Carbon sequestration in soils and climate change mitigation—Definitions and pitfalls

The term carbon (C) sequestration has not just become a buzzword but is something of a siren's call to scientific communicators and media outlets. Carbon sequestration is the removal of C from the atmosphere and the storage, for example, in soil. It has the potential to partially compensate for anthropogenic greenhouse gas emissions and is, therefore, an important piece in the global climate change mitigation puzzle. However, the term C sequestration is often used misleadingly and, while likely unintentional, can lead to the perpetuation of biased conclusions and exaggerated expectations about its contribution to climate change mitigation efforts. Soils have considerable potential to take up C but many are also in a state of continuous loss. In such soils, measures to build up soil C may only lead to a reduction in C losses (C loss mitigation) rather than result in real C sequestration and negative emissions. In an examination of 100 recent peer‐reviewed papers on topics surrounding soil C, only 4% were found to have used the term C sequestration correctly. Furthermore, 13% of the papers equated C sequestration with C stocks. The review, further, revealed that measures leading to C sequestration will not always result in climate change mitigation when non‐CO2 greenhouse gases and leakage are taken into consideration. This paper highlights potential pitfalls when using the term C sequestration incorrectly and calls for accurate usage of this term going forward. Revised and new terms are suggested to distinguish clearly between C sequestration in soils, SOC loss mitigation, negative emissions, climate change mitigation, SOC storage, and SOC accrual to avoid miscommunication among scientists and stakeholder groups in future.


| INTRODUC TI ON
At the 21st United Nations Framework Convention on Climate Change (COP21) in Paris, socio-environmental challenges arising from the anticipated effects of global warming, such as increased risk of droughts and flooding, were identified and nature-based solutions for combating these effects were discussed (IPCC, 2021).
The reduction of greenhouse gas (GHG) emissions through sustainable management of ecosystems is regarded as a key component in strategies for achieving this goal.Agricultural land, forests and wetlands have become increasingly prominent as land-use types that have the potential to store additional C in soils and biomass, thereby decreasing atmospheric CO 2 concentration and helping to mitigate climate change (Griscom et al., 2017).This focus has resulted in programs such as the 4per1000 initiative and various carbon (C) farming schemes (Minasny et al., 2017;Rumpel et al., 2020) aiming at increasing terrestrial C storage in managed ecosystems.Together, these ecosystems are seen as an actionable basis for achieving net zero GHG emissions by 2050 and preventing the rise of global temperatures beyond 2°C (IPCC, 2021).
Soil organic carbon (SOC) is dynamic in time and space and is continuously built up but also continuously decomposed and mineralised.Both SOC and soil inorganic carbon (SIC) occur in soils but this paper's focus is on SOC, since this is the main area of discussion in science and politics.SOC comprises all organic matter in soils that is dead.Changes in SOC stocks are small relative to existing large SOC stocks.Biomass in the form of aboveground and belowground litter, including woody material such as dead roots is entering the soil, thereby renewing parts of the SOC pool.
Simultaneously, microbes decompose SOC, releasing a portion of carbon as respired CO 2 into the atmosphere.Observed changes in SOC stocks are thus mostly the consequence of two major fluxes: the fraction of net primary production entering the soil, for example, as litter and the respiration flux releasing C from the soil (Bondeau et al., 2007).The difference between the two fluxes is referred to as the net C balance of the soil.The net balance changes over time as a result of temporal variations in these two

