Distinct changes in carbon, nitrogen, and phosphorus cycling in the litter layer across two contrasting forest-tundra ecotones

Abstract. At treeline, plant life forms and species are abruptly changing from low stature plants in the tundra to trees in forests. Our study assesses how this shift in vegetation affects the quality and elemental composition of the litter layer and consequently, the microbial processing and nutrient release during decomposition. We sampled litter layers along elevation gradients across conifer and broadleaf dominated treelines in the Russian subarctic Khibiny mountains and hemiboreal South Urals. In incubation experiments using microlysimeters at 5 and 15 °C, we measured carbon (C) mineralization and the release of inorganic nitrogen (N) and phosphorus (P), reflecting net N and P mineralization. We also measured releases of dissolved organic C and N and analyzed the functioning and stoichiometry of microbial biomass. Results showed that the chemical characteristics of the litter layer fundamentally changed across both treeline ecotones. On average, C:N and C:P ratios decreased by 56 and 65 % from tundra to forest, whereas lignin contents showed a 110 %-increase. The consistent decrease in C:N:P ratios in the litter layer from tundra to forest was paralleled by pronounced increases in net N and P mineralization from the tundra towards the low elevation forest in both treeline ecotones. In contrast to net nutrient mineralization, C mineralization and the release of dissolved organic C and N did not change across both treeline ecotones. In microbial biomass colonizing the litter layer, C:N and C:P ratios decreased on average by 26 and 74 % from tundra towards forest. The metabolic quotient (qCO₂) correlated positively with C:N and C:P ratios of the litter layer. In support, the substrate-use efficiency estimated by the microbial use of ¹³C labelled glucose-6-phosphate increased from the tundra to the forest with decreasing C:N:P ratios. In contrast, potential activity of a range of C-N-P-acquiring extracellular enzymes showed no consistent pattern. Overall, our results give evidence that the vegetation shift from tundra to forest is associated with an abrupt increase in net N and P mineralization in the litter layer, accelerating nutrient cycling and increasing N and P availability. Experimental warming by 10 °C was less important for net N and P mineralization than litter composition. This implies that indirect effects of climatic warming through changes in plant community composition with treeline advances seem to be more important for soil N and P cycling than direct temperature effects.