Litter Decomposition in Wet Rubber and Fruit Agroforests: Below the Threshold for Tropical Peat Formation

Round 1

Reviewer 1 Report

The manuscript describes a study on tropical peat formation thresholds in Indonesia using litter bag experiments in several land cover types.

 

Introduction

The introduction provides a basic overview of the unknowns regarding the peat formation process in general, but often contains extraneous information, does not cite specific studies on peat formation in artificial or commodity based ecosystems, and does not link concepts such as the home field advantage to the objectives of the study. The specific questions listed in the introduction do not have a clear connection to the study title. Perhaps most egregious, the authors do not explain how litter bag studies relate to peatland formation rates. The introduction must provide studies explaining the efficacy of litter bag techniques to estimate peat formation beyond a simple mass balance. Litter residues after a few months to years are likely to have a unique chemical composition that differs from peat which forms over hundreds of years.

 

Lines 47-59. These paragraphs do not flow well from the first paragraph. Overall they list information on Indonesian peatland with little connection to the topic, peat formation thresholds. Adding “the thresholds for initial, aerobic excess of necromass formation over its decomposition remain poorly quantified and triggered our current re-58 search (Fig. 2).” to the first paragraph would flow better.

 

Figure 2. Overall the figure is quite confusing. It is not clear why factors on the mineral soil side are not mirrored on the peat formation side. For instance, litter is color coded into above and belowground and grouped in boxes on the mineral soil side, yet the same factors have arrows going in the opposite direction, have question marks attached to them, and are not boxed. It is not clear if the width of the arrows is scaled. Climate change also indicates an arrow which may suggest moving towards mineral soils, but it contains the concept of fertilization.

 

The methods should describe the litter quality indicators used in this study (i.e. (L+P)/N) and the findings of other studies which have used this indicator.

 

 

Methods

The methods do not provide sufficient detail for replication and does not contain any mention of the equipment utilized (e.g., name and manufacturers information), the chemical purity, and the techniques utilized. Furthermore the methods section contains study results, Figure 5, table 2, table 3, figure 6 and table 3 are all results and should be moved with their accompanying text to the results section.

The axis text on most of the figures is too small.

Line 211. Before describing how litter was processed it should be clear what the source of the litter was. I am assuming leaves? What were the litter bags made out of?

Line 226. Briefly state the methods used for lignin and polyphenol determination. How was cellulose quantified?

 

Results

Figure 7. Some data points suggest a weight greater than the initial weight. Explain.

Table 4. It is not clear from the text what the litter sources are or what the acronyms stand for.

Lines 278-282. A statistical test must be used to determine whether a specific kd had a statistically significant difference.

Line 305. It is not clear what the 80% refers to. Clarify.

 

Discussion

The discussion provides some additional context for the experiment, but misses discussing some key details. Althougth the introduction specifically highlights the discussion section as answering the missing link between litter bags and peat formation, there is only one sentence that really addresses this issue and it basically states there maybe a connection. The manuscript would be fine as a documentation of litter decomposition rates, but by suggesting that these measurements are relevant for peat formation increases the evidentiary requirements. Essentially, the discussion should relate to the challenge of inferring peat formation thresholds on the basis of litter bag studies. 

Line 324. The text suggests that earthworms could be responsible for the low residence time, but the results section has not presented any evidence linking decomposition rates to earthworm abundance. The discussion of why the RMy litter had a different decomposition rate should be limited to actual analysis or a discussion of other non-quantified factors.

Lines 324-326. It is not clear why the estimates are reasonable. The authors cite the 8 Mg ha yr-1 sustainability target of soil organic matter maintenance, but a target is not a reasonable estimate. The authors should place their estimated in context with other studies and make greater effort to explain why their values align or differ with other studies.

Line 346. Implies that the threshold for peat formation is known.

Lines 347-355. I am not sure I understand the point of this paragraph.

Lines 356-360. The authors cite a study but do not place the results of their study in context. Greater effort must be made to make their findings in relation to the state of the art explicit.

How does the choice of litter (leaves vs stems) affect the peat formation rates?

 

Conclusions

The conclusions are not justified by the evidence and manuscript. 

