This chapter reports on progress in quantifying North American carbon stocks to depths that corresponding to carbon storage, rather than earlier estimates of surface carbon or carbon at arbitrary depths. New models include improved representation of soil processes, but models continue to differ on the sign and magnitude of soil carbon changes. Recent simulations contain results for high latitude soils that contradict experimental results, suggesting carbon gain while empirical studies suggest loss. Considerable uncertainty remains about the impact of lateral transport, erosion, and riverine movement on eventual carbon storage or release. Numerous studies suggest warming will on balance release soil carbon; but increased agricultural production and CO2 fertilization may increase storage, thus reducing or counterbalancing this effect. The vast reservoirs of carbon, and their apparent temperature sensitivity, indicate that soils could release large amounts of carbon to the atmosphere, but there are large uncertainties regarding the amount and rate of release.
Some key issues to bring to the attention of the chapter authors:
- The authors state that there is no possibility of improving carbon stock estimates. This seems like a very strong statement, given that improvements have been realized and could be improved with increased effort.
- The authors focus on temperature impacts, but other chapters (e.g. grasslands and agriculture) emphasize rainfall impacts.
- In various places in the chapter, soil carbon stabilization is linked to microbial processes, to inputs from plant growth, and to physical stabilization; but there is no overall conceptual framework for how these different factors interact.
- The organization of the chapter by a mix of process and region means that some information and quantification may be overlapping and can’t easily be combined.
- The chapter could use more discussion of experimental studies related to factors such as soil warming, tillage, rainfall enhancement and exclusion. There is a large literature on such matters that is barely referenced.
Statement of Task Questions
- Does the report accurately reflect the scientific literature? Are there any critical content areas missing from the report?
The report seeks to cover an enormous amount of ground and does not reconcile the many conflicting views about the mechanisms and magnitude of soil carbon changes. In some sections, the report reads like a primer on soil biology rather than a focused assessment. As one example, the section on soil fauna indicates that fauna are important to soil—undoubtedly true, but this does not consider interaction effects of higher trophic levels or ecosystem engineers (e.g. earthworms) on soil carbon changes related to ongoing climate and land use.
The section on nitrogen is weak and does not reflect conflicting evidence on N impacts through plant growth and through soil biological-chemical interactions.
Soil respiration is part of the annual cycle of uptake and release, yet the section on respiration does not distinguish between controls over the amount and phenology of the annual cycle versus decadal trends. Soil respiration is the major mechanism for carbon stock changes, so this section is confusing in conjunction with discussions of stock changes that result from altered respiration (e.g., due to climate change or tillage).
Given the importance of modeling acknowledged in the chapter findings and introduction, the modeling section is weak and sparsely supported by literature. Some of the relevant literature is covered in the agriculture chapter, but the vast body of work from the leading soil modeling groups (e.g., USGS, Colorado State, DOE) is poorly captured compared to the far less mature Earth System Model literature. Limiting the modeling section to the ESMs does not fully portray the state of knowledge.
- Are the findings documented in a consistent, transparent and credible way?
The regional and continental budget numbers are not reconciled into a North American budget. Are U.S., Canadian, and Mexican numbers consistent with overall continental budgets?
- Are the report’s key messages and graphics clear and appropriate? Specifically, do they reflect supporting evidence, include an assessment of likelihood, and communicate effectively?
Do the areas in Table 10.1 represent total area? If so, the area of grasslands would be more appropriate.
- Are the research needs identified in the report appropriate?
The research needs identified seem fairly generic and not well linked to the key uncertainties identified. There would seem to be at least two major classes of uncertainty that could be targeted in the chapter: (i) the uncertainty in stocks and fluxes as a basis for understanding soil feedbacks to atmospheric CO2, and (ii) the impact of region- and ecosystem-specific management practices on SOC enhancement or sustainment. Regarding the later, the significant literature on modeling of management regimes to assess impacts on SOC storage seems underrepresented.
