Death of a spruce: Soil decomposition processes under dying spruce trees at the forest-tundra ecotone

Arctic warming is facilitating the encroachment of trees into tundra landscapes. Trees at the forest-tundra ecotone are typically small, slow-growing and show high mortality rates. Tree necromass enters the soil as root, leaf and stem litter. This material can be decomposed or contribute to long-term soil organic matter stocks, as well as change decomposition of native soil organic matter (priming). We here tested whether decomposition processes change under dying spruce trees in tundra soils in a controlled laboratory experiment. The opportunity for addressing this question came up within a laboratory macrocosm experiment on the effect of various living tundra plants on carbon and nitrogen cycling in a tundra soil. For this experiment, root-picked soil was homogenized and filled into macrocosms, reconstructing the original horizon sequence. Plants with washed roots were placed in the soil and macrocosms watered regularly from the top. The small (ca. 50 cm) spruce trees died early in the experiment, and we kept the experiment running to assess changes in carbon and nutrient cycling resulting from the decomposition of spruce necromass compared to the plant-free control soil. Pore gas CO₂ concentrations at 7 cm depth were significantly higher in spruce than in the control soils. We further observed significantly lower pH values, as well as significantly, ca. 25% lower potential activities of hydrolytic (leucine-aminopeptidase, cellobiohydrolase, N-acetyl-beta-D-glucosaminidase), but not oxidative extracellular enzymes. These findings suggest that the input of root and needle-leaf litter altered the functioning of the soil decomposer community. These differences extended into the deeper soil below the rooting zone of spruce plants, pointing at an important role of leaching. These first observations will be compared with surface CO₂ fluxes, dissolved organic and microbial carbon concentrations to dissect the decomposition dynamics of spruce necromass at the forest-tundra ecotone.