 

Comments for author File: Comments.pdf

Author Response

We appreciate the detailed review and the many suggestions to improve the manuscript. The major change we implemented was a change in the title, as apparently the original one gave a wrong impression on the topic of our research. Further detail can be found in the attached "response to reviewer 1" file.

Author Response File: Author Response.pdf

Reviewer 2 Report

only minor comments

51 should be put as (51)

I dont see any sentences to justify why this study must be conducted since many similar studies has been done for agroforestry, in other words whats the new finding that make this study distinguished than the others?

in conclusion 'Wet agroforests support landscape-level rewetting strategies and protect peat dome hinterlands from drying and fire. ' is important finding for peat conservation in climate mitigation, is there any other studies suppports on this statement? 

 

 

 

Author Response

We thank reviewer 2 for the interest in the manuscript and for the detailed suggestions to improve the manuscript. Further detail can be found in the attached "response to reviewer 2" file.

Author Response File: Author Response.pdf

Reviewer 3 Report

The manuscript reports on the compatibility of agroforestry and paludiculture and peat formation

The comments below are minor suggestions

one of the conclusions 'Maintaining wet agroforest conditions adjacent to peat domes support peatland rewetting and restoration, but does not contribute to on-site peat formation processes' - is there a risk that wetland adapted agroforestry species may encroach on the peat dome with impacts for native peatland vegetation?

line 172 'Rubber Monoculture ( RMO), also planted around 30 years ago, with a higher rubber density, but still containing some local trees such as durian, rambutan and cempedak.'  not strictly speaking a monoculture, so perhaps 'high rubber density' is more appropriate as a description

line 356 'A study of plant litter decomposition in boreal peatlands ([68]) found that the direct effects of decreasing water tables were relatively small and were overruled by the indirect effects through changes in litter type composition and production of the changing vegetation. Increased litter inputs from a modified vegetation resulted in large accumulation of organic matter in spite of increased decomposition rates.'  I suggest this paragraph is expanded.  What direct effects are being referred to?  how does a boreal peat system differ to the one under evaluation e.g. climate.  A single reference is cited here, the impact of drainage may not be reported as being 'relatively small' by all studies

line 409 'The authors concluded that 409 site enrichment effects were stronger than those of litter quality although both were highly 410 significant.' does this support / disagree with the findings of your study?  at present it just reads as a statement, there is not critical engagement with your own data

line 422 'Longer dry seasons and/or higher ET rates make it impossible to match the mandated rules.'   so is the conclusion here that agroforestry is not a viable option due to the potentially higher evapotranspiration rates?

Author Response

We thank reviewer 3 for the interest in our study and the suggestions to improve the manuscript.  Further detail can be found in the attached "response to reviewer 3" file.

Author Response File: Author Response.pdf

Reviewer 4 Report

The paper is well formatted, with interesting and well-structured content. The scientific justification is correct and the bibliography used is up to date and supports the conclusions and results obtained. The research design is sound and the results are interesting. The following issues are commented on:

Figure 2. Report if it is self-made. It is suggested to add more details about the information that appears in the graph.

Figure 3. The following changes are suggested:

• Add the graphic scale to the figure on the right.
• Include the font of the images in the description of the figure.
• Improve the typography and the legend of the main figure.
• Correct typo: RMy
• It is suggested to differentiate the typography for each of the types of place names (road, river-channels, etc.).

MATERIALS AND METHODS

Line 158. It is suggested that the ages of the cited specimens be dated more precisely and scientifically, for example by means of pressler-type augers.

Figure 4. Correct typo RMy. It is recommended that this figure be moved to the Litter bags section. Include if own production.

Figure 5. Corrigendum RMy.

Line 197 and figure 6: it is suggested to move to results as it is not methodology.

Figure 6. Correct typo RMy; correct typo Layeer.

RESULTS

Figure 7. Correct typo RMy;

Table 4. Unify RMy / RMO designation

Figure 8. Correct typo RMy;

 DISCUSSION

Lines 379-380. Include the scientific names of the species cited.

It might be of interest to further elaborate on the difference in management and type of silviculture of the four types of formations analysed. For example: relationship between maturity of the formations, presence of forest elements of high biological value, litter, edaphic characteristics and agroforestry uses.