| DEFINITI ON OF C S EQUE S TR ATI ON: NE T UP TAK E OF CO FROM ATMOS PHERE
Carbon sequestration is defined by the IPCC as the process of increasing the C content of a C pool other than the atmosphere (IPCC, 2001).
Furthermore, for soil specifically, C sequestration in soils is described by Olson et al. (2014) as the "process of transferring CO 2 from the atmosphere into the soil of a land unit through plants, plant residues and other organic solids, which are stored or retained in the unit as part of the soil organic matter" (Table 1).The term SOC sequestration is frequently used with the same meaning despite it not being entirely correct since it involves the sequestration of atmospheric CO 2 rather than soil organic carbon (SOC).This means that for C sequestration in soils to occur, CO 2 must be drawn from the atmosphere and be converted into organic C via autotrophs metabolic activities, and then must enter the soil either directly (plant matter and plant residues produced on the same site) or indirectly (plant-derived organic matter such as manure or compost that derive mostly from other sites) in sufficient quantities to outweigh losses caused by respiration and lead to a net C stock increase in the soil (see example in Figure 1a).Consequently, as this C is derived from atmospheric CO 2 , there is a commensurate net removal of C from the atmosphere, referred to here as negative emissions if the emission of other greenhouse gases (GHGs) is not simultaneously enhanced and the sum of all GHG fluxes (in CO 2 equivalent) is negative.To produce negative emissions, a measure needs to change the soil from a GHG source to a GHG sink, considering possible leakage.Furthermore, the terms C sequestration and SOC storage are often used synonymously, but have different implications (Baveye et al., 2023).SOC storage is used as either (i) a quantity, that is, the amount of SOC (e.g., SOC stock) or (ii) a process, that is, an increase in SOC stocks over time for a given land unit.The former is not associated with net removal of C from the atmosphere, and thus does not constitute actual C sequestration in soils (Chenu et al., 2019;Guenet et al., 2021), while the latter implies it.In order to reduce miscommunication potential, it is proposed that clear definitions of the terms are used.We propose definitions in Table 1.Mathematical formulations of the definitions are presented in Data S1 .

| CURRENT US E OF THE TERM C SEQUE S TR ATION
Use of the term C sequestration in the context of soils was surveyed and analyzed in 100 recent peer-reviewed publications with the aim of comparing the broader use of the term in current scientific TA B L E 1 Proposed definitions of key terms.literature with the definitions outlined above (Table 1).With this review we wanted to get a representative overview of how the term C sequestration is currently used.Publications were selected using the search string ("soil") AND ("carbon sequestration" OR "C sequestration") in the title or abstract in the Web of Science database on 12 October 2022.This produced 10,601 peer-reviewed publications, dated between 1945 and 2022, covering the fields of environmental sciences (28%), soil science (22%), ecology (12%), agronomy (8%), forestry (7%), plant sciences (6%), geosciences (6%), agriculture (5%), biodiversity conservation (3%) and other multidisciplinary sciences (3%).The top ten countries publishing 8794 (84%) of these studies were China (26%), USA (23%), Germany (7%), Australia (5%), India (5%), Canada (5%), UK (4%), Spain (3%), France (3%), and Italy (3%), with the remaining 16% of studies published in other countries.The 100 most recent publications covering April 2022 to October 2022 were then selected (see Data S1).The broad range of scientific disciplines and countries represented in the search results indicates the importance of the term C sequestration in soils across scientific communities, and the urgent need for standardization of the terminology surrounding the uptake, storage, and release of C in and from soils.Subsequently, based on the definitions given above, the following nine aspects were looked at when surveying the literature (Table 2).
After surveying the selected papers, it was found that 93% re-  Soils are generally a relatively thin layer between the atmosphere and bedrock; however, in some regions they can be several decameters deep.The highest SOC stocks are found in topsoils which are also most susceptible to SOC changes because of their proximity to the atmosphere as well as anthropogenic impacts (Poeplau & Don, 2013).In 72% of the 100 analyzed papers representative soil units were studied.Subunits, such as aggregates, that are not necessarily representative of either topsoil or total soil were studied in 8% of the publications.The remaining 20% did not mention the soil unit that was analyzed (e.g., sieved <2 mm).Further, most papers referred only to the topsoil to 30 cm soil depth (55%) or made no mention of the soil depth for which their results were reported (34%).Only 11% of the studies explicitly included subsoils below 30 cm soil depth, demonstrating the systematic neglect of subsoil's contribution to soil C dynamics (Börjesson et al., 2018).
Following the definitions given in Table 1, none of the 42 papers that only used C sequestration in soils to put their study into a broader context without data or discussion of the topic applied the term correctly and, of the 58 remaining publications involving soil C-related topics, only 7% applied the term correctly (Figure 3b).
As many as 67% of the 100 papers compared experimental soil C values to a BAU scenario assuming that SOC stock is in a steady state.This assumption is oversimplified and unrealistic in most cases since SOC stocks change over time (Minasny et al., 2017;Sanderman et al., 2017).Thus, it is unclear whether they are showing a net SOC increase (C sequestration) or a relative increase compared to a reference treatment or BAU scenario.Finally, for 29% of the 100 papers no distinction could be made between correct or incorrect use of the term C sequestration as SOC was not the main topic of these papers and the information necessary to make this distinction was not given.
Overall, almost two-thirds of 100 recent peer-reviewed publications referred to SOC storage as an SOC stock increase, while 13% referred to SOC storage as SOC stocks.The remaining 25% of studies did not mention SOC stocks or SOC stock changes, despite discussing C sequestration (Figure S1a).These proportions did not change significantly when excluding publications in which C sequestration in soils was only used to put their study into a broader context without data or discussion of the topic (75%, 12% and 14% respectively, Figure S1b).
A value for total C sequestration within a time period (e.g., in Mg C ha −1 ) was found in 40% of 100 recent peer-reviewed publications, while 57% gave a C sequestration rate (e.g., in kg C ha −1 year −1 ) and 3% did not give any information at all.No large differences were found after excluding those publications that only used C sequestration in soils to put their study into a broader context without data or discussion of the topic (47%, 51% and 2%, respectively).

