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Peer review report 1 On “Trenching reduces soil heterotrophic activity in a loblolly pine (Pinus Taeda) forest exposed to elevated atmospheric [CO2] and N-fertilization”
Agricultural and Forest Meteorology 201S (2015) 482–483
Contents lists available at ScienceDirect
Agricultural and Forest Meteorology journal homepage: www.elsevier.com/locate/agrformet
Peer Review Report
Peer review report 1 On “Trenching reduces soil heterotrophic activity in a loblolly pine (Pinus Taeda) forest exposed to elevated atmospheric [CO2] and N-fertilization”
Original Submission Recommendation Minor Revision Comments to Author Overall, the authors present significant findings with regard to the effects of a root-exclusion experiment on metrics of carbon (C) and nitrogen (N) cycling in a loblolly pine forest. The results are new and original but are somewhat expected based on the body of literature that has been provided for this forest (as seen with the number of refs provided by the same or associated authors). The results presented in this study, however, significantly contribute to a better understanding of the impacts of root-derived C inputs into the soil environment on soil C processing by the heterotrophic community. On this basis, I believe this manuscript is suitable for publication in Agricultural and Forest Meteorology. The topic definitely fits in the broad scope of the journal and will be an excellent contribution to our understanding of ecosystem carbon dynamics. I hope the comments I provide will help the authors at improving their manuscript. Technically, the analysis presented in this study is very robust with no apparent flaws in the experimental procedure. The manuscript is very well-written. The tables and figures are clear, efficient and easy to interpret, except maybe for the technical comments presented below. The methods are also clear and repeatable. Enough information is given with respect to the experimental design and the measurements of soil respiration (Rsoil). I personally have a limited knowledge on enzymatic analyses, but everything looks in order from what I can read. Minor criticisms that should be addressed 1- Should the authors have used deeper trenches for their experiment? Probably so. 30-45 cm is very shallow and it is very unlikely that all roots wood be severed in this type of forest ecosystem. 2- Was October a representative date for soil coring and sampling and subsequent lab analyses? Debatable. Probably if one looks
DOI of published article: http://dx.doi.org/10.1016/j.agrformet.2012.05.017. 0168-1923/$ – see front matter http://dx.doi.org/10.1016/j.agrformet.2015.07.075
at the time it takes for a root-exclusion experiment to reach its full effect. Probably not if one looks at maximizing the amount of heterotrophic activity during the year. 3- Should the authors have reported differences between locations A and B for the ACES system? Yes but it has probably be done already in previous publications. Maybe refer to these publications. 4- Should the authors have “speculated” on what would have happened during drought conditions with regard to Rsoil? Could have been interesting since there was a significant increase in Rsoil following re-wetting of the soil in October and that is when the differences in treatments were the most apparent. This could imply that rhizosphere priming effects on decomposition are waterdependent, which would be a new and original finding. P18 L394. Contrary to what previous studies have shown, the results of this study do not show that root biomass increased under elevated CO2 conditions (Fig 1). Why is that and why are these results different than previous results at that site? Also, these results do not provide an independent support for the previous body of literature as stated in following sentence. P18 L398. How is this possible that Rroot/Rsoil is near 0 in ambient CO2? Conceptually, this is very difficult to understand. I understand the concept of reduced C allocation to root systems in improved soil N conditions but to say that root respiration falls to near 0 (∼10% according to Fig 6) is near impossible. Please expand on this. From my understanding of root-exclusion experiments, here are some possible outcomes (and I am probably missing some) that need to be taken into account when interpreting the results of such manipulative experiments on the heterotrophic activity of soil organisms. Overall, the trenching: 1- Removes live roots and associated fungi, (-) effect on heterotrophic activity; 2- Adds organic matter through dead roots, (+) effect on heterotrophic activity; 3- Removes exudation of labile C, (-) effect on heterotrophic activity; 4- Removes part of the decomposer community directly associated with the rhizosphere, (-) effect on heterotrophic activity; 5- Changes the decomposer community in the overall soil environment, (- or +) effect on heterotrophic activity, hard to determine. To my belief, outcomes #4 and 5 presented above, i.e. an overall change in the decomposer community, seem very important but are addressed only very briefly in the discussion section (P16 L345-349)
Peer Review Report / Agricultural and Forest Meteorology 201S (2015) 482–483
and not in direct relation with other results of the soil heterotrophic activity. From all the results presented in this manuscript and in previous studies, it appears that N fertilization alone has a dramatic impact on C and N cycling in this type of forest ecosystem. N fertilization decreases root biomass, Rhet metrics, N mineralization and nitrification but has no apparent effects on Rsoil. It is interesting to note that trenching never enhanced the effect of N fertilization on the various metrics presented. The results for nitrification alone following N fertilization or trenching strongly indicate a switch in microbial community or pool following these treatments and I think that topic should be addressed more appropriately in the discussion section and should be the basis for further studies. Maybe these results are showing a switch from a fungal to bacterial-dominated community. Specific comments Remove square brackets around [CO2] in title. Italicize et al. throughout the manuscript.