- Are there other key findings that are missing? Any critical literature missing?
The literature cited lacks depth in soil modeling, the nitrogen section is light, and the discussion of methane is very brief given the soil origin of this gas. The literature review on high latitude soils, identified as a key issue, seems sparse and does not fully reflect relevant literature, including inference from experiments and observational studies and the growing literature based on flux and atmospheric measurements. Are the results here consistent with discussions of the Arctic/Boreal Zone elsewhere in the report?
In a number of sections (e.g., on the Arctic/Boreal Zone), the literature review emphasizes papers from 5-10 years ago and does not seem to include recent papers based on extensive research efforts by NASA and DOE. For instance, see publications stemming from the NASA-led programs such as the Arctic-Boreal Vulnerability Experiment (ABOVE) and the Soil Moisture Active Passive (SMAP)
satellite mission, and from DOE-led programs such as the Next Generation Ecosystem Experiment (NGEE), and Spruce and Peatland Responses Under Climatic and Environmental Change (SPRUCE).8
- Are there any broader questions, such as the selection of the evidence and findings, weight of evidence, or the consistency of the application of uncertainty language?
Key Finding 2 notes the wide range of Earth System Model projections, which would indicate low confidence in model projections. High confidence should be applied to the result or not to the discrepancy among approaches. This could instead be listed as the last Key Finding.
Key Finding 3 needs to state a time frame for the soil carbon loss.
- What other significant improvements, if any, might be made in the document?
The writing is uneven across various subsections with some inconsistencies and duplications among sections. Also there are some inconsistencies in values used in different figures.
In the discussion of protection mechanisms, there is no mention of freezing and waterlogging as an important protection mechanism, especially in high latitude regions (boreal and Arctic).
P459, Line 14-20
Do increasing crop yields increase SOC?
P459, Line 22-24
Specify if this focuses on U.S. soils or North American soils. Specifically calling out Canada without calling out the U.S. creates confusion. The repeated presentation of discrete numbers for Canada, while a range is reported for the U.S., gives the impression that the Canadian data is more definite than the U.S. data.
P459, Line 37
Change to “between 16 Pg C and 78 …”
P460, Line 5-9
Here it seems that the terms “peatlands” and “permafrost” are used interchangeably. They are different despite some overlap (e.g., permafrost peatlands vs. non-permafrost peatlands vs. permafrost mineral soils).
P460, Line 11
It would be good to clearly state the scope and objectives of the chapter at the beginning.
P460, Line 32-36
Is this sentence useful? It could be deleted.
8 ABOVE (https://above.nasa.gov/cgi-bin/above_pubs_list.pl), SMAP (https://smap.jpl.nasa.gov/science/publications/); NGEE (https://ngee-arctic.ornl.gov/publications); SPRUCE (https://mnspruce.ornl.gov/content/project-publications).
P461, Line 32
Change to “The researchers found that the largest….”.
P461, Line 2-4
“There is no possibility” is a strong phrase when it is known that better estimates could be made with better data. This makes it sound as if we will never have any better idea of North American carbon stocks.
P461, Line 42
Change “huge” to “large”? Delete “larger-scale” as ESMs are global scale, and thus “large-scale” is redundant.
P461-466, Subsection 12.2.
The subsection could be better organized, perhaps as follows:
12.2.1 Plant Litter Inputs
12.2.2. Soil Microbes
12.2.3 Macrofauna (foodweb)
12.2.4 Rhizosphere Interactions
12.2.5. Protection Mechanisms
12.2.6 Nitrogen Effects on SOM Dynamics
P462, Line 27
Need to discuss freezing and waterlogging as protection mechanisms.
P463, Line l8
Is better to use the phrase “parent materials” rather than “source materials”, because in soil literature, “parent materials” refer to bedrock or other materials that soil develops on.
P463, Line 11
Change “outsized” to “important”.