F I G U R E 2
Main topics of 100 recent peer-reviewed publications following the search string ("soil") AND ("C sequestration" OR "carbon sequestration") in the title or abstract and divided into two groups: "C sequestration data and/or discussion of the topic" (dark bars) and "C sequestration used without data or discussion of the topic" (light bars).

C SEQUE S TR ATION IN SOIL S
The literature review underlines the cross-and intra-disciplinary confusion about the terms C sequestration, negative emissions, climate change mitigation, and C storage related to soil C. In this chapter, we want to show the pitfalls surrounding these words and how to avoid them.

| C sequestration in soils versus C loss mitigation
As a result of the definitions of IPCC (2021) and Olson et al. (2014) for the term C sequestration in soils, not every measure to enhance SOC will result in C sequestration and negative emissions with a net uptake of C from the atmosphere (Figure 4a).The SOC stocks of many agricultural soils are currently declining due to changes in land management, such as drainage, and historic land use changes but also to climate change (Ciais et al., 2010;Poeplau & Dechow, 2023;Sanderman et al., 2017).Agricultural measures can help reduce (Figure 4b) or stop this SOC loss or even achieve SOC build-up (Figure 4c,d) and thus true C sequestration in soils (Paustian et al., 2016).However, in cases where SOC losses are only reduced compared to a business-as-usual scenario (i.e., a SOC stock increase compared to a BAU scenario) this cannot be called C sequestration in soils as the soil is still losing SOC.Instead, use of the term SOC loss mitigation is proposed but also the term C stock protection is used (Whitehead et al., 2018; Figures 1c and 4b).
Previous literature referred to this as relative C sequestration (Peralta et al., 2022), but we considered this to be misleading since it does not represent a net removal of atmospheric CO 2 but uses the term C sequestration.Regardless of the underlying reasons for the decline in SOC, if SOC stocks do not increase over time true C sequestration in soils cannot be achieved.Therefore, a comparison of the difference in SOC stocks between a treatment that enhances SOC and a control treatment should not be interpreted as C sequestration; the initial SOC stock also needs to be considered.The difference in SOC stocks between SOC-enhancing treatments and a control treatment can instead be referred to as SOC accrual or SOC increase.Some ecosystems are particularly SOC rich, such as peatlands and some grasslands and forests (Jobbagy & Jackson, 2000).The protection of their SOC stocks is important for climate change mitigation since many of them are actually C sources due to land use conversion or the agricultural use of peatland after drainage (Leifeld et al., 2019).Therefore, a business-as-usual scenario in which SOC is in decline needs to be defined in order to include measures to protect these SOC stocks as climate change mitigation measures, for example, if peatlands are rewetted to reduce or stop their CO 2 emissions compared to a business-as-usual scenario with drained peatlands (see Figure 1c).In these ecosystems the major aim is to achieve climate change mitigation by reducing GHG emissions rather than through C sequestration.

