the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 09 Apr 2024
Comprehensive increase in CO2 release by drying-rewetting cycles among Japanese forests and pastureland soils and exploring predictors of increasing magnitude
Abstract. It is still difficult to precisely quantify and predict the effects of drying-rewetting cycles (DWCs) on soil carbon dioxide (CO2) release due to the paucity of studies using constant moisture conditions equivalent to the mean water content during DWC incubation. The present study was performed to evaluate overall trends in the effects of DWCs on CO2 release and to explore environmental and soil predictors for variations in the effect size in 10 Japanese forests and pastureland soils variously affected by volcanic ash during their pedogenesis. Over an 84-day incubation period including three DWCs, CO2 release was 1.3- to 3.7-fold greater than under continuous constant moisture conditions (p < 0.05) with the same mean water content as in the DWC incubations. Analysis of the relations between this increasing magnitude of CO2 release by DWCs (IFCO2) and various environmental and soil properties revealed significant positive correlations between IFCO2 and soil organometal complex contents (p < 0.05), especially pyrophosphate extractable aluminum (Alp) content (r = 0.74). Molar ratios of soil total carbon (C) and pyrophosphate-extractable C (Cp) to Alp contents and soil carbon content-specific CO2 release rate under continuous constant moisture conditions (qCO2_soc) were also correlated with IFCO2 (p < 0.05). The covariations among Alp, total C, and Cp to Alp molar ratios and qCO2_soc suggested Alp as the primary predictor of IFCO2. Whereas soil microbial biomass C and nitrogen (N) levels were significantly lower in DWCs than under continuous constant moisture conditions, there was no significant relation between the microbial biomass decrease and IFCO2. The present study showed a comprehensive increase in soil CO2 release by DWC in Japanese forests and pastureland soils, suggesting that Alp is a predictor of the effect size likely due to vulnerability of organo-Al complexes to DWC.
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RC1: 'Comment on egusphere-2024-419', Anonymous Referee #1, 11 May 2024
The authors investigated the effects of drying-rewetting cycles (DWCs) on soil CO2 release and explored the controlling environmental and soil predictors for variations in the effects based on incubation experiments using 10 Japanese forests and pastureland soils. The topic of this study is interesting and important, and fits well the scope of this journal. The manuscript is well-structured and presented clearly. Nonetheless, I still have two major concerns on the methods and results of this study:
- To give a fair comparison of DWC-induced change in CO2 release rates between different soil samples, the constant soil water content and DWC for each soil samples should be same. However, the soil water contents for different soil samples (Fig. 3) are very different in this study. This might disturb the results of this study. The difference in responses of CO2 release rate to DWC might just because the different constant soil water content, rather than the environmental and soil preditors. In addition, the authors claimed that the soils were incubated aerobically. Yet from Fig. 3, we can find that the soil water contents during the rewetting period could be 1 to 2 times of the WHC. To my understanding, the soil will be in anaerobic environment when the actual soil water content exceeds WHC.
- The explanation on the higher CO2 release rate under DWC than that under constant soil water content might be not that convinced. It is surprising to see that the CO2 release rate is very high during the rewetting period when the soil is in anaerobic environment. The authors argue that the destruction of microbial cells is expected to release soluble organic matter available for microbes that have survived the DWC and to cause a marked increase in CO2 release after rewetting. But will this mechanism continue for a long time? Microbial biomass only accounts for very limited fraction of SOC. Even the destruction of microbial cell can contribute to the release of soluble organic matters, I am doubt if this contribution can result in such a significant increase in the CO2 release rate, in particular under anaerobic environment. In addition, the microbial biomass would declined quickly during the DWC experiment, will the higher CO2 release rate continue for a longer time?
Please add the identification number for each for the sub-panel in figures with more than one sub-panels.
Citation: https://doi.org/10.5194/egusphere-2024-419-RC1 - AC1: 'Reply on RC1', Hirohiko Nagano, 24 May 2024
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RC2: 'Comment on egusphere-2024-419', Anonymous Referee #2, 28 Jun 2024
The current study deals with a relevant and timely topic: How would moisture fluctuation impact organic matter mineralization in volcanic soils vs. under constant moisture regime. Some intriguing result comes out: the contrast in CO2 emission from soil depends on the pyrophosphate or NH4-oxalate extracted Al (and Fe) level. The authors provided a first interpretation of the found results and also tried to link observed higher soil CO2 emission with lower microbial biomass C under fluctuating vs. under constant moisture. While mostly the experiments seem to have been properly carried out, it is impossible to appreciate this with no details provided onto how soil moisture was monitored during the experiment.
