Comparing soil carbon loss through respiration and leaching under extreme precipitation events in arid and semiarid grasslands

Liu, Ting; Wang, Liang; Feng, Xiaojuan; Zhang, Jinbo; Ma, Tian; Wang, Xin; Liu, Zongguang

doi:https://doi.org/10.5194/bg-15-1627-2018

Articles | Volume 15, issue 5

https://doi.org/10.5194/bg-15-1627-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/bg-15-1627-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 15, issue 5

Research article

|

16 Mar 2018

Research article |

| 16 Mar 2018

Comparing soil carbon loss through respiration and leaching under extreme precipitation events in arid and semiarid grasslands

Ting Liu, Liang Wang, Xiaojuan Feng, Jinbo Zhang, Tian Ma, Xin Wang, and Zongguang Liu

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Final revised paper (published on 16 Mar 2018)
Supplement to the final revised paper
Preprint (discussion started on 18 Jul 2017)
Supplement to the preprint

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Review of bg-2017-264', Anonymous Referee #1, 24 Aug 2017
- AC1: 'Response to Referee #1's comments on "Comparing soil carbon loss through respiration and leaching under extreme precipitation events in arid and semi-arid grasslands"', Xiaojuan Feng, 18 Sep 2017
RC2: 'Review of bg2017-264', Anonymous Referee #2, 12 Oct 2017
- AC2: 'Response to Referee #2's comments on "Comparing soil carbon loss through respiration and leaching under extreme precipitation events in arid and semi-arid grasslands"', Xiaojuan Feng, 24 Oct 2017

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (29 Oct 2017) by Kees Jan van Groenigen

AR by Xiaojuan Feng on behalf of the Authors (02 Nov 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (22 Nov 2017) by Kees Jan van Groenigen

RR by Anonymous Referee #1 (07 Dec 2017)

RR by Anonymous Referee #2 (09 Jan 2018)

Suggestions for revision or reasons for rejection

General Comments:
The manuscript was significantly improved by the authors, they responded to all comments properly.
There are two points which should be more discussed, first: the transferability of the results to field conditions, especially the low water content in arid and semi arid climates and what does it mean for soil carbon leaching. Second the comparison of soil respiration from bare vs. litter-covered soil.
Additionally there are some points which are confusing and for a final publication these parts should be rewritten.

Specific comments:
page 7, line 29:
The authors present the soil respiration normalized for different SOC contents, with 2.2, 2.6 and 7.6 [µg C g⁻¹ SOC h⁻¹], it is not clear which values are presented, maximum or average values – please specify.
Furthermore, I the specific respiration rate of the XLHT site is to high with 7.6 [µg C g⁻¹ SOC h⁻¹], given the maximum respiration rate of 13.7 [mg C m⁻² h⁻¹] (same page line 27) and SOC stocks of 7.4 [kg C m⁻²] (based on the numbers given in table 1). There must be a mistake in the calculation or in the presented numbers. In consequence, SOC of the XLHT site is not easier to degrade.

page 8, line 3 ff.:
page 8, line 3 ff. “Total respired CO 2 was higher in the litter-amended than non-amended soils before and after EPEs (Fig. S3), likely caused by one or two following reasons: (1) the degradation of labile components in the fresh litter; (2) increased degradation of SOC primed by litter additions (Fröberg et al., 2005; Ahmad et al., 2013). These two reasons might affect CO 2 release in litter-amended soils in two ways: (1) both of them had positive effects; (2) one of them had positive effects and the other one had negative ones, but the total net effects was positive.”

The second sentence is confusing and makes no sense, I’d suggest to delete it and rewrite the first a little bit.
Suggestion: “… (1) the degradation of labile compounds in the fresh litter; (2) induced priming effects due to the addition of an easily available energy source.“

page 8, line 3-18:
In that paragraph I really miss the comparison between soil carbon loss from bare soil versus litter covered and the resulting discussion. What are the field conditions for the studied area, is the soil normally covered by litter or not? What does is mean for SOC losses due to respiration and leaching? - As mentioned in the introduction the authors want to compare soil carbon loss from bare vs. litter covered soil.

page 8, line 25 ff.:
“The specific EPE-induced CO2 was significantly different for the litter-amended soils among sites (p < 0.05), showing a pattern of KQ > XLHT > GC.”