| C sequestration in soil, SOC storage or SOC stocks?
Only C that is additionally stored in soils or that is additional compared to a business-as-usual scenario can be relevant for climate change mitigation.Thus, C sequestration in soils is not the same as SOC stocks.Soil organic carbon sequestration refers to a net flux of C from the atmosphere to the soil.The rate unit should be mass C per area per time period, for example, Mg C ha −1 year −1 , while SOC stock is a mass per area, for example, Mg C ha −1 .In summary, the size of any C pool can be referred to as a C mass, C pool or C stock, F I G U R E 3 Use of the terms carbon (C) sequestration or soil organic carbon (SOC) sequestration in 100 recent peer-reviewed studies following the definitions shown in Table 1.In the studies, soil C was assessed either relative to a business-as-usual scenario in a steady state (orange) or as C sequestration in soils (green), as defined in this study for (a) all 100 publications and (b) a selection of 58 recent peer-reviewed publications excluding those that only use C sequestration without data or discussion of the topic.In all cases where study conditions could not be determined, these were considered "Not specified" (grey).

(a) (b)
| 7 of 14 while C sequestration is the process of removing CO 2 from the atmosphere over time, which is always a C flux.
The present analysis showed that in 12% of the 58 publications that focused on C sequestration in soils, the term SOC storage was used synonymously with SOC stocks, while two thirds used it to describe an increase in SOC stocks (Figure S1).This highlights the potential for miscommunication resulting from a single term with two different definitions (as shown in Section 1).While the two uses are correct as storage describes both the act of storing or the state of being stored (Collin, 1982), we suggest that: (i) SOC content or stock is referred to as SOC storage and (ii) an increase in SOC stocks is referred to as SOC accrual (Table 1).While SOC accrual is related to C sequestration in soils, there is potential for differences in the source of C.Where C sequestration in soils requires the atmosphere to soil pathway of C accumulation, SOC accrual does not require atmospheric C as the source of C, and instead relates to any increase in SOC stocks at a given site.For example, SOC accrual can be due to eroded sediment that is deposited and increases SOC at one site.However, this is causing SOC depletion at another site where the sediment was derived from and thus, no net SOC increase (C sequestration) is achieved.
Additionally, climate change mitigation effects were often claimed to be related to the size of the existing SOC stock at a single site (Baveye et al., 2023); however, the protection of existing SOC stocks is only a measure for climate change mitigation if in the business-as-usual scenario soils are losing SOC.In either case, C sequestration in soils is not achieved since there is no net uptake of C from the atmosphere.

| C flux or global warming potential
Differentiating between the global warming potential (GWP) of CO 2 and C mass is essential for understanding the effect of a measure on C fluxes and for climate change mitigation.The common unit to express the effect of GHGs on the climate is CO 2 -equivalents (CO 2 -eq).This converts N 2 O and CH 4 emissions into equivalent units relative to the cumulative radiative forcing of CO 2 over a given period, usually 100 years.In contrast, C sequestration in soils refers to C mass which is representative of the number of C atoms removed from the atmosphere and is thus different from removals calculated in CO 2-eq , which are key when focusing on comparing the radiative effect of different GHGs.This is particularly important if methane (CH 4 ) is part of the evaluation, since it contains C but has a 28-fold higher global warming potential than CO 2 over a 100-year period.
Paddy soils and peatlands are systems that emit large amounts of CH 4 (Jackson et al., 2020).Thus, C sequestration can be achieved with C removal from the atmosphere, but if CH 4 emissions increase at the same time, they can easily offset the climate change mitigation effect of C sequestration (Figure 5).Thus, C may be removed from the atmosphere (number of C atoms, Figure 5b) without negative emissions being achieved (unit CO 2-eq , Figure 5a).

| Permanence of additional SOC storage
There are diverging views on the conditionality of permanence for C sinks: CO2 storage in geological formations or the deep ocean is generally considered to stay for millennia whereas biomass C sinks in afforestations are considered as C sinks even though their storage time is often only measured in decades (Gren & Zeleke, 2016;Tyka et al., 2022).Thus, the permanence of C storage is context specific.
Ideally, additional SOC is stored permanently but even temporary storage is beneficial.For instance, the climate benefit of SOC that is stored for 40 years before release is still 66% of that for SOC stored for 100 years before release (Leifeld & Keel, 2022).Thus, the time period for which additional SOC is stored is pivotal for its climate impact.
Carbon bound in soil organic matter is in a continuous flow.
About 80%-90% of the C entering soils in the form of plant biomass is respired within a timespan of months to a few years in a temperate climate (Angers et al., 2022).Nevertheless, even such a dynamic C pool can be enhanced, and C stocks can probably be preserved in the long-term by adopting SOC increasing management practices (Johnston et al., 2017).Only 7% of 100 recent peer-reviewed publications considered or ensured long-term storage, for example, via stabilization as biochar.Furthermore, permanence was considered by 11%, at least mentioned by 46%, and not mentioned at all by 36% of all studies considered.No notable differences were found when excluding publications that use C sequestration in soils only to put their study into a broader context without data or discussion of the topic (7%, 17%, 46%, 30%, respectively).

