the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Oct 2024
Methane oxidation potential of soils in a rubber plantation in Thailand affected by fertilization
Abstract. Forest soils, as crucial sinks for atmospheric methane in terrestrial ecosystems, are significantly impacted by changes in ecosystem dynamics due to deforestation and agricultural practices. This study investigated the methane oxidation potential of rubber plantation soils in Thailand, focusing on the effect of fertilization. The methane oxidation activity of the top soils (0–10 cm) in the dry season was found to be extremely low and slightly increased in the wet season, with lower activity for higher fertilization levels. The potential methane oxidation potential of the topsoil was too low to explain the in-situ methane uptake. Soils below 10 cm depth in unfertilized rubber plantations showed higher activity than the surface soils, and methane oxidation was detected at least down to 60 cm depth. In contrast, soils under the high-fertilization treatment exhibited similarly low activity of methane oxidation up to 60 cm depth as surface soils both in dry and wet seasons, indicating that fertilization of para rubber plantation negatively impacts the methane oxidation potential of the soils over the deep profile without recovery in the off-season with no fertilization. Methane uptake per area estimated by integrating the methane oxidation potentials of soil layers was comparable to the field flux data, suggesting that methane oxidation in the soil predominantly occurs in depths below the surface layer. These findings have significant implications for understanding the environmental impacts of tropical forest land uses on methane dynamics and underscore the importance of understanding methane oxidation processes in soils.
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RC1: 'Comment on egusphere-2024-2937', Anonymous Referee #1, 13 Nov 2024
The authors measured seasonal in-situ methane fluxes and estimated methane oxidation potential from a microcosm experiment under four fertilization treatments, and found that fertilization negatively affected methane oxidation potential in both topsoil and subsoils. In general, despite the inconsistent sampling scheme, this study provided new insights, such as subsoil had equivalent or higher methane oxidation potential compared to surface soil and fertilization affected methane oxidation up to 60cm, etc., contributing to the knowledge on overlooked subsoils. In the meantime, more information on methods and a clearer distinction between in-situ methane oxidation rate and estimated oxidation potential are needed especially when the authors discuss their results with the literature. Below are my comments for consideration.
Major concerns
The field fluxes measurements and microcosm experiment lack important information such as soil moisture and physical properties, which are highly relevant to methane processes in the soil. Considering the sandy-loam texture at the SKRS site, low methane oxidation potential in topsoil in the dry season might be due to low moisture and temperature. Did the authors measure and adjust moisture for soils from different sampling layers before incubation?
It is worthy of adding more discussions on the gaps between in-situ methane fluxes and estimated PMORs, and possible reasons why the surface soil layer had lower PMORs than the subsoil layers, as well as how fertilization suppressed methane oxidation. Alternatively, adding an outlook after the conclusion about what needs to be done in future to address these questions.
Specific comments
line 28: change off-season, use consistent terms for the seasons
line 23: delete the first potential
lines 28-30: Although the integrated potential numbers (Figure 5) might match with in-site measured methane fluxes, considering very different methane and oxygen concentrations in deeper soil layers under the field condition and incubation setting, be cautious to conclude that methane oxidation in the soil predominantly occurs in depths below the surface layer based on only one site. I suggest the authors present the integrated numbers as an additional column in Table 2. A clearer distinction between potential and in-situ rate should be made throughout the texts.
lines 64-71: could you formulate them into hypotheses? Line 165 mentioned the hypothesis.
lines 75-78: the duration of wet season and dry season is unclear, please specify the start and end of each season.
lines 81-83: how long have been the fertilizer treatments set up in the rubber plantation at SKRS? What are the fertilizer forms especially N applied in the treatments? If the fertilization treatments have been carried out for a long time, a gradient of soil properties might be already established between treatments.
lines 95-99: it is interesting to see how field sampling progressively changed over time, at the same time, it limited what statistical analysis could test, e.g. seasonal effect, land use effect, interactions, etc.
lines 106-107: sieved fresh soil? Which samples were put into 50-ml GC vials? Considering the long incubation time (30 days) in this study, was it possible oxygen became limited during the incubation? The limitation of using high initial methane concentration in incubation should be communicated to readers, i.e. not favoring high-affinity methanotrophs that oxidize low concentrations of methane (more dominant in aerated soils). This might be one of the reasons for the low estimation of oxidation potential.
lines 152-155: higher total N correspond to higher PMORs? This seems contradictory to the negative fertilization effect on PMORs and in-site methane fluxes (figure 2, lines 142-144). Could the authors add the surface soil (0-10 cm) properties by treatment to Table 2 or in supplement? I do not understand the argument here either, is organic fertilizer applied in this study?
