the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Warming accelerates belowground litter turnover in salt marshes – insights from a Tea Bag Index assay
Stefanie Nolte
Kai Jensen
Roy Rich
Julian Mittmann-Goetsch
Peter Mueller
Abstract. Salt marshes play an important role in the global carbon (C) cycle due to the large amount of C stored in their soils. Soil C input in these coastal wetland ecosystems is strongly controlled by the production and initial decomposition rates of plant belowground biomass and litter. This study used a field warming experiment to investigate the response of belowground litter breakdown to rising temperature (+1.5 °C and +3.0 °C) across whole-soil profiles (0–60 cm soil depth) and the entire flooding gradient ranging from pioneer zone via low marsh to high marsh. We used standardized plant materials, following the Tea Bag Index approach, to assess the initial decomposition rate of (k) and the stabilization factor (S) of labile organic matter (OM) inputs to the soil system. While k describes the initial pace at which labile (= hydrolyzable) OM decomposes, S describes the part of the labile fraction that does not decompose during deployment in the soil system and stabilizes due to biochemical transformation. We show that warming strongly increased k consistently throughout the entire soil profile and across the entire flooding gradient, suggesting that warming effects on the initial decomposition rate of labile plant materials are independent of the soil aeration (i.e. redox) status. By contrast, negative effects on litter stabilization were less consistent. Specifically, warming effects on S were restricted to the aerated topsoil in the frequently flooded pioneer zone, while the soil depth to which stabilization responded increased across the marsh elevation gradient via low to high marsh. These findings suggest that reducing soil conditions can suppress the response of belowground litter stabilization to rising temperature. In conclusion, our study demonstrates marked differences in the response of initial decomposition rate vs. stabilization of labile plant litter to rising temperature in salt marshes. We argue that these differences are strongly mediated by the soil redox status along flooding and soil-depth gradients.
Hao Tang et al.
Status: final response (author comments only)
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RC1: 'Comment on bg-2022-189', Md. Golam Rakkibu, 26 Oct 2022
The paper represents a very important perspective of coastal ecosystem carbon dynamics which is not adequately understood. The findings and arguments of the study are critical and very important for future studies exploring coastal soil carbon dynamics which is susceptible to climate change. The manuscript is very well written, explained and discussed. Data are very well presented and clearly interpreted.
Experimental design and methods used to cover the whole soil profile is the significant development in the study. But the issue of whether TBI materials represent the real world scenario with regard to belowground biomass, litter and organic matter turnover remains critical.
Whether the solid PVC posts and perforated holes in which tea bags were placed had any impact that could lead to different conditions in terms of soil moisture, temperature and microbial activity compared to natural soils around need to be discussed.
Besides mean elevation other edaphic characteristics of the three marsh types could be described under site description to signify the zonations.
Citation: https://doi.org/10.5194/bg-2022-189-RC1 - AC1: 'Reply on RC1', Hao Tang, 06 Jan 2023
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RC2: 'Comment on bg-2022-189', Inge Althuizen, 24 Nov 2022
The manuscript presents a study of belowground decomposition processes in salt marshes in response to climate warming, which are underexplored. The experiment has a novel expermental design that makes use of two warming treatments along a flooding gradient, where decomposition rates and stabilization factor were assessed throughout the soil profile using a standardized litter bag method (tea bag index;TBI). In addition, soil redox index was measured along this gradient to deduce whether changes in hydrology and redox conditions affect decomposition.
The study design has multiple aspects and gives interesting new insights to decomposition processes in this system and the manuscript is in general well written, though there are some sections that need further clarification. I also have more substantial comments regarding the methodology and performed analyses that I will detail below.
Since the experiment was performed in salt marshes, leaching could play a large role in mass loss due to high soil moisture/inundation and might influence the findings, see for example (Gessner et al. 2010, Lind et al. 2022, Marley et al. 2019). While the authors do state that they used a tidal wetland-adapted TBI protocol, I would like to see details on what adjustments this protocol has for the k and S calculations within the manuscript and whether this takes into account leaching. In addition, it might be good to raise and discuss this point already in the introduction.