| Leakage can prevent C sequestration in soils from achieving climate change mitigation
Leakage of GHGs may determine whether or not an agricultural measure for C sequestration in soils is able to mitigate climate change or even achieve negative emissions.Leakage occurs if a measure to enhance SOC stocks leads to an increase in GHG emissions either on site (i.e., from the soil where SOC stocks are increased) or off site.Some agricultural measures for SOC accrual also increase on-site N 2 O emissions to such an extent that the potential climate change mitigation effect of added SOC is completely negated or even reversed (Guenet et al., 2021;Lugato et al., 2018).This may be the case, for example, when SOC stock increases are obtained by promoting biomass production and thus C inputs to the soil as a result of additional N fertilization (Poeplau et al., 2018).Potential additional off-site emissions can be caused, for example, by increased energy consumption required for   (Kløverpris & Mueller, 2013;Searchinger et al., 2008).It is, therefore, not reasonable, or perhaps even possible, to fully account for all leakage effects in studies on C sequestration in soils due to the complexity of the topic.However, we encourage an increased awareness of such leakage effects when evaluating C sequestration measures.
Another form of leakage is the transfer of C from one site to another through organic amendments such as farm-yard manure or compost (Paustian et al., 2019).The level of these organic amendments is limited by the amount of biomass available to produce these organic fertilizers in the first place (Janzen et al., 2022).Thus, SOC cannot be increased with organic amendments at one site without a commen- or sewage sludge that is currently incinerated) or some transformation increases the persistence of the resulting SOC.The latter is true for biochar, which has been shown to increase the mean residence time of organic C in soils significantly compared with its feedstock and therefore achieves C sequestration in soils (Schmidt et al., 2021).However, additional GHG emissions from processing of biochar and transport should be accounted for when evaluating its C sequestration potential.
Notably, of the 100 recent peer-reviewed publications reviewed, only 9% fully consider leakage by reporting a complete GHG balance, while 3% partially account for this (e.g., by accounting for just some GHGs but not all).Meanwhile, 20% mention that leakage effects exist or are possible, while the majority (68%) do not mention it at all.This does not change very much when excluding those publications that use C sequestration in soils only to put their study into a broader context without data or discussion of the topic (14%, 5%, 21% and 60%, respectively).Owing to the widespread potential for leakage in SOC-enhancing measures, spatially explicit accounting of SOC stock changes is not sufficient to capture the climate change mitigation potential of a measure, and therefore the global scale view is required to establish the true effect of C sequestration.That being said, for plot scale studies such global view is hardly possible and thus estimates on off-site and leakage effects may only be roughly estimated, discussed or mentioned.

| The temporal dimension of C sequestration: C stocks versus GHG fluxes
The implementation of management changes can induce SOC stock increases, but the achieved SOC accrual will decrease over time with SOC stocks approaching a new steady state between SOC formation and respiration (Chenu et al., 2019;Sommer & Bossio, 2014).
Although the measure continues to be implemented, there is no further buildup of SOC once the new steady state is reached (Figure 1).
In particular, if agricultural measures are implemented to increase SOC that also enhance GHG emissions such as N 2 O (i.e., on-site leakage), the respective time scale for judging these agricultural measures on their potential to mitigate climate change or cause negative emissions is important (Lugato et al., 2018) and needs to be clearly reported.For illustration, here is one example.Reduced tillage can enhance SOC stocks mainly in dry regions (Bai et al., 2019).
A total C sequestration in soils of 3 Mg ha −1 (11 Mg CO 2 ha −1 ) may be reached after 30 years in the topsoil compared to a reference scenario with conventional ploughing (Lugato et al., 2018).Thus, each year 0.1 Mg C ha −1 is sequestered for 30 years (0.37 Mg CO 2 ha -1 ).

