Carbon for soils, not soils for carbon

In the paper

“The most recent estimate (using data-­ driven statistical modelling) puts global C losses at 116 Gt C (430 Gt CO 2 ) over the last 12,000 years of human land use (Sanderman et al., 2017), suggesting a global SOC sequestration potential of 285 Gt CO 2 under the two-­ thirds recovery assumption.”

What the sources say

• Disputed

  The Results and Discussion report a cumulative agricultural land-use loss of approximately 133 Pg C, not 116 Gt C. The same section gives 322 Pg CO2, not 285 Gt CO2, as the potential offset if two-thirds of lost SOC can be recovered. The paper supports the modelling approach and the conditional two-thirds assumption, but states different figures for both estimates. (LLM, conf=0.90)

  Checked by a reasoning model — may be wrong.

  cites Sanderman et al., 2017 sourceExperimental — accuracy not yet measured

In the paper

“We also observe that depth is inconsistently reported in review papers.”

• Also stated in: § 3.1 | Estimates of global soil organic carbon sequestration potential · p. 3 · hedged

  “These reviews, therefore, potentially combine estimates of sequestration rates that are not directly comparable (i.e. sequestration rates estimated to a depth of 2 m cannot be averaged directly with sequestration rates estimated to 30 cm depth; Knotters et al.,  2022). This also contributes to the discrepancies between global estimates.”

What the sources say

• Partly supported

  Section 4.4 (page unavailable) says sampling depths vary between countries and points to reviews that compare monitoring methods. It does not say that review papers report depth inconsistently, or that combining incomparable sequestration rates contributes to discrepancies between global estimates. (LLM, conf=0.78)

  cites Knotters et al., 2022 sourceExperimental — accuracy not yet measured

In the paper

“The authors found a global technical SOC sequestration potential of 3.3 Gt CO 2 year -1 , far less than previous estimates which did not constrain the available land area (Fuss et al., 2018), and with greater recognition of the need to use multiple solutions and address multiple objectives concurrently.”

What the sources say

• Partly supported

  In ‘Soil contribution to Natural Climate Solution Pathways’ (page unavailable), Bossio et al. report 5.5 Gt CO2e per year of soil-carbon mitigation, including 2.2 Gt from avoidable losses; subtracting those losses yields 3.3 Gt for the remaining potential, though the paper does not state that figure directly. ‘Soil science knowledge gaps’ (page unavailable) says its sequestration estimates are lower than Fuss et al.’s, but attributes unconstrained cropland-area availability to Zomer et al., not Fuss et al. The claim conflates those two comparisons. (LLM, conf=0.84)

  cites Bossio et al. (2020) sourceExperimental — accuracy not yet measured
• Supported

  The citation supports the comparison with earlier estimates, not the finding attributed to Bossio et al. In “Permanence and saturation,” Fuss et al. describe estimates above 7 Gt CO₂ per year as unconstrained and not realistically achievable; 3.3 Gt CO₂ per year is substantially lower. The paper does not report Bossio et al.’s later estimate. (LLM, conf=0.88)

  cites Fuss et al., 2018 source

In the paper

“The authors estimated global photosynthetic C inputs and, from this, estimated a global SOC sequestration potential of around 0.52 Gt CO 2 year -1 .”

What the sources say

• Supported

  The Abstract and Section 3 report a photosynthesis-constrained estimate of net SOC sequestration in croplands globally of about 0.14 Pg C per year. Converting carbon to CO₂ gives about 0.51 Gt CO₂ per year, consistent with the citing sentence’s approximate 0.52 Gt. The paper characterizes the estimate as rough and gives an uncertainty of ±0.1 Pg C per year. (LLM, conf=0.94)

  cites Janzen et al. (2022) source

In the paper

“Although some authors argue that this is realistic (Rumpel et al., 2019), SOC remains vulnerable to losses and is sensitive to a range of environmental factors. This vulnerability remains poorly understood.”