I also wondered whether the use of PVC tubes could influence the conditions in which the decomposition experiment was performed, as the solid pipes might prevent water flow and could potentially also hinder the warming treatment used. Could the authors address this point. Potentially assess whether the temperature treatment was affected by the use of solid PVC tubes?
This also leads me to another point that I would like clarification on being the temperature monitoring. The authors only provide a mean temperature during the deployment time for the different zones. Which leaves me to wonder whether temperature was also monitored in the various treatments, to check whether treatments were effective. I could also not find at which depth temperature was monitored, only 1 depth or throughout soil profile? Could the authors provide a figure with the temperatures for the different zones and treatments throughout the deployment times to give a better representation of the treatments used?
The description of the statistical analyses is different from the results you present. From the results I deduce that you have performed separate analyses of warming and zone and their interaction for the different years (between subject), excluding the effect of depth.
Then there is separate analysis of the effect of depth, depth*zone and depth*zone*warming (within subject). In the statistical analyses section these are lumped together and it is not clear that the analyses were performed for each year separately. Furthermore, it is not clear to me why these analyses are split and what the authors mean with the indication “between subject” and “within-subject”? The “within subject” analysis would still need warming and zone included as separate factors to account for their effect.
Furthermore, there is no mention of analysis done to produce figure 5 and it is hardly discussed in the results. Lacking this information it is hard to properly assess the results.
Personally, I think Figure 5 gives a much better look into how the different factors influence decomposition rate k and S. I think expanding on this analysis would improve the manuscript as it sheds more light what factors/conditions affect decomposition in salt marshes. This could also be a way Figure 3 and 4 could then potentially be moved to supplementary material.
I also wondered why the authors did not use the measured soil reduction index in their analysis, as they as they discuss the influence of redox a lot in the discussion, but have not directly tested these links in their analysis. Why not use reduction index as predictor of k and S?
Specific comments
line 68. What do the authors mean with short- and mid-term warming?
Line 96 Can the authors also indicate the location of the different warming treatments. And were these treatments randomly assigned?
Line 132. Not clear if it is 1 PVC stick per zone or whether there is replication? The transect in the next line adds to my confusion. Figure 6 legend states “n = 6 observations per zone, deployed over four consecutive deployment campaigns (July-October)”. Clarify in methods.
Figure 3 Hard to read this figure as the error bars of the different treatments are overlapping. Please adjust figure so it is possible to discern the different treatments per soil depth.
Line 114 Incubation period different in 2018 and 2019, June-Sept vs May-July. Why? This does explain why temperatures were higher in 2018 as it is later in the season.
Line 150-153 It is not clear to me what the authors are trying to say. Clarify
Figure 3c Why high marsh higher k in +1.5 vs +3.0 treatment?
Why k higher in warming treatments in 2019 vs 2018? Bigger difference in temperature?
Technical corrections:
Line 49 “in” question
Line 50. Do the authors mean labor intensive instead of efficient? Constructing a lot of litter bags is labor intensive in my opinion.
Line 54 rephrase: represents a widely used standardized litter bag approach
Line 68 replace “over” with “for”. Over implies the incubation time was either 1 or 2 years, but there were separate 3-month incubations in each year.
Line 81 Can the authors use a more widely known standard like meter above sea level for to indicate elevation instead of NHN?
Line 132 “.” After citation
Line 132 remove “from pioneer…. High marsh.”
