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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Status: open (until 05 Dec 2024)
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RC1: 'Comment on egusphere-2024-2937', Anonymous Referee #1, 13 Nov 2024
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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
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