In my opinion you can not compare the specific EPE-induced CO2 release of the litter amended soils, because you have no information about the source of CO2 especially on the XLHT and KQ site. Due to the coarser soil texture of these two sites, it might be also possible that labile C is dissolved from the litter, transported down column and led to positive priming effects in the deeper parts of the column. However, since you have no information about the contribution of SOC mineralization to total CO2 this statement is highly speculative. You can only compare the non-amended sites. It would be more interesting if you could show the reduction of SOC mineralization of the GC site, after EPE. Here you have the isotopic data.

page 8, line 32 f.:
“These results suggest that SOC contents and SOC:N ratios are important factors influencing the total EPE-induced CO2 release”

I disagree, figure 3b shows that there is no significant difference in specific respiration of the non-amended sites. Therefore, differences between site might be less important.

page 9, line 2
For me it it not clear what the authors mean with “ … a total of 0.57−0.71, 0.56−0.94, and 0.73−0.89 L of leachates were collected for the XLHT, KQ, and GC soils, respectively.”
Does it mean e.g.
• that after 3 EPE in total 0.57 L of leachates were collected for the XLHT site?
• or does it mean a range, that after the first EPE 0.57 L were collected for the XLHT site and after the third EPE 0.71 L
Is there any explanation why the XLHT soil retain more water than the GC soil, due to the lower sand content in GC I would expect the highest retention on this site.

page 9, line 16 f.:
”However, for the KQ soil having a relatively low SIC content which was similar to the added litter-OC (0.7 g per column; Table 1), the influence of litter addition on the DIC flux was quite obvious. Therefore, although the contribution of dissolved CO 2 to DIC fluxes should be more important in high-pH soils, the relative effect of litter amendment on DIC fluxes under EPEs seemed more significant in soils with a low SIC content.”

I do not see the “quite obvious influence” of litter addition to DIC flux for the KQ site in figure 4b. Therefore I would recommend to give a number on the relative effect of litter amendment on DIC fluxes. Furthermore, comparing the GC with KQ site, the absolute / relative effect of litter addition on DIC fluxes was higher / equal on the GC site. Which contradicts the statement “the relative effect of litter amendment on DIC fluxes under EPEs seemed more significant in soils with a low SIC content.”

page 10, line 1:
“… despite their higher SOC contents and CO2 release rates (fig S5).”
Figure S5 does not show a CO2 release rate, See comment to Figure S5.

page 10, line 12 ff.:
“In addition, DIC loss exclusively resulting from SIC dissolution or weathering was also a significant fraction of soil carbon loss, equivalent to 219% SOC loss in the form of CO 2 during EPEs (Fig. 5). These results collectively corroborate that inorganic carbon loss is the main form of soil carbon loss in alkaline soils during EPEs.”

In my opinion this paragraph is a little bit confusing and needs more information especially the meaning of CO2 loss. I guess the authors mean here EPE induced CO2 release, however in the current form I would understand “SOC loss in form of CO2” as total respiration. I’d suggest to stay with the term “EPE induced CO2 loss”.
But if the authors mean total CO2 release during EPE the presented numbers are wrong.
E.g. In figure 5 the CO2 loss during the first EPE was ca. 1 g C m⁻², however given the numbers from figure S3 for the first EPE for the XLHT site, basal respiration with ca. 7.8 mg C m⁻² h⁻¹. Calculating a CO2 loss for the first 20 days, would lead to 3.7 g C m⁻² for the site.

Furthermore, it is not clear which treatment is shown in figure 5 – non-amended or amended?

page 12, line 7 ff.:
Here I miss the discussion about the adjusted WHC of the soil columns, which might be important to estimate the losses of soil carbon in these arid and semi arid climates. In fact, under field conditions I’d expect a much lower DIC loss because under field conditions the water content in soils might be lower < 60 % of WHC and soils can retain more water.
On page 9 line 2, the author mentioned that the sum of the collected leachates during 3 EPE was between 0.57 - 0.94 L, after the addition of in total 3 L precipitation . Therefore, this indicates a strong retention of rain water in the investigated soils, which of course depends on soil texture and water content before an EPE. It would be interesting to know how much water was added to adjust the WHC of the different soils. This would allow to give a more realistic estimation about soil carbon leaching losses under field conditions. Further, the time between two EPE might be also an important factor for DIC losses, because the longer the time between two EPEs the lower the soil water content. In consequence DIC losses might be smaller. Therefore, leaching of soil carbon might be smaller under field condition. In addition, after an EPE under field conditions the moisture content is altered and may provide better conditions for SOC mineralization which would increase CO2 losses.

Figure S4: Which bulk density was used for the GC site? There is no bulk density given in table 1.
How did you calculated the numbers in figure S4.

Figure S5: They y axis label and figure caption is wrong. The figure does not show a rate rather it show a sum of total CO2 produced during the three EPE.

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ED: Publish subject to minor revisions (review by editor) (22 Jan 2018) by Kees Jan van Groenigen

AR by Xiaojuan Feng on behalf of the Authors (02 Feb 2018) Author's response Manuscript

ED: Publish subject to minor revisions (review by editor) (18 Feb 2018) by Kees Jan van Groenigen

ED: Publish subject to technical corrections (22 Feb 2018) by Kees Jan van Groenigen

AR by Xiaojuan Feng on behalf of the Authors (22 Feb 2018) Author's response Manuscript

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Short summary

Compared to the respiration process, few studies have examined soil carbon leaching possibly enhanced by extreme precipitation events (EPEs). We show that soil carbon leaching was much higher than CO₂ loss through respiration under EPEs in grassland soils through incubation experiments. The soil carbon leaching process should be incorporated into soil carbon models when estimating carbon balance in grassland ecosystems, especially considering the projected increase in EPEs with climate change.