Line 248 known
References
Mark O. Gessner, Christopher M. Swan, Christian K. Dang, Brendan G. McKie, Richard D. Bardgett, Diana H. Wall, Stephan Hättenschwiler, Diversity meets decomposition, Trends in Ecology & Evolution, Volume 25, Issue 6, 2010,Pages 372-380, ISSN 0169-5347, https://doi.org/10.1016/j.tree.2010.01.010
Lind, L., Harbicht, A., Bergman, E., Edwartz, J., & Eckstein, R. L. (2022). Effects of initial leaching for estimates of mass loss and microbial decomposition—Call for an increased nuance. Ecology and Evolution, 12, e9118. https://doi.org/10.1002/ece3.9118
Marley, A. C. R. G., Smeaton, C., & Austin, W. E. N. (2019). An assessment of the Tea Bag Index method as a proxy for organic matter decomposition in intertidal environments. Journal of Geophysical Research: Biogeosciences, 124. https://doi.org/10.1029/2018JG004957
Citation: https://doi.org/10.5194/bg-2022-189-RC2 - AC2: 'Reply on RC2', Hao Tang, 06 Jan 2023
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RC3: 'Comment on bg-2022-189', Anonymous Referee #3, 07 Dec 2022
This paper examines patterns of initial decomposition rate and stabilization factor over a flooding gradient and depth profile in a tidal marsh. The focus on decomposition over an elevation gradient and belowground, where the greatest contribution by plants to blue C accumulation occurs, represents an important contribution to the literature. Further, the use of standard substrate to control for litter quality allows for a focus on abiotic drivers of decay. Overall, this is a good paper that, with some clarifications to the methods and stats and additional interpretation of results, will add valuable insights to decomposition processes in tidal marshes that are especially vulnerable to climate change.
Specific comments and questions:
L60-61: clarify by explaining what their proposed mechanism is for how the lack of oxygen inhibits warming effects.
L63: reiterate that the use of a standard substrate is needed to understand warming effects.
L70: is the first hypothesis expected regardless of soil depth?
Site description – Figure 1 is difficult to see, and the zones are not clearly defined in the text. It would be helpful to explain how the zones are oriented relative to the open water, with the pioneer zone along the shoreline and the high marsh farthest inland. Also, how is “pioneer zone” defined?
Experimental design – How was the soil warming established and verified along the soil depth gradient? Was there uniform warming of the soil column or did it vary with depth? It would be nice to see a graph of these data. Did you confirm treatment conditions of +1.5 and +3 degrees warming? Why was the average soil temperature from -10 and -60 cm used as opposed to looking at temperature along the soil depth gradient at the same intervals at which decomposition and soil reduction were measured?
Decomposition – why was this examined across two different periods (June-Sept vs. May-July) in the two years? It is not surprising that ambient temperatures were cooler in year 2 (late spring/early summer) than in year 1 (late summer), which may have contributed to the larger effect sizes of warming in year 2 compared to year 1. Address why these time periods were selected, and later discuss how this could have affected results.
Statistics – This section is lacking details and does not fully track with the results presented. Were years compared statistically or tested separately? Why or why not? How were the effect sizes determined and analyzed, and why was this only examined in year 2? How was soil reduction analyzed?
For the discussion and methodological considerations, how much could leaching be contributing to the results and different findings for k and S along the flooding gradient? How did the PVC influence the hydrology or connectivity of the tea bags with their surroundings? Was the temperature gradient verified within those PVC pipes? It would also be useful to revisit the importance of litter quality, as well as species-specific differences in decay with species turnover along the elevation gradient. While this study was designed to avoid plant influences, brief discussion of how it could affect these patterns, and how shifts in community composition with sea-level rise is another climate change driver to be considered that, if species differ in their contributions to blue C, could have implications for marsh resilience.
Technical comments:
L13: clarify “plant production”
L15: suggest “entire intertidal flooding gradient”
L17: delete “of” before “(k)”
L54: delete “probably”
L59: offset “and thus strongly reducing” with commas
L69: what is short- and mid-term warming effects? Is this in reference to projected warming of +1.5 vs. 3 degrees?
L71: combine sentences so that it reads “…soil, and (2) that warming…”
L77: “has operated” instead of “operates”
L79: change comma after climate to semicolon
L148: should this be “appear to be consistent”?
L150: this is unclear. What do you mean by “refer the significant interaction”?
L152: clarify that the relationship is “with increasing soil depth”
L192: change to “a large” instead of “an”
L209: add a comma after the citation
L248: “known”
Citation: https://doi.org/10.5194/bg-2022-189-RC3 - AC3: 'Reply on RC3', Hao Tang, 06 Jan 2023
Hao Tang et al.
Hao Tang et al.
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