P463, Line 18-22
While this may be true, it is a logical leap that some readers may not be ready to take at face value. Yes, SOC stabilization by microbes affects the plant community, but the plant community also drives the microbial community (a chicken/egg scenario).
P463, Line 18-28
This appears to replicate subsection 12.2.2. Move/merge to that subsection?
P463, Line 31-33
Can 2001 be called a “recent” paper?
P464, Line 12-37
Fauna: This section reads more like a primer than an assessment. Need discussion of the direction and magnitude of effects, and how fauna can amplify or moderate other effects.
P464, Line 41 – P465, Line 3
What is the cause of this diverging trend in nitrogen deposition— fertilizer use changes? air pollution control measures? other?
P465, Line 12
This should be “23 grams of carbon per “gram” of nitrogen”
P465, Line 16 – P466, Line 28
In discussing gas fluxes, need to be more explicit about time periods and spatial domains, to provide context for the numbers.
P465, Line 19-21
State during what time period, or state “is released annually”.
P465, Line 33-37
Could reference the Agriculture chapter here.
P465, Line 29-38
Need to use consistent units.
P466, Line 2-3
What about CH4 oxidation/sink in upland soils?
P466, Line 12
Only one component “of” net SOC changes.
P466, Line 30 – P467, Line 14
The literature cited in this section is particularly light and limited Also, some terminology needs to be defined (priming, sorption, etc).
P466, Line 38
The “other ecosystem compartments” shouldn’t include “atmosphere”. Correct?
P467, Line 31-32
Change the statement to “…captures change in the carbon content of soils across CONUS over time.”
P467, Line 37-38
Is density the correct term for a unit that does not include volume?
P468, Line 15 – P469, Line 5
Check on depths and context for the numbers cited.
P468, Line 17-20
Is 9.13 Pg C the 20 cm stock? Is it 73% of the 30 cm stock? Clarify to make the 18 Pg number (in line 19) make sense.
P469, Line 7
“153.7 Pg were in organic (peat) soils” is different from Chapter 13 (Terrestrial Wetlands), where peatland soil in Canada is stated to contain 130 PgC in Table 13.1.
P469, Line 7-37
This first paragraph presents conflicting data without smoothly transitioning between estimates: Tarnocai’s total is 262.3 Pg C, Kurz’s estimate places boreal forest alone at 208 Pg (which is ~80% of Tarnocai’s total). If the boreal forest is 208 Pg, then Tarnocai’s estimate is low for total Canadian soil carbon, because of significant contributions from other regions, therefore Tarnocai’s estimated total will likely increase as further research on permafrost continues.
P469, Line 8-9
Reiterate that the remaining carbon stocks are those estimated by Tarnocai, as to not imply that all of Canada’s soils are peat soils, tundra, forest, and agriculture.
P469, Line 9
Total soil carbon estimates for Canada likely will increase...
P470, Line 31-32
“Causes of soil loss in agricultural soils include...” This list is true for all soils, except for the tillage part, which is specific to agricultural soils.
P470, Line 31-41
Quotes losses without specifying time periods? Over what period? Are these losses fast or slow?
P472, Line 24
Reword as “moisture disturbances”?
P473, Line 10
Remove the phrase “types of”.
P473, Line 11-14
Could mention perennialism here.
P498, Table 12.1
The value given for “other” (11.2 PgC) is smaller than the value given in Table. 13.1 for wetlands in conterminous U.S. (13.5 PgC). Please check for consistency between these values.
P500, Table 12.3
Add a new row at the bottom of the table for “Total”. Why is there no table for Canada?
P501, Figure 12.1.
- The letterings are too small to read
- What about CH4 emissions, especially from “peatlands” (are they peatland near snow/ice? Is hard to tell).
- Label “tundra” right below “snow/ice”, and label “land-use” at location between forest and agriculture?
- Cannot comment on other boxes and meanings of all the arrows, as it is difficult to read what the authors intend to show.