Also, the interpretation of the findings does not really go in depth and we may only guess about the nature of the dependence of Birch-effect magnitude on Al and Fe contents in these soils. The main cause of this is probably that some essential soil information is missing, viz. soil texture, soil moisture retention characteristics and any appreciation of the soil organic matter quality; Without these, it may well be that soil Al and Fe contents covaried with soil texture which should have a big impact on the moisture fluctuation under the imposed drying and rewetting. It is well known that it is notoriously hard to just even quantify soil texture in volcanic soils precisely owing to the very strong binding of soil particles by Al and Fe. Content of Fe and Al (hydr)oxides thus likely strongly impacts the soil moisture retention characteristic of such volcanic soils. The resulting effect is that magnitude of drying and rewetting might have been very different between the 10 investigated soils. In my view this paper is now to be resubmitted after soil textural and moisture retention characteristic have been provided and accounted for in the interpretation. Lastly, since mostly forest soils were included, a substantial part of the SOM may be present under the form of POM – it thus seems relevant enough to also carry out a limited soil fractionation and include POM and MAOM proportions as potential predictor variables of the IFCO2.
More specific comments:
The introduction section starts off well, but from L44 till 56 its added value becomes limited: a listing of several studies that have tried to quantify the differences in soil CO2 emissions at constant and variable moisture level is not enough. In this introduction at least some brief overview needs to follow on current explanations for the generally observed higher overall CO2 efflux with variable vs. constant moisture level. Of particular relevance is to see if there were already any previous ideas on the fate of Fe/Al associated OM under variable vs; constant moisture? This then needs to lead towards formulation of a research hypothesis specifically connected to the potential Birch effect size for SOM in the Japanese soils, particularly considering the abundant presence of short-range ordered Fe/Al and its role in stabilizing OM. Without such parts it is not clear what the added value would be of this study.
Surprisingly, no motivation is given as to why the experiment was set up with these 10 particular soils. It is also difficult to compare these soils as some essential information to interpret the results is missing: soil particle size distribution, soil water retention characteristics and basic information on the soil organic matter quality. Especially in the forested sites it could be that much of the SOM occurs as particulate organic matter and that could form a contrast to the grassland sites. But without some basic soil fractionation data we cannot appreciate this.
It is striking that soil moisture content was apparently not monitored during the soil incubation experiment – unless it was (?) - But at least that was not described anywhere in the M&M. Given that only very small cores were used (containing but 8-10g) inside 1L jars it seems probable that in the constant soil moisture treatments actually soil dried out. But no data is provided to check this. The presented course of soil water during the DWC treatments with linear drying of soil and constant levels in between drying events seems unrealistic and should have been replaced by actually recorded moisture. Because of this lack of moisture data we cannot be certain that the observed correlations between the IFCO2 and contents of Alo and Feo and Alp were not largely or in part indirect. Is it not conceivable that in soil having more OM and more pedogenic oxides the fluctuation of moisture differs to soils with less? Variation in these properties likely also causes a contrast in the soil moisture retention curve of these soils and that in turn will directly impact the magnitude of the imposed drying to actual soil moisture fluctuation.
Another major shortcoming is the non-continuous follow-up of soil CO2 emission: 1° CO2 emission from the constant moisture treatments was apparently only measured in the first 29 days; 2° Moreover no CO2 emissions were measured during the drying stages. Without these data, can we really compare emissions at constant and fluctuating moisture properly?
Details:
L44 “…in comparison with the medium level of constant moisture content” is not clear, what is meant by medium level here?
L46 “Another 29 data were calculated…” sounds awkward and furthermore with this sentence you are not bringing any message: what was now the outcome of this comparison?
It is not clear really what is intended by “soil carbon content-specific CO2 release rate under continuous constant moisture conditions (qCO2_soc)” – requires further clarification
Fig 2 would be useful to indicate the point in time what interval this ‘after rewetting in first cycle’ now precisely ended
L94 to measure field capacity, likely also soil was allowed to leach out after its saturation? But that is not well described here.
L194 “Especially, the importance of Alp for variations in IFCO2… “ this link between Alp content comes in too early and is best omitted from this start of the discussion section.
L196 Better not directly make a leap towards podzols, safer to just restrict the interpretation to volcanic soils.
L 234 the link to CH4 and N2O emission is best not made.
L238 A strange starting sentence ‘insight in the precise quantification’ needs to be revised
Citation: https://doi.org/10.5194/egusphere-2024-419-RC2 -
AC2: 'Reply on RC2 (particularly about the measurement of soil water content during the incubation)', Hirohiko Nagano, 02 Jul 2024
Thank you very much for your kind evaluations of our manuscript.
