the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Soil organic carbon mineralization is controlled by the application dose of exogenous organic matter
Abstract. Substantial input of exogenous organic matter (EOM) may be required to offset the projected decline in soil organic carbon (SOC) stocks in croplands caused by global warming. However, information on the effectivity of EOM application dose in preserving SOC stocks is surprisingly limited. Therefore, we set up a 90-day incubation experiment with large soil volumes (sandy loam and silt loam) to compare the mineralization of EOM (13C-labelled ryegrass) and SOC as a function of three EOM application doses (0.5, 1.5, and 5 g dry matter kg-1 soil). In the sandy loam soil, the percentage of mineralized EOM was not affected by EOM dose, while SOC mineralization increased proportionally with increasing EOM dose (+49.6 mg C per g EOM). In the silt loam soil, the percentage of mineralized EOM decreased somewhat with increasing dose, while SOC mineralization increased at a higher rate than in the sandy loam soil (+117.2 mg C per g EOM). In both textured soils, increasing EOM dose possibly supplied energy for microbial growth and enzyme production, which in turn stimulated mineralization of native SOC (i.e. co-metabolism). Higher soil macroporosity at higher EOM doses in the silt loam soil could have contributed to sustaining aerobic conditions (indicated by soil Eh) and promoting SOC priming as shown by positive relationships between pore neck size classes 43–60, 60–100 and >300 μm and SOC priming, suggesting a new mechanism for understanding SOC priming. In sum, this experiment and our previous research suggest that EOM mineralization is mostly independent of EOM dose, but EOM dose modulates mineralization of native SOC. These findings tentatively indicate that using larger EOM doses could help preserve more of added EOM-C in silt loam soils, but longer-term confirmation in the field will firstly be required before we could draw any conclusion for soil C management.
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RC1: 'Comment on egusphere-2024-107', Anonymous Referee #1, 13 Feb 2024
The authors present the results of a laboratory study, in which the mineralization patterns of plant litter is investigated in two soils of different texture. The aim of the study is to investigate how different doses of plant litter may influence the mineralization rate and priming of the initial SOC.
The study is nicely written, easy to read and follow. The experiment itself is interesting and fits into the scope of the journal. However, and I am really sorry to say this, I see several major critic points that make the manuscript unsuitable for publication in my opinion:
- The authors investigate the mineralization of added OC and initial SOC and distinguish between them with a 13C label. This is valid to do so. However, I could not find any data on the native C concentration in the soil and the changes therein. It seems to me, that only respired C has been measured. By this, a calculation of the overall net C balance in the soils is impossible (and has not been done by the authors). This means that it is impossible to detect any C losses other than as CO2 (e.g. by methane, leaching, etc.). The priming effect seems to have quite an influence, but the supplementary materials show, that it is quite limited in time. The authors should therefore provide a clear balance (table, etc.) showing how much overall C can be added to the SOC by the various C inputs and whether the overall balance would be positive or negative. It is really necessary to show, how much of the overall external carbon ended up in the soil and how much the priming effect influenced the overall carbon storage in the soil. The fate of added OC and whether it is causing priming or SOC formation is currently a hot topic at the moment, although there is a big debate about its impact on the net C balance. Therefore, without this data I cannot see how the presented study adds any novel aspect to the current scientific knowledge.
- There is a quite similar study published by the same authors in the journal Biol. Fertil. Soils (2022). According to my understanding, this ealier study uses the same plant species and the same soils. The only difference I could find is the author’s explanation that the previous study was “sampled at different times in the field?” (L.426) and there were two levels of disturbance. It seems to me that this previous study covers the same aspects than the presented study, but is much more comprehensive and better explained. I am sorry to say, but I cannot really see the novel aspect of the presented manuscript. It seems to be rather a subchapter or preliminary study to the already published study.
- There is no information about the used soils and vegetation. The soil type, current vegetation or crops, and soil management, farming history (or at least the present use) should be provided. (The authors want to publish in a journal called “SOIL”.) Without this information, the discussion section cannot be evaluated, especially not the part about initial SOC quality and soil structure. It is not enough to give only soil texture, pH and some total elemental concentration. In addition, there is no information on the C input by the ryegrass. Is this input realistic for those soils under the respective management? Has other literature suggested that the concentration will induce priming in similar soils? Also, how big where the ryegrass pieces? Could they be incorporated in aggregates?
- The introduction and motivation of the study is based on very, very old SOC models from 1945. This is clearly not up to date. What about e.g., the AMG model (1999) or the roth-C model? I admit that they are also quite old, but they are at least still used and updated. The same goes for the literature on aggregation processes. Although these are fundamental aspects of soil science and it is therefore fine to cite the fundamental (and old) papers, they are also topics of current science and the knowledge has developed tremendously in recent years. In addition, the authors claim on several occasions that certain aspects have not yet been studied, while neglecting the existing literature on this. E.g., (a) (L.70) The authors claim that no general comparison of the size of organic residues and their decomposition has been done. This is simply not true, as there are several experiments as well as recent modelling studies on this. (b) (L.425) Mapping O2 availability in pores is indeed difficult. However, there are several studies trying to do this by combined approaches of µCT and modelling or micro-sensors. The authors should try to include this literature instead of suggesting that this has not been done. By not including it, one might fear that the authors are not aware about recent developments in research on soil structure, aggregation and C cycling.
