Distinct changes in carbon, nitrogen, and phosphorus cycling in the litter layer across two contrasting forest–tundra ecotones
<p>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 d...
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| Main Authors: | , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2025-06-01
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| Series: | Biogeosciences |
| Online Access: | https://bg.copernicus.org/articles/22/2959/2025/bg-22-2959-2025.pdf |
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| Summary: | <p>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 <span class="inline-formula">°C</span>, 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, <span class="inline-formula">C:N</span> and <span class="inline-formula">C:P</span> ratios decreased by 56 % and 65 %, while lignin contents increased by 110 % from tundra to forest. The consistent decrease in <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">C</mi><mo>:</mo><mi mathvariant="normal">N</mi><mo>:</mo><mi mathvariant="normal">P</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="41pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="926cd20b050a0d2708fe37004cbe0c28"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-22-2959-2025-ie00001.svg" width="41pt" height="10pt" src="bg-22-2959-2025-ie00001.png"/></svg:svg></span></span> 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 <span class="inline-formula">C:N</span> and <span class="inline-formula">C:P</span> 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 <span class="inline-formula">C:N</span> and <span class="inline-formula">C:P</span> ratios by 26 % and 74 % from tundra to forest, while potential activities of <span class="inline-formula">C–N–P</span>-acquiring extracellular enzymes showed no consistent pattern. Mineralization of <span class="inline-formula"><sup>13</sup>C</span>-labelled glucose-6-phosphate decreased with decreasing <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M11" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">C</mi><mo>:</mo><mi mathvariant="normal">N</mi><mo>:</mo><mi mathvariant="normal">P</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="41pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="e72c6c2c453a39ebd69c0d2d9f81eb7e"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-22-2959-2025-ie00002.svg" width="41pt" height="10pt" src="bg-22-2959-2025-ie00002.png"/></svg:svg></span></span> ratios from tundra to forest. As the <span class="inline-formula"><sup>13</sup>C</span> 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 <span class="inline-formula">°C</span> 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.</p> |
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| ISSN: | 1726-4170 1726-4189 |