First, I want to clarify our measurement of soil water content during the incubation period. Yes, we have periodically measured soil water content during the incubation, even in the drying stage under the DWC treatment for Day 1 to Day 8 and Day 18 to Day 25. For each drying stage, we conducted measurements of soil water content once to twice. The measurements were performed by weighing those soils. Based on these data, we confirmed that the mean soil water content during DWC incubation was equal to that during constant moisture incubation. Figure 3 (bottom panels) shows these data on soil water content. We have written some explanations for measuring soil water content at L129-L131, but this was somewhat unclear. In the revision, we will provide more explicit explanations for measuring soil water content during the incubation, as described above.
Additionally, under the constant water treatment, we surrounded the small vial with 20 mL of water within the incubation jar to prevent the soil from drying. Sorry for lacking this explanation. We will also add this explanation to the Materials and Methods.
We expect our explanations on measuring soil water content during the incubation will solve some of your primary concerns in our study. Then, we will respond to your other comments later.
Citation: https://doi.org/10.5194/egusphere-2024-419-AC2 -
AC3: 'Reply on RC2 (particularly about insufficient soil information and evaluation for soil texture, soil moisture retention characteristics, and soil organic matter quality)', Hirohiko Nagano, 12 Jul 2024
Thank you for your suggestions. We additionally measured soil textures as particle size distributions, i.e., relative compositions of clay, silt, and sand-sized particles. Then, we found that the amounts of clay and sand-sized particles showed significant correlations with IFCO2 after rewetting in 1st cycle (p < 0.05, r = -0.66 for clay and 0.71 for sand particles). However, those correlations between the particle contents and IFCO2 were insignificant for IFCO2 for a total of three cycles. Nevertheless, pyrophosphate-extractable Al (Alp) content showed significant correlations with IFCO2 for all incubation stages (r = 0.84 to 0.74 with p < 0.05), as described in the manuscript (Figure 5 and L165-L171). This result supports our argument that pyrophosphate-extractable Al (Alp) content is likely the primarily important factor for the magnitude of CO2 release increase by DWC. We will add these results and descriptions to the revised manuscript.
Regarding water retention, we consider the water-holding capacity (WHC) of soils. As presented in Table 2, we have measured WHC in addition to soil water content as soil properties used for the experiment. Whereas there was a significantly positive correlation between WHC and soil water content (r = 0.87, p<0.01; thus, soil water content at the soil sampling also reflected the ability of soil to hold the water and the usual water contents in the field), none of the soil water content at the soil sampling and the WHC showed a significant relation with IFCO2 not only as a linear correlation but also as a nonlinear relation, which was examined visually (Table 4; please also see the figure in our reply to RC1). These facts from the obtained data support us in stating that the variations in IFCO2 were significantly associated with soil metal-humus complexes and soil microbiology rather than different WHC among soils (Tables 4 and 5, Fig 5). We will refine our sentences for the results of exploring predictors of IFCO2 in the manuscript (L165-L171), adding the positive correlation between the soil water content and WHC to the material and method section of the soil sampling (L80 to L88).
For the soil organic matter quality, we can consider the C/N ratio of K2SO4 extractable organic matter in addition to the C/N ratio of total organic matter. Their correlations with IFCO2 were statistically insignificant, as presented in Tables 4 and 5. We will add this description to L165-L176. Whereas we currently have no data on POM, we are now conducting the measurement of POM, and the results will be presented in the revised manuscript. Moreover, given the similar IFCO2 values between the forest and pasture soils of Kuju (Figure 4, less than 12% differences between them), possibly substantial differences in POM content between the forest and pasture grassland might be negligible in the currently investigated soils. We will add this discussion to the revised manuscript with the result of the POM contents of investigated soils.
We sincerely hope those explanations can also satisfy your concerns about insufficient soil information and evaluation for soil texture, soil moisture retention characteristics, and soil organic matter quality.
Citation: https://doi.org/10.5194/egusphere-2024-419-AC3 -
AC4: 'Reply on RC2 (particularly about the more specific points and the details)', Hirohiko Nagano, 13 Jul 2024
Much more information about mechanisms for CO2 release increases under DWC:
Because our first primary purpose is to clarify the overall trend of DWC effect on soil CO2 release under the comparison between DWC and constant moisture conditions, which have the same mean water content during the incubation, we less mentioned the proposed mechanism for CO2 release increases under DWC in the introduction section. Roughly three mechanisms are proposed: (i) increase in available carbon source via the releases of cellular metabolites from microbial cells destroyed by rewetting after the strong drought, (ii) increase in available carbon source by the releases of carbon from macroaggregates destroyed by repeated DWC, and (iii) changes in the microbial communities in response to transient moisture conditions. Whereas the DWC-induced destructions of macroaggregates might be related to changes in association between mineral/metal and organic matters, we cannot find any literature which specifically mentioned the organo-Al complexes, which is the primary predictor for IFCO2 in the present our study. We will add the brief description of currently proposed mechanisms for CO2 release increase under DWC to the introduction section, especially after the paragraph to describe the current knowledge about the trend of DWC effects on soil CO2 release in L39-L56.