 

Author Response

We appreciate the detailed review and the many suggestions to improve the manuscript. Further detail can be found in the attached "response to reviewer 4" file.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors have made moderate revisions and adequately addressed some of my concerns. The change in title is useful, but I still think there is a connection missing between the litter bag experiment and peat formation. 

 

In light of providing more information more concerns have been raised. 

The study uses litterbag studies to assess the thresholds where C accumulation and peat formation starts. In the abstract the authors suggest that litter residence time is below the apparent peat formation threshold and thus suggest that the threshold is a time period which is known. Yet lines 415 suggest this is unknown, and lines 511 suggest that other factors aside from time are important. This concept of a threshold is not clear. 

 

The authors use the Van Soest procedure to quantify the various major fractions. Yet on lines 235 they indicate using acid detergent fiber as the remaining mass of lignin plus cellulose. However, in table 3 they have clearly separated cellulose, lignin, and ash which would require additional steps for ADL and then furnacing to find the ash weight. It is not clear how the authors could get these measurements without conducting the full procedure. 

 

Author Response

Response to reviewer comments:

"The authors have made moderate revisions and adequately addressed some of my concerns.  ** Thank you

1. The change in title is useful,  ** Thank you 
2.  but I still think there is a connection missing between the litter bag experiment and peat formation.

In light of providing more information more concerns have been raised.

2. The study uses litterbag studies to assess the thresholds where C accumulation and peat formation starts. In the abstract the authors suggest that litter residence time is below the apparent peat formation threshold and thus suggest that the threshold is a time period which is known. Yet lines 415 suggest this is unknown, and lines 511 suggest that other factors aside from time are important. This concept of a threshold is not clear.

[we have added two paragraphs (see below) to the discussion that reflect on this connection and missing pieces of the puzzle]

3. The authors use the Van Soest procedure to quantify the various major fractions. Yet on lines 235 they indicate using acid detergent fiber as the remaining mass of lignin plus cellulose. However, in table 3 they have clearly separated cellulose, lignin, and ash which would require additional steps for ADL and then furnacing to find the ash weight. It is not clear how the authors could get these measurements without conducting the full procedure"

[we have added further detail on the method – at risk that the material and method section is becoming unnecessarily complex – a comment we just received on another manuscript for this same journal…]

New text in M&M section:

Samples of the initial litter dried at 60 °C for 48 hours [[i]] were analyzed for the concentration of polyphenols [extraction of 0.1 g of plant material with 50 mL of a 50% (v/v) methanol-water mixture, response to Folin-Ciocalteu reagent calibrated with a tannic acid standard after 14], N (Kjehldal method in [14,[ii]]), cellulose and lignin [[iii]]. The latter method is based on the residual weight of samples following successive removal of various tissue constituents: The first step oxidizes soluble organic components and defines Acid-detergent fibre (ADF: Lignin + cellulose + insoluble minerals) based on boiling for 1 h in CetylTrimethylAmmonium Bromide 1 N-H₂SO₄, followed by filtering, drying and weighing. The next step breaks down the cellulose during 3 h at 20 ^oC in 72% H₂SO₄ and estimates lignin+insoluble minerals (‘ash’). After filtering, drying and weighing, the sample is heated in a muffle furnace at 550 ^oC to determine the insoluble mineral residue. Cellulose and lignin estimates are based on weight differences between 1^st and 2^nd and 2^nd and 3^rd step, respectively. Part of the literature describes these fractions as proximate cellulose and lignin, respectively [[iv],[v]].

 

[i] Walela, C., Daniel, H., Wilson, B., Lockwood, P., Cowie, A., Harden, S. The initial lignin: nitrogen ratio of litter from above and below ground sources strongly and negatively influenced decay rates of slowly decomposing litter carbon pools. Soil Biology and Biochemistry 2014, 77, 268-275.

[ii] Vanlauwe, B., Gachengo, C., Shepherd, K., Barrios, E., Cadisch, G., Palm, C.A. Laboratory validation of a resource quality‐based conceptual framework for organic matter management. Soil Science Society of America Journal, 2005, 69(4), 1135-1145.

