Source and Accumulation of Soil Carbon along Catena Toposequences over 12,000 Years in Three Semi-Natural Miscanthus sinensis Grasslands in Japan

Round 1

Reviewer 1 Report

The aim of this work is to examine the contribution of a kind of C4 species Miscanthus to the soil carbon accumulation and also the years of carbon with soil depth. The authors took soil samples from field in three different sites and then measure the 14C years and carbon isotope to distinguish the carbon from C4 or non-C4 species. At the same time, authors try to explain the carbon accumulation difference with along with the slope of research site. The data revealed three research sites showed different carbon stock and also significant difference along the catena position. This is a useful study, but there are several problems need to be clarified.

Firstly, the study involves several questions unclear. (1) catena position of each site, this data displayed the different soil carbon stock, but there is not many data to support the assume reason (Al and Fe). The story will be more credible if these relative data could be supplied. (2) three research sites are close to volcano site, which definitely affect the soil carbon accumulation. But the present data did not support the difference coming from volcanoes or vegetation. The authors should clarify this problem in results and discussion. (3) C4 species contribution to soil carbon stock, the vegetation various in history as authors mentioned. The mechanism of how the organic carbon of C4 herbaceous plants reaches to the soil layer with a depth of 120 cm is not clear, and the decomposition process between C4 and C3 plants in the soil are not clear. So it is arbitrary to conclude that C4 plants contribute most of the C stock in the soil.  

Secondly,the abstract can be improved by more results and data.   

Thirdly,Line 328 the title should be “source and rate of carbon accumulation”.

If the author could clarify these questions and provide more data about to support C stock difference with the catena position, this would be a good story for soil carbon accumulation.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

 Mescanthus, an extremely productive C4 grass, demonstrates what might be considered an upper end estimate of the amount of carbon that grasslands might be able to add to the soil.  Results presented here provide three values for three sites varying in precipitation. I find the work interesting and relevant for the current discussion of the abilities of pseudo-natural areas to function in carbon draw-down scenarios.  I also find the concept of ‘pyrotourism’ to be ….interesting, but I don’t recommend using the term in the western US.

This brings me to what is a more basic and significant concern, we have no error estimates for soil dates? (e.g., Figure 4, 5?)  We have error terms for accumulation but not for dating? I confess I have not processed soil isotope data as was done here.  That said, I’m a bit concerned that model fits provide negative year dates for surface soils in six of the nine fitted curves.  I realize that happens when data are noisy, but we’re not given access to see how variable individual depth samples were in terms of estimated age? While I’m not interested in a statistical test that tells me that deeper soils are older than younger soils, one does wonder about the importance (or absence?) of bioturbation in affecting age distributions? This information is arguably not essential to the key questions addressed, but does establish a necessary baseline of uncertainty regarding patterns.

I would strongly urge the addition of a data document URL or supplemental table showing means, std deviations, n of the data needed for the authors to calculate these results.

Relating these findings to others, even with just a quick table, would be useful for readers. We realize that depth increments are unlikely to be consistent across studies, but even the amount of C deposition of surface soils would be valuable.  This also might help us appreciate the value of the last sentence of the conclusions.  If that can’t be done, then the last sentence of the conclusion needs a rewrite, as it comes across as ‘results are context specific’…. There’s an implication about a soil texture interaction…but we’ve got no comparisons here to judge this statement? Relating their findings to the most similar, current and relevant results would enhance the value of this effort.

Abstract, line 24:  add “in three annually burned grasslands”

Figure 3.  Is graph (b) identified and described in figure legend?

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

It is improved greatly and satisfy for the Journal to published.