Nitrogen and phosphorus enrichment effects on CO 2 and methane fluxes from an upland ecosystem

Reactive nitrogen (N) deposition can affect many ecosystem processes, particularly in oligotrophic habitats, and is expected to affect soil C storage potential through increases in microbial decomposition rate as a consequence of greater N availability. Increased N availability may also result in changes in the principal limitations on ecosystem productivity. Phosphorus (P) limitation may constrain productivity in instances of high N deposition, yet ecosystem responses to P availability are poorly understood. This study investigated CO2 and CH4 flux responses to N and P enrichment using both short(1 year) and long-term (16 year) nutrient addition experiments. We hypothesised that the addition of either N or P will increase CO2 and CH4 fluxes, since both plant production and microbial activity are likely to increase with alleviation from nutrient limitation. This study demonstrated the modification of C fluxes from N and P enrichment, with differing results subject to the duration of nutrient addition. On average, relative to control, the addition of N alone inhibited CO2 flux in the short-term (9%) but considerably increased CO2 emissions in the long-term (+35%), reduced CH4 uptake in the short term (-90%) and reduced CH4 emission in the long term (-94%). Phosphorus addition increased CO2 and CH4 emission in the short term (+20% and +184% respectively), with diminishing effect into the long term, suggesting microbial communities at these sites are P limited. Whilst a full C exchange budget was not examined in the experiment, the potential for soil C storage loss with long-term nutrient enrichment is demonstrated and indicates that P addition, where P is a limiting factor, may have an adverse influence on upland soil C content. Response to Reviewers: Reviewer #1: Comments on "Nitrogen and phosphorus enrichment effects on CO2 and methane fluxes from an upland ecosystem" I appreciated for a revision done by the authors and it got improved. Some of my comments have been addressed, yet I did not see much improved quality of the manuscript. Another round of moderate revision is needed. I understand the authors asking for some specific comments, yet as some organization and representation are throughout the manuscript, I have to list few of them, leave the authors to work on most. I also encourage the senior authors spend time on the representation if the leading author is reluctant for revision. >> We thank the reviewer for taking the time to review this manuscript and are grateful for their helpful comments. We have attempted to refine the text throughout in line with the above suggestion. 1) For the interactive impacts of N and P, Figure 2 and Figure 7 do have those results, worth a brief discussion. >> We are grateful for this suggestion, but as we stated in the first round of comments, the power of the study to assess the interactive effects of N and P was constrained by the original design of one of the two experiments, which did not include a P-only addition treatment. The conclusions of the study are robustly based on contrasts between the effects of N-only and N&P treatments. 2) First sentence in abstract in my previous round of comments. This paper addresses N and P impacts on CO2 and CH4 fluxes; of course, the soil organic carbon is important and relevant to CO2 and CH4 flux, but it should not be the opening sentences for abstract. >> Entire first sentence now deleted. 3) Comparison with other studies will enhance the impact of this paper; it is same for the model implications. >> Comparisons with the findings of other studies are drawn frequently in the discussion section of this paper – for instance, line 324/325, line 345/346, line 370, line 387, line 414/415 and others. With regard to ‘model implications’, as we stated in the first round of comments: We consider that the study contributes to scientific understanding and as such the results and inferences will be useful for testing and design of ecosystem models. However, reviewing the potential applications of the insights gained within specific models is beyond the scope of this paper. 4) Figure 1, a & b are missing. >> Figure 1, parts a) and b) are in reference to the nutrient addition treatment squares (bottom right of the figure). We have also added a) and b) labels to the map to help the reader distinguish between trial plots. 5) Line 331, "sizable" not professional phrase. >> All use of the word ‘sizeable’ deleted or replaced with significant, where appropriate. Similar subjective phrases were identified in other places in the text (e.g. markedly, considerable, slightly) and have been substituted with statistically validated, objective terms. We hope that this has therefore addressed the concern raised by the reviewer in the opening comment since the example given helped us to appreciate where concern lay in the presentation of the work in the earlier version of the manuscript. 6) At the beginning of the discussion, a small paragraph to highlight the key findings of this study is needed. >> Paragraph added to the discussion as requested. Reviewer #1: Comments on "Nitrogen and phosphorus enrichment effects on CO2 and methane fluxes from an upland ecosystem" I appreciated for a revision done by the authors and it got improved. Some of my comments have been addressed, yet I did not see much improved quality of the manuscript. Another round of moderate revision is needed. I understand the authors asking for some specific comments, yet as some organization and representation are throughout the manuscript, I have to list few of them, leave the authors to work on most. I also encourage the senior authors spend time on the representation if the leading author is reluctant for revision. >> We thank the reviewer for taking the time to review this manuscript and are grateful for their helpful comments. We have attempted to refine the text throughout in line with the above suggestion. 1) For the interactive impacts of N and P, Figure 2 and Figure 7 do have those results, worth a brief discussion. >> We are grateful for this suggestion, but as we stated in the first round of comments, the power of the study to assess the interactive effects of N and P was constrained by the original design of one of the two experiments, which did not include a P-only addition treatment. The conclusions of the study are robustly based on contrasts between the effects of N-only and N&P treatments. 2) First sentence in abstract in my previous round of comments. This paper addresses N and P impacts on CO2 and CH4 fluxes; of course, the soil organic carbon is important and relevant to CO2 and CH4 flux, but it should not be the opening sentences for abstract. >> Entire first sentence now deleted. 3) Comparison with other studies will enhance the impact of this paper; it is same for the model implications. >> Comparisons with the findings of other studies are drawn frequently in the discussion section of this paper – for instance, line 324/325, line 345/346, line 370, line 387, line 414/415 and others. With regard to ‘model implications’, as we stated in the first round of comments: We consider that the study contributes to scientific understanding and as such the results and inferences will be useful for testing and design of ecosystem models. However, reviewing the potential applications of the insights gained within specific models is beyond the scope of this paper. 4) Figure 1, a & b are missing. >> Figure 1, parts a) and b) are in reference to the nutrient addition treatment squares (bottom right of the figure). We have also added a) and b) labels to the map to help the reader distinguish between trial plots. 5) Line 331, "sizable" not professional phrase. >> All use of the word ‘sizeable’ deleted or replaced with significant, where appropriate. Similar subjective phrases were identified in other places in the text (e.g. markedly, considerable, slightly) and have been substituted with statistically validated, objective terms. We hope that this has therefore addressed the concern raised by the reviewer in the opening comment since the example given helped us to appreciate where concern lay in the presentation of the work in the earlier version of the manuscript. Responses to Reviewers Comments