Figure 2: what does 'corrected' mean?
line 157: medium is more suitable than middle
line 161-163: very important observation, it is worth discussing possible reasons for the gap between PMOR and methane flux in situ.
Figure 3: I think keeping one set of legends is sufficient here because of the same sampling depths.
lines 196-199: what are the bases for this statement? The correlation in Figure S2 was total N and the authors did not mention organic fertilizers in the methods description at all.
Citation: https://doi.org/10.5194/egusphere-2024-2937-RC1 -
RC2: 'Comment on egusphere-2024-2937', Anonymous Referee #2, 30 Nov 2024
General comments:
This manuscript presents data on soil potential methane oxidation rates (PMORs) from incubation experiments of rubber plantation soils in Thailand. PMORs are assessed using lab incubations of soil samples with enhanced methane concentrations. PMOR appears to be negatively correlated with fertilizer application rate. In-situ measurements of methane uptake also appear to negatively correlate with fertilizer application rate. Interestingly, PMORs from the top 10 cm of soil are found to be smaller than in-situ soil methane uptake. PMORs measured at several depths down to ~50 cm indicate that PMORs may be higher in the subsurface. These measurements also tend to show PMOR throughout the soil profile with the higher fertilizer application rates. PMORs are aggregated throughout the soil profile to produce a single per-area value.
This manuscript provides some insightful data on the oxidation of methane by soils in tropical rubber plantations. The finding of fertilizer suppressing methane oxidation in the study area is presented more or less convincingly. Additionally, the discussion of PMORs throughout the soil column potentially sheds light on the interplay of biological and physical processes leading to methane uptake by soil. The introduction is well-organized and relevant, providing good context for the study.
However, this manuscript has shortcomings in several areas. The experimental design and methods are lacking thorough description. The in-field treatments, the conditions of the incubations, and the steps for aggregating soil profiles are all rather opaque. The discussion does not adequately address complex and potentially interesting findings, as in the finding that total soil nitrogen is correlated with higher methane oxidation. The dataset could benefit from some editing, such as with the inclusion of some data from a palm plantation. The data visualizations are straightforward, but have room for improvement to concisely and meaningfully present the main findings. In terms of writing, the overall organization is good, but the manuscript would benefit from thorough copy editing to improve the use of English language for clarity and readability.
Specific comments:
- The verb tenses vary between past and present throughout the manuscript. Please standardize here
- Line 24: “potential methane oxidation potential”
- Line 70-71: results should be removed from the discussion
- Line 81: The type of fertilizer and its N-P-K values should be reported, as well as its approximate application rate per hectare.
- Line 103-104: The preparation of MORP samples should be much expanded, using something like the following reference as an example for writing:
- Chan, Alvarus SK, and Timothy B. Parkin. "Methane oxidation and production activity in soils from natural and agricultural ecosystems." Journal of Environmental Quality 30.6 (2001): 1896-1903.
- Line 107-108: The amount of methane (0.5 or 1.0ml into 50ml or 100ml) doesn’t add up with the reported 50ppm in the incubation experiments - it should be 100ppm, unless I’m missing something. Also, the use of such high methane concentration comes with some cause for concern and should at least be discussed, since it is much higher than atmospheric mixing ratios.
- Oxygen limitation is another potential issue with these incubations, but it seems promising that at least some of the incubations fully oxidized the methane.
- Line 113: “Adding up the methane oxidation rates” needs to be described mathematically to show what has been done. More broadly, I’m not sure this technique fully respects the actual field processes, ie the concentrations of methane and oxygen at depth, and the exchange of gas with the atmosphere.
- Line 119: Methane production potential is mentioned in the discussion but not measured here, as in other works. This should also be at least discussed, as it potentially confounds some of the main findings.
- Line 120-124: The sample collection, preservation, preparation, and analysis all need to be better described. The sample state gives important context to the chemical analysis.
- Table 2 can and should be converted to a figure, as it represents the main findings of the manuscript
- Figure 4 is presented inconsistently - why are all treatments lumped together for February 2024, but Tr1 and Tr4 are separate panels in August?
Citation: https://doi.org/10.5194/egusphere-2024-2937-RC2
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