[iii] Goering, H.U., Van Soest, P.J. Forage fiber analyses (apparatus, reagents, procedures, and some applications). Report No 379. US Agricultural Research Service, 1970.

[iv] Van Soest PJ, McQueen RW. The chemistry and estimation of fibre. Proceedings of the Nutrition Society 1973, 32(3), 123-130.

[v] Gessner MO. Proximate lignin and cellulose. In:: Graça MAS, Bärlocher F, Gessner MO (Eds) Methods to study litter decomposition (pp. 115-120). Springer, Dordrecht. 2005

New text in discussion:

It is clear that in these agroforests no peat is currently formed, with a litter layer of maximum 4 cm (Fig 5) and a mean residence time of around two years. We interpret the relatively high C_org concentrations of in the young RAF_y and RMO_y shown in Table 1 as remnants, while C_org in the older FAF_O and RAF_O  is about at the level predicted by a texture-based pedotransfer function for mineral soils [59]. Yet, at what mean residence time an accumulating litter layer could cross the threshold to peat formation is unknown. “By most definitions, the peat thickness must exceed 40 cm before a soil profile can be called peat, but there is also C contained in shallower organic deposits where peat is currently accumulating or has the potential to form if peat-forming plants are present” [[i]]. Current understanding goes beyond  “peat forms whenever C inputs to a peatland exceed C outputs.“ and emphasizes for Southeast Asia that “The peatlands of this region typically form large domes located behind mangroves along coastal plains and between major rivers. During their development, the peat surface of these systems became increasingly elevated above the surrounding land. As peat accrued, rainwater replaced tidal and riverine flood waters as the principal water source, and the domed peat surface … became increasingly nutrient poor and acidic. The peat-forming vegetation is a type of tropical rainforest—peat swamp forest—that, despite the stressful environmental conditions of anoxia, acidity, and low nutrient availability, has a relatively high tree species diversity” [87].

Our litterbag experiments focused on the early stages of aboveground litter decomposition, while it is recognized that much aboveground production in tropical peats—the leaves especially—is readily decomposed and does not contribute much to the makeup of the peat [[ii]]. Although overall litter production may be dominated by the aboveground component in the tropics, it may actually be the belowground component—fine roots in particular—that is critical in peat formation. This certainly seems to be the case for Southeast Asian peatlands [[iii]]. Decomposition of tree roots depends on water tables. A recent review found sub-surface water-table fluctuations from pristine forested tropical peatlands have been reported to vary from about 25 to 90 cm [[iv]] – a range similar to what we documented for the agroforests. Peat swamp forests have a remarkable microtopographic  differentiation between hummocks and hollows [[v]] – more than we found in the agroforests.

 

[i] Page, S.E., Baird, A.J. Peatlands and global change: response and resilience. Annual Review of Environment and Resources, 2016, 41, 35-57.

[ii] Chimner, R.A., Ewel, K.C.. A tropical freshwater wetland: II. Production, decomposition and peat formation. Wet. Ecol. Manag. 2005. 13, 671–84

[iii] Brady, M.A. Organic matter dynamics of coastal peat deposits in Sumatra, Indonesia. PhD Thesis, Dep. For., Univ. BC, Vanc., Can.  1997. https://open.library.ubc.ca/cIRcle/collections/ubctheses/831/items/1.0075286 [accessed: 25 Jan 2022].

[iv] Bacon, K.L., Baird, A.J., Blundell, A., Bourgault, M.A., Chapman, P.J., Dargie, G., Dooling, G.P., Gee, C., Holden, J., Kelly, T.J. and McKendrick-Smith, K.A. Questioning ten common assumptions about peatlands. Mires and Peat 2017, 19,1-23. https://eprints.whiterose.ac.uk/119944/1/map_19_12.pdf [accessed: 25 Jan 2022].

[v] Jauhiainen, J., Takahashi, H., Heikkinen, J.E.P., Martikainen, P.J., Vasander, H. Carbon fluxes from a tropical peat swamp forest floor. Global Change Biology 2005, 11, 1788–1797.

Author Response File: Author Response.pdf