What the sources say

• Partly supported

  The citation in the printed sentence is attached to the assertion that the 4p1000 target is realistic, not directly to the subsequent vulnerability statements. The Critiques of the 4P1000 Initiative section describes that target as aspirational and uncertain rather than established as achievable. The Biophysical limits and barriers section does support SOC’s vulnerability to losses and changing conditions, but does not clearly state that this vulnerability remains poorly understood (page numbers unavailable). (LLM, conf=0.82)

  cites Rumpel et al., 2019 sourceExperimental — accuracy not yet measured

In the paper

“Contributions decrease to 11.1% and 4.5% for the most optimistic SOC sequestration rate estimate (Lal et al., 2018) under the fast and slow assumptions, respectively; and to 4.0% and 1.6% for the more realistic initial SOC sequestration rate (3.3 Gt CO 2 year -1 by Bossio et al., 2020; Figure 1d,e).”

What the sources say

• ?Not checked — no full text of the sourcecites Lal et al., 2018 source
• Supported

  In “Soil contribution to Natural Climate Solution Pathways” (page unavailable), Bossio et al. estimate 5.5 Gt CO2e per year of soil-carbon mitigation, of which 2.2 Gt is avoided losses. The remaining 3.3 Gt CO2e per year supports the sequestration-rate figure attributed to this paper. The percentage contributions are the citing authors’ calculations, not figures attributed to Bossio et al. (LLM, conf=0.88)

  cites Bossio et al., 2020 source

In the paper

“An increase would confirm the hypothesis that larger SOC stocks lead to improved soil structure, water holding capacity, or soil quality in general (although we acknowledge that the term is controversial; Bünemann et al., 2018), leading to more plant production per unit N applied.”

What the sources say

• Supported

  Split into 2 atoms — 2 confirmed, 0 partially supported, 0 disputed, 0 misattributed, 0 unverifiable. Expand the claim to see per-atom verdicts.

  cites Bünemann et al., 2018 source

In the paper

“Terrer et al. (2021) showed a trade-­ off between SOC sequestration and plant biomass production in a meta-­ analysis of 108 elevated CO 2 experiments across forests and grassland ecosystems, most likely due to competition for N between soil and plants.”

What the sources say

• Supported

  Split into 2 atoms — 2 confirmed, 0 partially supported, 0 disputed, 0 misattributed, 0 unverifiable. Expand the claim to see per-atom verdicts.

  cites Terrer et al., 2021 source

In the paper

“Another example is that of biochar, for which beneficial effects on crop yields are mainly observed in tropical acid soils (Jeffery et al., 2017), most likely due to the amelioration of aluminium toxicity (Shetty et al., 2021).”

What the sources say

• Supported

  Jeffery et al. is cited for the claim that biochar's crop-yield benefits are mainly observed in tropical acidic soils. The Abstract reports an average 25% yield increase in the tropics but no average effect in temperate latitudes, and identifies low-nutrient, acidic tropical soils as especially likely to benefit. The aluminium-toxicity explanation is attributed to Shetty et al., not to this source. (LLM, conf=0.94)

  cites Jeffery et al., 2017 source
• ?Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

  cites Shetty et al., 2021

In the paper

“It is increasingly recognised that greater scientific knowledge and concurrent management of these soil functions will be critical (Kopittke et al., 2022; Schulte et al., 2014).”

What the sources say

• ?Could not verify

  Searches for scientific knowledge, joint management of soil functions, and soil multifunctionality found no relevant passage from the paper’s body. The available matches are bibliographic material and an abstract; under the evidence rule, they cannot establish what the cited paper says about the claim. (LLM, conf=0.65)

  cites Kopittke et al., 2022 sourceExperimental — accuracy not yet measured
• Supported

  Section 5.5 calls for further research to quantify soil multifunctionality and identify management options. The Conclusions say meeting food-security and environmental goals requires maximising the supply of soil functions, while Section 5.2 describes managing the total suite of those functions. Together, these support the broad paraphrase. (LLM, conf=0.88)

  cites Schulte et al., 2014 source

In the paper

“Furthermore, SOM is central to the numerous functions delivered by soils (Kopittke et al., 2022).”

What the sources say

• Supported

  The title in our copy identifies organic carbon as central to balancing the multifunctional nature of soils. That supports the citing sentence’s broad point about soil organic matter and the functions soils deliver; no section or page was available in the retrieved text. (LLM, conf=0.82)

  cites Kopittke et al., 2022 source

In the paper

“More generally, relationships between the multiple functions of soil are context-­ specific and numerous trade-­ offs exist (Schulte et al., 2014; Vazquez et al., 2021; Zwetsloot et al., 2021).”