Motivation for using 10 Japanese soils:
This was that these soils were variously affected by volcanic ash during their pedogenesis, and therefore include several Andisols, which are known to have a high SOM storage capacity, likely due to the protection of SOM from microbial decomposition by enrichment of reactive minerals and metals in these soils. Our previous study showed the significant increase in CO2 release by DWC for two Japanese forest soils. Especially, a volcanic ash soil showed a substantially large increase. This was why we used those 10 Japanese soils differently affected by volcanic ash. Whereas we have presented those information in L52-L67, this might be not clear. We will thoroughly refine those sentences in the revised manuscript, to present clear description of our motivation using 10 Japanese soils.
Monitoring soil moisture content:
Please see our reply entitled as 'Reply on RC2 (particularly about the measurement of soil water content during the incubation)'.
CO2 measurement during the incubation:
Sorry for confusing you. Our CO2 measurement for constant moisture condition were conducted periodically during the 84-day incubation. Namely, in the incubations with the constant moisture condition, the CO2 release rates were measured for Day 1 to Day 12, Day 13 to Day 28, Day 29 to Day 40, Day 41 to Day 56, Day 57 to Day 68, and Day 69 to Day 84, as shown in Figure 3. We will revise sentences in L128-L129 to describe the measurement of CO2 release for the constant moisture conditions. Because we observed significant absorption of CO2 by silica gels in preliminary experiments, we did not conduct the measurement of CO2 release during the drying stages. Other previous studies also assumed the linear changes in CO2 release rate during the drying. Furthermore, significant relationships between IFCO2 and organo-Al complex was also observed in IFCO2 after the rewetting in addition to the IFCO2 for total 1st cycle and three cycles. Therefore, shortcomings from unmeasured CO2 release during the drying periods should be minor as the uncertainty in our main findings in the present study. We will add those description for Discussion section.
Details:
L44 “…in comparison with the medium level of constant moisture content” is not clear, what is meant by medium level here?:
This means that the comparison was made with the medium level of constant moisture content which was equivalent to the mean water content during DWC incubation. We carefully revise this sentence for clear understanding.
L46 “Another 29 data were calculated…” sounds awkward and furthermore with this sentence you are not bringing any message: what was now the outcome of this comparison?:
This sentence emphasizes the lacking of actually measured data of CO2 release rate under DWCs which have the same mean water content during the incubation with that for constant moisture conditions. We carefully revise this sentence for clear understanding.
It is not clear really what is intended by “soil carbon content-specific CO2 release rate under continuous constant moisture conditions (qCO2_soc)” – requires further clarification:
The qCO2_soc should be an index for availability of carbon substrate to microbes under normal moisture conditions. High qCO2_soc suggests that substantial parts of carbon in soils are available for microbes, and lower values suggests low availability of carbon substrate. We will add this explanation.
Fig 2 would be useful to indicate the point in time what interval this ‘after rewetting in first cycle’ now precisely ended:
Thanks. We will add an arrow suggesting the end point of the rewetting stage to Fig. 2.
L94 to measure field capacity, likely also soil was allowed to leach out after its saturation? But that is not well described here.:
WHC was measured by the Hilgard method using capillary action of air-dried soils to soak the water (e.g., Ahn et al., 2008 Water Management). After the weighing saturated soils, we oven-dried soils and weighed those soils again, to obtain the amount of water soaked in soils. We will add description about measuring the WHC by the Hilgard method.
L194 “Especially, the importance of Alp for variations in IFCO2… “ this link between Alp content comes in too early and is best omitted from this start of the discussion section.:
Thanks. We will omit this from this start of the discussion section, then move to the end of third paragraph (L216-L223) in the Discussion section.
L196 Better not directly make a leap towards podzols, safer to just restrict the interpretation to volcanic soils.:
Thanks. We will remove the sentence related to podzols and restrict the interpretation to volcanic soils.
L234 the link to CH4 and N2O emission is best not made.:
Thanks. We will remove the sentence for the link to CH4 and N2O emission .
L238 A strange starting sentence ‘insight in the precise quantification’ needs to be revised:
Thanks. Our meaning is that “The present study provides significant insights into precisely quantifying the effects of DWCs on soil CO2 release”. We will revise the sentence.
Citation: https://doi.org/10.5194/egusphere-2024-419-AC4
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AC2: 'Reply on RC2 (particularly about the measurement of soil water content during the incubation)', Hirohiko Nagano, 02 Jul 2024
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