The authors have dealt in an extensive and critical way with the comments posed by the reviewer and the paper this warrant publication. There is a very minor issue to be clarified after which the paper is ready for publication in BG (see below)

The authors should in their next upload clarify the issue of the mean residence time of C in these forest soils as raised their response to the reviewer on this point is not 100% clear.

Reviewer
Line 258: I did expect this as these are relatively “old” soils. The long aging process accumulates old and possibly recalcitrant organic matter along with very stable iron minerals… Also, aging results in more stable iron-organic matter associations as shown by Kaiser et al 2007 (doi:10.2136/sssaj2006.0189).

Author response
We have limited information about aging. But our 14C data indicates the age of soils (AL and TK) ranged from 1993 to 2008 (modern soil) (Zhao, unpublished). We did not observe old soils.

AE comment: In which year were these soils sampled. You imply that based on the (bomb-)14C content in these forest soil (0-20cm) you estimated that most C had entered and were present in the soils when samples was from between 1993 and 2008 (so if soils were sampled in 2011 the MRT would be 3 to 8 years. Indeed very recent. Clearly there is/was also a discrepancy between what the reviewer expected (older soils) and what was found (very recent soil material) what could be the reason for this ?

Dear Authors,
Even without looking in detail at the main manuscript, more work is needed. I would start with the Table and Figures. Normally these are self explanatory or mostly, some of yours lack this. These are my suggestions for improvement.
Table 1 I would not put the 14 sites in alphabetically order, but select a parameter relevant to the explaining the observations in your work, e.g. you could list them in order of low to high precipitation or temperature or the other way.
Fig 1. The placing of the locations and displayed parameters on the USA map is nice, but why do I need to look at them on a map ? Is there more C up North more bound Fe-C, maybe indicate what it shows with a view arrows .... is it related to T or precipitation, see comment Table 1 maybe this is the parameter to list Table 1 under.
Fig 2 what relationship are shown, if the are there they are not obvious. If there are no relationship why put in a figure at all
Fig 3 Do we need all the show subsfigures ? I would prefer a selection of the most pertinent examples which link to the text, with the rest going to supplementary material.
Fig 4 This is very poor Figure, messy and unclear it gives a bad impression of overall standard of the manuscript, it lets your paper down.
Fig 5 see comment Fig 3
Fig 6. A lot of information but no self evident message from both Figures, please condense the information more and better.