What the sources say

• Supported

  Section 2.1 says the capacities of soil functions vary with soil properties and land use, and gives an example in which grassland favors carbon sequestration and water purification while tillage favors agricultural output. Section 2.2 describes production gains that come at the expense of water purification, while noting that other interventions can avoid such trade-offs. Together, these passages support the claim that relationships are context-specific and trade-offs occur. (LLM, conf=0.87)

  cites Schulte et al., 2014 source
• ?Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

  cites Vazquez et al., 2021
• ?Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

  cites Zwetsloot et al., 2021

In the paper

“Following this approach, the most optimistic global potential estimate of ca. 454 Gt CO 2 (Lal, 2018 ) represents a little under 12 years of global CO 2 emissions at the current rate of 39 Gt CO 2 year -1 , and 3.5 years of global CO 2 emissions under our assumption of a fast decreasing SOC sequestration rate.”

What the sources say

• ?Not checked — no full text of the sourcecites Lal, 2018 source

In the paper

“This exercise led us to three main conclusions, successively elaborated in the following sections: (i) establishing causality between SOC sequestration and crop yield is problematic, (ii) the response variables used vary among the studies and all have limitations, and (iii) the outcomes vary with space, time and methods used.”

In the paper

“We, therefore, argue that a lack of consideration for nitrogen use efficiency is a missed opportunity to strongly reduce the greenhouse gas footprint of agriculture as much as it is a trade-­ off with SOC sequestration (Powlson et al., 2011).”

What the sources say

• ?Not checked — no full text of the sourcecites Powlson et al., 2011 source

In the paper

“Agriculture is responsible for approximately 70% of global anthropogenic N 2 O emissions (Tian et al., 2020), largely due to the inefficient use of nitrogen fertilisers.”

What the sources say

• ?Not checked — no full text of the sourcecites Tian et al., 2020 source

In the paper

“Second, 'measuring SOC' often entails measuring concentrations (in unit mass of C per unit mass of soil), which do not inform on the quantities of SOC stored in the soil. Concentrations need to be converted to stocks (in unit mass of C per unit of land area to a sufficient depth to avoid the effects of soil compaction).”

In the paper

“This is misleading and an unfair comparison because the potential of SOC sequestration will decline and reach zero over time, while it will be continued through time for most other options, including transitioning to non-­ CO 2 emitting energy production.”

In the paper

“The large variability in the responses of SOC stocks to soil warming (van Gestel et al., 2018) suggests a context-­ dependent response and a highly uncertain future for newly sequestered SOC, even when best management practices are maintained.”

What the sources say

• ?Not checked — no full text of the sourcecites van Gestel et al., 2018 source

In the paper

“SOC sequestration may occur along this pathway and contribute to climate change mitigation and should be regarded as a co-­ benefit.”

In the paper

“This 'sink saturation' is due to either a finite availability of mineral surfaces and other physico-­ chemical properties responsible for the persistence of C in soils, defining a maximum storage capacity, or to environmental constraints on C input stabilisation and decomposition rates leading to a new equilibrium value for SOC and defining an 'effective' storage capacity (Stewart et al.,”

What the sources say

• ?Not checked — no full text of the sourcecites Stewart et al., 2007 source

In the paper

“First, SOC stocks measured at a given point in time provide no information on a trend-­ SOC sequestration involves a dynamic change in SOC stocks that can be quantified only by monitoring the stocks over time or by quantifying the balance between soil C uptake and release.”

In the paper

“Yet already more than 15 years ago, Janzen ( 2006) highlighted that a trade-­ off exists between storing C in soils for the benefit of climate change mitigation, and using soil C during soil organic matter (SOM) decomposition for the release of nutrients to support plant production.”

What the sources say

• ?Not checked — no full text of the sourcecites Janzen ( 2006) source

In the paper

“This implies a total net reduction in emissions of around 3000 Gt CO 2 .”

