Distinct changes in carbon, nitrogen, and phosphorus cycling in the litter layer across two contrasting forest-tundra ecotones
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2025-06-25
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Journal Article
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Abstract
At treeline, plant life forms and species change abruptly from low-stature plants in the tundra to trees in forests. Our study assesses how the vegetation shift 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. Using microlysimeters at 5 and 15 degrees C, we measured carbon (C) mineralization and the release of inorganic nitrogen (N) and phosphorus (P), reflecting net N and P mineralization. Additionally, we quantified releases of dissolved organic C and N and analysed the stoichiometry and ecophysiology of microbial biomass. Our findings showed significant shifts in the chemical characteristics of the litter layer across both treeline ecotones. On average, C:N and C:P ratios decreased by 56 % and 65 %, while lignin contents increased by 110 % from tundra to forest. The consistent decrease in C:N:P ratios in the litter layer was paralleled by pronounced increases in net N and P mineralization from tundra to lower-elevation forest in both treeline ecotones. The negligible nutrient release from tundra litter was likely due to immobilization of mineralized N and P at molar C:N and C:P ratios exceeding 35 and 1100, respectively. In contrast to net nutrient mineralization, C mineralization and the release of dissolved organic C and N remained largely unchanged. Microbial biomass colonizing the litter layer showed average decreases of C:N and C:P ratios by 26 % and 74 % from tundra to forest, while potential activities of C-N-P-acquiring extracellular enzymes showed no consistent pattern. Mineralization of 13C-labelled glucose-6-phosphate decreased with decreasing C:N:P ratios from tundra to forest. As the 13C incorporation into microbial biomass remained unaffected, substrate-use efficiency (SUE) increased along the same trajectory. 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. In contrast, experimental warming by 10 degrees C was less important for net N and P mineralization than litter composition. This indicates 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.
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published
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Journal / series
Volume
22 (12)
Pages / Article No.
2959 - 2977
Publisher
Copernicus