483
P4 L69. Replace Phllips with Phillips. P4 L76. Remove double dots at the end of sentence. P9 L177. Something wrong with the end of the sentence. Rephrase. P9 L187. How do you know that the correction did not later any of the results of the study? P17 L362. Replace Rhetl with Rhet. Figures Figure 4 Change the appearance of the tick marks and labels on these panels. It is very confusing right now where October falls, for example, on the x axis. The month label should be located between two major tick marks. Change the line type for soil moisture in panel e as well. Anonymous Available online 6 August 2015
Contents lists available at ScienceDirect
Agricultural and Forest Meteorology journal homepage: www.elsevier.com/locate/agrformet
Peer Review Report
Peer review report 1 On “Trenching reduces soil heterotrophic activity in a loblolly pine (Pinus Taeda) forest exposed to elevated atmospheric [CO2] and N-fertilization”
Original Submission Recommendation Minor Revision Comments to Author Overall, the authors present significant findings with regard to the effects of a root-exclusion experiment on metrics of carbon (C) and nitrogen (N) cycling in a loblolly pine forest. The results are new and original but are somewhat expected based on the body of literature that has been provided for this forest (as seen with the number of refs provided by the same or associated authors). The results presented in this study, however, significantly contribute to a better understanding of the impacts of root-derived C inputs into the soil environment on soil C processing by the heterotrophic community. On this basis, I believe this manuscript is suitable for publication in Agricultural and Forest Meteorology. The topic definitely fits in the broad scope of the journal and will be an excellent contribution to our understanding of ecosystem carbon dynamics. I hope the comments I provide will help the authors at improving their manuscript. Technically, the analysis presented in this study is very robust with no apparent flaws in the experimental procedure. The manuscript is very well-written. The tables and figures are clear, efficient and easy to interpret, except maybe for the technical comments presented below. The methods are also clear and repeatable. Enough information is given with respect to the experimental design and the measurements of soil respiration (Rsoil). I personally have a limited knowledge on enzymatic analyses, but everything looks in order from what I can read. Minor criticisms that should be addressed 1- Should the authors have used deeper trenches for their experiment? Probably so. 30-45 cm is very shallow and it is very unlikely that all roots wood be severed in this type of forest ecosystem. 2- Was October a representative date for soil coring and sampling and subsequent lab analyses? Debatable. Probably if one looks
DOI of published article: http://dx.doi.org/10.1016/j.agrformet.2012.05.017. 0168-1923/$ – see front matter http://dx.doi.org/10.1016/j.agrformet.2015.07.075
at the time it takes for a root-exclusion experiment to reach its full effect. Probably not if one looks at maximizing the amount of heterotrophic activity during the year. 3- Should the authors have reported differences between locations A and B for the ACES system? Yes but it has probably be done already in previous publications. Maybe refer to these publications. 4- Should the authors have “speculated” on what would have happened during drought conditions with regard to Rsoil? Could have been interesting since there was a significant increase in Rsoil following re-wetting of the soil in October and that is when the differences in treatments were the most apparent. This could imply that rhizosphere priming effects on decomposition are waterdependent, which would be a new and original finding. P18 L394. Contrary to what previous studies have shown, the results of this study do not show that root biomass increased under elevated CO2 conditions (Fig 1). Why is that and why are these results different than previous results at that site? Also, these results do not provide an independent support for the previous body of literature as stated in following sentence. P18 L398. How is this possible that Rroot/Rsoil is near 0 in ambient CO2? Conceptually, this is very difficult to understand. I understand the concept of reduced C allocation to root systems in improved soil N conditions but to say that root respiration falls to near 0 (∼10% according to Fig 6) is near impossible. Please expand on this. From my understanding of root-exclusion experiments, here are some possible outcomes (and I am probably missing some) that need to be taken into account when interpreting the results of such manipulative experiments on the heterotrophic activity of soil organisms. Overall, the trenching: 1- Removes live roots and associated fungi, (-) effect on heterotrophic activity; 2- Adds organic matter through dead roots, (+) effect on heterotrophic activity; 3- Removes exudation of labile C, (-) effect on heterotrophic activity; 4- Removes part of the decomposer community directly associated with the rhizosphere, (-) effect on heterotrophic activity; 5- Changes the decomposer community in the overall soil environment, (- or +) effect on heterotrophic activity, hard to determine. To my belief, outcomes #4 and 5 presented above, i.e. an overall change in the decomposer community, seem very important but are addressed only very briefly in the discussion section (P16 L345-349)
Peer Review Report / Agricultural and Forest Meteorology 201S (2015) 482–483
and not in direct relation with other results of the soil heterotrophic activity. From all the results presented in this manuscript and in previous studies, it appears that N fertilization alone has a dramatic impact on C and N cycling in this type of forest ecosystem. N fertilization decreases root biomass, Rhet metrics, N mineralization and nitrification but has no apparent effects on Rsoil. It is interesting to note that trenching never enhanced the effect of N fertilization on the various metrics presented. The results for nitrification alone following N fertilization or trenching strongly indicate a switch in microbial community or pool following these treatments and I think that topic should be addressed more appropriately in the discussion section and should be the basis for further studies. Maybe these results are showing a switch from a fungal to bacterial-dominated community. Specific comments Remove square brackets around [CO2] in title. Italicize et al. throughout the manuscript.
483
P4 L69. Replace Phllips with Phillips. P4 L76. Remove double dots at the end of sentence. P9 L177. Something wrong with the end of the sentence. Rephrase. P9 L187. How do you know that the correction did not later any of the results of the study? P17 L362. Replace Rhetl with Rhet. Figures Figure 4 Change the appearance of the tick marks and labels on these panels. It is very confusing right now where October falls, for example, on the x axis. The month label should be located between two major tick marks. Change the line type for soil moisture in panel e as well. Anonymous Available online 6 August 2015