply after litter application and then decreased again. The pasture soil shows higher cumulative respiration for each temperature compared to the forest soil incubated at the respective temperature. After the highest litter-induced respiration of the pasture soil at 20.5°C at the beginning, this is surpassed by that of the pasture soil with 16.5°C from about 40 days after the beginning of incubation. This could indicate a temperature optimum of the current soil microbial community closer to 16.5°C rather than to 20.5°C. These initial results indicate a different sensitivity of the soil microbial community and consequently also carbon cycling in alpine soils to future rising temperature depending on vegetation cover.

Nazaries, L., Tottey, W., Robinson, L., Khachane, A., Al-Soud, W. A., Sørenson, S., & Singh, B. K. (2015). Shifts in the microbial community structure explain the response of soil respiration to land-use change but not to climate warming. Soil Biology and Biochemistry, 89, 123–134.

WSL-SLF (2021), IMIS Weather Station Fochsen-Jaun, WSL, Davos/Switzerland.

Zierl, B., & Bugmann, H. (2007). Sensitivity of carbon cycling in the European Alps to changes of climate and land cover. Climatic Change, 85(1–2), 195–212.