Some smaller and more specific comments:- Soil redox potential: I don’t really understand why you expect anoxic conditions in an aerated soil adjusted to a water content of 55% water filled pore space? I understand that this might be the case in fine pores, but you can only rely on bulk soil measurements. Can you explain this in more detail?
- L.71: I don’t see why the general processes of decomposition should be different in the lab
- L.110: It is not correct to say that the “initial soil structure is intact” when the soil has been sieved to <10 mm. By this, the original soil structure is completely disrupted. The aggregates might be preserved, but they are only one part of the soil structure.
- The authors describe the study as a macrocosm study. This is not correct in my opinion. I would rather say that it is a microcosm or maybe even a mesocosm study, but never a macrocosm study because it is entirely in the laboratory.
- Pore size distribution: When were the pore sizes measured? At the beginning or end of the experiment?
- While I do not question the results of course, I am nevertheless surprised about the low total pore space and amount of coarse pores in the sandy loam. Usually a sandier texture is connected with more coarse pores. Could you please elaborate a little, why the situation could be like this in your soils?
- L.340: You discuss NO3- data. Where can this data be found?
- L.390: It is no news that SOC quality and texture influence SOM mineralization and priming. There are studies about that (which especially disentangle these processes)
- L.388: If you relate the effects on SOC quality, it is even more striking that you don’t even mention the crops of the fields and did not test any SOC quality parameters
- L.395: I would delete the whole paragraph. The authors mention themselves that they do not have data about repeated doses and the priming effect was very short-lived. Therefore, I cannot understand how you can recommend a certain soil carbon management under field conditions with repeated doses vs. one time application.
- L.412: You cannot discuss changes in the pore network with a one-time measurement. It is obvious that litter causes a coarser pore space. There is literature about that. You don’t even mention if the pore size measurement has been at the beginning or end of the experiment
- L.435: Although speculative, it is a genuinely very interesting result that the coarse pores and better aeration probably counterbalanced the low redox potential due to O2 consumption. If the paper is resubmitted, it would be nice to elaborate this discussion a bit more.
Citation: https://doi.org/10.5194/egusphere-2024-107-RC1 -
RC2: 'Comment on egusphere-2024-107', Julia Schroeder, 28 Feb 2024
The authors of the manuscript SOIL_2024-107 investigated how the amount of exogenous organic matter (EOM) added to two different textured soils (sandy loam, silt loam) affects its decomposition and priming of SOC using 13C-labelled ryegrass. The authors expected that EOM mineralisation at high dose would be limited in silt loam due to lower O2 availability in microbial hotspots (strong consumption), whereas this limitation would not occur in sandy loam. This would result in an asymptotic increase in EOM mineralisation with dose in silt loam, and a linear increase in sandy loam. However, effects of EOM dose on soil structure may counteract this limitation. The focus of this study lies on potential feedback mechanisms of EOM addition on decomposition by effects on soil structure (pore volume) and thus O2 availability.
The authors find that EOM mineralisation increases with EOM dose, where EOM mineralisation rate is decreasing at high dose in silt loam, but not in sandy loam, which would indicate a limitation of EOM in fine-textured soils as hypothesised. However, they misinterpret their results in the latter case (see comment major concerns 1). SOC priming was found to be higher at higher EOM dose, with larger stimulation in silt loam (see comment major concern 2). They find that EOM addition increases the pore volume in silt loam but not in sandy loam, while Eh (indicator for redox-potential and thus O2 availability) was not altered by EOM dose in none of the soils. They conclude that the effect of EOM dose on macroporosity in silt loam had stimulated SOC priming at high dose. MBC was increased in both soils with EOM dose, but the authors state that the per unit increase in MBC was higher at lower EOM dose in both soils. Again, this is in my opinion a very brave interpretation of the non-significant results (see comment major concern 3). Taken together, they conclude that the same amount of EOM applied in several smaller doses of EOM would result in less C losses due to mineralisation as compared to the same dose being applied once.
The investigated research question is of broad interest and adds to the current discussions. Understanding how application dose will affect the fate of C entering the soil, has also implications for agricultural recommendations.
As reviewer #1 indicated, the authors published similar work in Biology and Fertility of Soils. The submitted manuscript, however, presents a new laboratory experiment with a new focus and research question, emerging from the results of the previous study. Therefore, it is novel. However, I do agree that the authors should better introduce the novelty of this study in comparison to Mendoza et al. 2022b (e.g. 3 doses instead of 2, microbial biomass determined, larger ryegrass fragments, only <10mm sieved).