| CON CLUS IONS
The fluxes depending on various drivers.If the net C balance is positive soil takes up C. If soil C is increased relative to initial SOC as a result of reducing atmospheric carbon (e.g., via photosynthetic pathways), C sequestration is achieved.What is ultimately of importance in relation to climate change mitigation is the SOC stock change on annual, decennial or centennial timescales, and the spatial domains in which this change occurs.In addition to the difficulty of measuring this net balance, the misuse of the terms related to C sequestration in soils can lead to misunderstandings and biased and unrealistic expectations of the role of agricultural and forest soils in their ability to contribute to climate change mitigation.Not every local or field-scale increase in terrestrial C stocks amounts to C sequestration, and not all C sequestration is a negative emission that contributes to climate change mitigation.Assessing the climate change mitigation potential of additional SOC stocks requires accounting for leakage effects (Lugato et al., 2018).Leakage describes additional GHG emissions caused by climate change mitigation measures that either reduce the strength of a C sink, or turn these measures into sources of GHGs.The results and discussion surrounding C sequestration have implications beyond discipline-specific research circles, including for stakeholders such as politicians and farmers.There are risks of miscommunication if the terminology around C sequestration is not adequately defined and correctly used.Therefore, the aim of this paper was to revisit existing definitions of the terms C sequestration, SOC sequestration, climate change mitigation, negative emissions, SOC storage, and SOC accrual, with the goal of clarifying their meaning and ensuring their appropriate and accurate usage going forward.The results are presented of an evaluation of 100 recent peer-reviewed publications that use the term C sequestration or SOC sequestration in relation to soils in order to explore current use of the term, identify pitfalls associated with use of these terms based on their definitions, and outline a pathway for accurate communication in the field of C sequestration and negative GHG emissions.

C
sequestration in soils is a term often used in the context of climate change mitigation because it is the process responsible for determining the flux of atmospheric C entering soils and turning them into C sinks.Climate change mitigation has been defined as "a human intervention to reduce emissions or enhance the sinks of greenhouse gases" (IPCC, 2021).Prior and post states of GHG fluxes need to be | 3 of 14 DON et al. compared (including C sinks) to quantify the climate change mitigation effect of such interventions.At the same time it is clear, a reduction of emissions does not equate to negative emissions but only of C sinks, and C sequestration in soils may not always lead to climate change mitigation, depending on past sink strength or past GHG emissions.

F I G U R E 1
Possible trends in soil organic carbon (SOC) stocks in business-as-usual (BAU) scenarios and following implementation of C sequestration measures.(a) SOC stocks are assumed to be in steady state with no change in a BAU scenario, (b) SOC stocks are predicted to increase even without C sequestration measures in the BAU scenario, (c) SOC stocks are expected to decline in the BAU scenario despite the implementation of C sequestration measures, and (d) SOC stocks are expected to decline if no C sequestration measures are implemented."SOC change of a measure" is relative to the BAU scenario.The dashed line indicates zero change.Calculations are provided in the Data S1.
practices that have the potential to increase C storage in soils (39%) or quantified the C sequestration potential of whole ecosystems (20%; Figure2).Surprisingly, 42% of the 100 papers appeared to use the term C or SOC sequestration only to apply a broader context to their study.While the topics of these papers often included C or soil C, there was insufficient data, context, and meaningful C or SOC sequestration related discussion linked to the term C sequestration.
Although C sequestration in soils refers to C fluxes per number of C atoms, in the wider context of climate change mitigation the global warming effect of different greenhouse gases expressed as CO 2 -equivalents is relevant.F I G U R E 4 Theoretical examples of management changes (a-e) and their site-specific impacts on soil and atmospheric carbon pools and N 2 O fluxes.Off-site fluxes (e.g., leakage) are not considered.Management examples are evaluated according to their effects on SOC loss mitigation, climate change mitigation, C sequestration and negative emissions.Arrow length represents the flux size in CO 2 -equivalents.✓: yes; ×: no; −: not applicable.