In the paper

“larger crop yields may lead to increased biomass inputs to the soil, through crop residues, roots and stubble, and may therefore positively affect the soil C balance.”

In the paper

“As discussed, clay soils have the largest potential to store SOC, and those that are depleted offer the greatest opportunity to increase SOC stocks (Angers et al., 2011; Kirschbaum et al., 2020; McNally et al., 2017)”

What the sources say

• ?Not checked — no full text of the sourcecites Angers et al., 2011 source
• ?Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

  cites Kirschbaum et al., 2020
• ?Not checked — no full text of the sourcecites McNally et al., 2017 source

In the paper

“In short, increasing SOC stocks does not always increase crop yields, and can also negatively affect them.”

In the paper

“SOC sequestration cannot substitute emission reductions, even temporarily, nor does it delay the need for immediate and aggressive cuts in greenhouse gas emissions if we are to keep global warming under 1.5 or well-­ below 2°C (Anderson et al., 2019).”

What the sources say

• ?Not checked — no full text of the sourcecites Anderson et al., 2019 source

In the paper

“Only one study (Oelofse et al., 2015) found unclear effects across eight soil types, with borderline positive effects on coarse sandy loam and fine sandy clay loam soils but negative effects on fine sandy soils.”

What the sources say

• ?Not checked — no full text of the sourcecites Oelofse et al., 2015 source

In the paper

“Two key points emerge from our review: (i) the climate mitigation potential of SOC sequestration is modest, at best, on a global scale, and context-­ specific”

In the paper

“Using the same data selection, the authors also found a positive yield effect in tropical climates, with no significant yield differences in the final experimental year in the other climate zones (subtropical and continental).”

What the sources say

• ?Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

  cites Chen et al., 2018

In the paper

“When experimental treatments receive sufficient P and K application, a change in agronomic N use efficiency is also representative of the long-­ term effect of SOC on crop yield.”

In the paper

“and (ii) correlations between SOC sequestration and crop yield are strongly context dependent and it is highly uncertain whether a causal link exists.”

In the paper

“For example, cultivation of a cover crop can increase SOC stocks and, at the same time, can reduce N losses, thereby increasing soil N supply and possibly crop yields (Thapa et al., 2018).”

What the sources say

• ?Not checked — no full text of the sourcecites Thapa et al., 2018 source

In the paper

“At the same time, the authors acknowledged that this approach is ambiguous as a control treatment with equal NPK supply from mineral fertiliser is required, while relative nutrient mineralisation rates of organic amendments vary strongly with the quality of the amendment and across locations (Schröder, 2005).”

What the sources say

• ?Not checked — no full text of the sourcecites Schröder, 2005 source

In the paper

“An additional complication that arises when assessing the contribution of SOC sequestration to crop yields is the composition of SOM, which changes over time.”

In the paper

“Similarly, whereas Hijbeek et al. (2017) found no overall yield effects of organic amendments, attainable crop yields were increased on soils with smaller clay content (an additional 0.13% increase in attainable yields when using organic amendments for 1% less clay particles).”

What the sources say

• ?Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

  cites Hijbeek et al., 2017

In the paper

“is not subject to such confounding environmental factors but has other limitations.”

In the paper

“We argue that including SOC sequestration in the narrative of global climate change mitigation is ill-­ suited and we call for a soil-­ smart approach, which will not always be 'climate-­ smart'.”

In the paper

“SOC sequestration is indeed one of many approaches, one small piece of a very large puzzle, and it should be treated as such.”

In the paper

“Without a doubt, soils have a critical role to play, and SOM, with the carbon it contains, is fundamental to maintaining soil fertility and supporting plant production.”

In the paper

“We conclude that, if we are to ensure food security in a changing climate, let alone other sustainable development goals, SOC sequestration is simply not up to the challenge.”

In the paper

“and large CO 2 emissions in clay soil and small SOC sequestration in the sandy soil with an overall SOC loss and aggravated climate change”

In the paper

“The reciprocal transfer, in panel b, leads to mirrored effects: Small yield loss in the clay soil and high yield gain in the sandy soil”

In the paper

“Therefore the net overall effect is that C is sequestered, to the benefit of climate (provided that no additional N 2 O or CH 4 emissions would arise).”