the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
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.
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RC1: 'Comment on egusphere-2024-2622', Anonymous Referee #1, 09 Oct 2024
This is a nice paper about microbial C, N and P cycling in the litter layer of forest and tundra ecosystems. I have two suggestions and some rather minor comments.
Concerning Fig. 8, I suggest adding a second panel that shows the percentage of respired 13C (derived from glucose-6-phosphate). Based on this figure, the authors could discuss the loss of C in comparison to the loss of P.
It seems that the authors conclude that the main driver of the differences in microbial element cycling between forest and tundra is the difference in litter stoichiometry between these two ecosystems. This raises further questions. For example, about how the plants are able to acquire higher nutrient contents in the forest than in the tundra, and specifically how the forests are able to build up a larger N stock than the tundra ecosystems. I think that the (formation of the) larger N stock in the forest ecosystems is very relevant for the discussion of microbial N cycling in these ecosystems. I would like to read some lines of text about this, including a short outlook about how this might change in response to global warming and what might happen with these element ratios when the treelines shift.
Further comments
L.50-60 the transport of nutrients (nitrogen and phosphorus) into decomposing litter by fungi is also be very important during the initial decomposition stages.
L. 59 Whether “overflow respiration“ really occurs in soil microorganisms is debatable. It might be a process that can only be observed under extreme conditions in the lab.
L. 71 and 81 It is not clear what exactly “litter quality” is. This term requires an explanation.
L. 95 In hypothesis 1, the term “litter quality” requires an explanation.
L. 98 What is “microbial functioning”? This sounds a little awkward. I guess the authors mean microbial ecophysiology or something related
L. 105 see above
Section 2.1 It would be helpful to see a map and a few photos of the sites.
L. 218/219 Was the glucose-6-phosphate uniformly labelled or was it only labelled in one C position?
L. 125 How was this done randomly. Please explain.
Fig. 1 The label on the y-axis is not clear. This should be improved and further explained in the caption.
Fig. 3 and corresponding text. Please indicate whether these ratios are based on mass or number of moles. It is confusing that in some parts of the text, it is indicated that these are molar ratios while in other parts the authors simply refer to ratios.
Fig. 8 I suggest to add a second panel to the figure that shows the percentage of respired 13C (derived from glucose-6-phosphate). This would allow the authors to discuss loss of C in comparison to loss of P.
Fig. 450/451 This sentence is not entirely clear. Specifically it is not clear what exactly “decreasing C:N:P ratios” refers to and what “effects” refers to.
L. 455 replace “mineral” by “inorganic”
L. 470 Microbial mineralization of what? I guess glucose-6-phosphate but it might be good to clarify this.
L. 474 It seems that rather the lab studies are biased (or artificial) because they exclude nutrient import and export.
L. 484 is mineralized (not becomes mineralized)
L. 502 Please replace “very likely” by “which might have been” (since this is rather speculative).
L. 505 see above. What is “microbial functioning”?
L. 552 remove “in the tundra”.
l. 556 This is not correct. While the litter has a lower nutrient content in the tundra than in the forest, 100% of the nutrients are released in both ecosystems after several decades
L. 572 remove “worldwide”Citation: https://doi.org/10.5194/egusphere-2024-2622-RC1 -
RC2: 'Comment on egusphere-2024-2622', Anonymous Referee #2, 29 Oct 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-2622/egusphere-2024-2622-RC2-supplement.pdf
Data sets
C-N-P-Release Treeline Frank Hagedorn https://doi.org/10.16904/envidat.536
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