Overall, the writing style is clear and easy to follow. But introduction could be further improved to clearly point out the novelty of this study and logically introduce the hypotheses. Adding a figure showing expected relationships between EOM mineralisation and dose for silt loam (asymptotic) and sandy loam (linear) will help to clarify the still somewhat confusing hypotheses. I highly appreciated the detailed M&M section, which allowed to follow the experimental design and indicated in most cases the reasoning behind decisions. However, reasoning for why different statistical approaches were used is lacking and should be added during revision. I have several major concerns regarding the interpretation and thus discussion of the results.
Major concerns:
- Large parts of the discussion are based on the interpretation of Figure 1, that the proportion of EOM mineralised decreases exponentially with increasing EOM dose. However, this conclusion cannot be made based on the study results. The authors find that EOM mineralisation increases linearly with EOM dose (Fig 1 top). The proportion of EOM C mineralised over EOM dose (shown in Fig 1 bottom) is basically the slope of EOM mineralisation over EOM dose (shown in Fig 1 top). If EOM mineralisation is linear, the slope of this function cannot decrease. Furthermore, while statistical metrics indicate high significance of the regression in Fig 1 top (i.e. the linear relationship between EOM mineralisation and EOM dose seems very solid), the relationship in Fig 1 bottom between the proportion of EOM mineralised and EOM dose is only marginally significant and should be therefore treated even more carefully. Therefore, the results only allow to conclude that EOM mineralisation increases proportionally with EOM dose in both soils, with different proportions of EOM being mineralised between sandy loam and silt loam. Reasoning should be given, why you decided to include only 3 different application doses instead of 4 to 5 equally distributed EOM doses. Including only 3 doses, where two are relatively close and one is far above the two other doses on the x-axis, makes it IMO impossible to find differences between linear and asymptotic fit and thus test your hypothesis of limitation of EOM mineralisation at high dose in silt loam.
- SOC priming is displayed as absolute increase in SOC mineralisation as compared to the non-amended control. Instead of showing this absolute priming in Fig 2 bottom, which is redundant with Fig 2 top (normalising for y-intercept; this is why statistics are almost the same), you should display the proportion of SOC priming as relative to the total cumulative CO2-C. Only this would allow to compare how EOM dose affects priming between both soils. The comparison between absolute priming is misleading, since absolute mineralisation differs between both soils, with higher rates in silt loam. In the end, SOC priming might just be proportional to EOM mineralisation. Therefore, the discussion on why SOC priming is stimulated to a larger extent in silt loam as in sandy loam needs revision.
- While results are mostly displayed as cumulative results after 90-days incubation, microbial biomass was only measured once after 45 days. The reasoning for this decision is lacking in the M&M. Also, this needs consideration in combined interpretation of EOM dose effects on MBC (45-days) and EOM mineralisation and priming (90-days). Also the information is lacking in the Fig 3 caption. However, my major concern regarding MBC is the interpretation of Fig 3 bottom. First, the unit is missing. It is unclear from looking at the figure and reading its caption how EOM-mediated MBC increase was calculated. It is not based on 13C-incorporation. I assume it is calculated similar to SOC priming, i.e. normalising for the y-intercept. On the other hand, this would not explain the low values at 5 g dose. Did you divide the delta MBC (to non-amended control) by EOM dose? Second, the relationship is non-significant. Therefore, the conclusion that the increase in MBC per unit EOM added decreases with EOM dose is not valid.
General comments:
Statistics
- Indicate reasoning for why different tests were used. Did you use an ANOVA to test for significant effects of dose x lime in your GLM? In lines 212-214 you state that you tested for normal distribution of residuals. However, with GLM residuals do not need to be normally distributed. Revise this section to clarify.
Figures
- Display uncertainties, i.e. confidence intervals. Remove redundant figures (see comments major concerns)
Figure 5/pore neck sizes
- In general, you missed to clearly indicate in the introduction in which pore neck sizes you expected changes to occur, and what would be the implication of such changes. Figure 5 is hard to interpret. It is unclear that EOM (g kg-1) is y-axis label. Different colours are hard to see and I would prefer to have one simple figure on total pore volume changes over having several pore neck sizes. Consider changing axes and aggregating all fractions to display total pore volume differences with error bars. If you are more interested in showing the distribution of pores normalise the total pore volume to better visualise the shift in proportions.
Figure 7
- Needs revision due to major concerns regarding the interpretation of your results.
Conclusion:
The study fits the scope of the journal and presents novel results, adding a piece to the puzzle in understanding C dynamics in soil. We learn that EOM mineralisation does increase linearly with its application and that SOC priming seems to linearly correlate with EOM mineralisation. Regarding the major concerns I have with the interpretation of the results, I recommend that the manuscript can only be accepted after a major revision. Given that I expect the manuscript to undergo large changes during its major revision, I will not add line to line comments to this version.
Citation: https://doi.org/10.5194/egusphere-2024-107-RC2
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