F I G U R E 5
Net-exchange of methane (CH 4 ) and carbon dioxide (CO 2 ) between the soil and the atmosphere for a theoretical example of a management change and its site-specific impact in (a) CO 2 -equivalents (CO 2-eq ) and in (b) number of C atoms.Off-site fluxes (e.g., leakage) are not considered.Arrow lengths in (a) represent the flux size in CO 2 -equivalents with a global warming potential of CH 4 of 28.CH 4 fluxes and CO 2 fluxes together result in the net greenhouse gas balance (net GHG) by assuming zero N 2 O fluxes.By only considering the C in CH 4 and CO 2 , we derived a net C balance (Net C).Arrow lengths in (b) represent the C flux size of CH 4 -C and CO 2 -C that together indicate SOC accrual (net C).
production and transport.Furthermore, leakage can occur due to indirect effects, for example, due to a reduced agricultural yield that may trigger the need for additional agricultural land and related C losses as a result of land-use changes.Such indirect land-use changes may even dominate the net GHG balance of agricultural measures for climate change mitigation and turn their balance from positive to negative window for preventing climate change from reaching an irreversible tipping point is closing and action needs to be taken to keep the effect of anthropogenic global warming under +2°C.Naturebased solutions that enhance soil C storage can contribute to climate change mitigation.In order to identify and quantify these measures and their climate change mitigation potential, multiple stakeholder groups need to be able to work together and communicate unambiguously.This kind of collaboration is only possible when key terms are carefully defined and accurately employed.It is therefore a priority for the scientific literature to be clear about definitions of important terms in order to guide discussions in the political world and wider society.This analysis of 100 recently published peer-reviewed research papers clearly demonstrates that the terms C sequestration and SOC storage are either used ambiguously or have multiple interpretations.Thus, a more rigorous use of the term C sequestration and related vocabulary is needed in order to avoid misunderstandings and biased perception of the true potential of nature-based solutions.Clearer definitions of such terms are proposed and new terms outlined to distinguish clearly between C sequestration in soils, SOC loss mitigation, negative emissions, climate change mitigation, SOC storage, and SOC accrual.In addition, based on examples, this paper highlights the pitfalls of inaccurately applying terminology associated with SOC storage, and provides guidance on the proper use of the more clearly defined terms suggested.Furthermore, it highlights the importance of transparent communication regarding the permanence of additional SOC and leakage effects.Without correct use of these key terms, misleading conclusions may be drawn regarding climate change mitigation strategies, ultimately leading to support for measures that do not have the intended benefits.By using a consistent vocabulary the foundations can be laid for much-needed climate change solutions.
Overview of the study details and survey criteria considered for 100 peer-reviewed studies involving carbon (C) or soil organic carbon (SOC) sequestration.The criteria (right column) are mutually exclusive.
specifically to SOC sequestration or C sequestration in soils in the title or abstract, while 7% referred to C sequestration where the word soil is mentioned independently in the abstract.The majority of studies either focused on agricultural management TA B L E 2 Is C/SOC sequestration in soils the major topic (regardless of definition)?•C/SOCsequestration is mentioned but no related data or discussion is presented OR • Specific or related data are presented (e.g., soil organic matter data) or discussed iii.Representativeness • Only a subunit of soil is considered (e.g., sequestration in aggregates) OR • a representative unit of soil (e.g., fine soil <2 mm) is considered reduction of SOC from another site.Organic amendments are thus likely to enhance SOC stocks on a local scale (SOC accrual) without achieving SOC stock increases on a global scale (C sequestration in soils).Organic amendments can only boost SOC stocks globally if they are not returned to soil in the business-as-usual scenario (e.g., compost (Guenet et al., 2021)ced tillage leads to enhanced annual N 2 O emissions by 0.2 Mg CO 2eq ha −1(Guenet et al., 2021).Over a 30-year period, this results in a ratio of 0.54 between additional N 2 O emissions and additional CO 2 uptake (see also Figure4c).Any ratio below 1 indicates climate mitigation effects.Thus, over a 30-year timescale, this agricultural measure would be considered to contribute to climate change mitigation (if there is no other leakage and if the reference scenario's SOC storage is in a steady state).This conclusion changes if the same example is viewed from a 100-year perspective, resulting in a ratio of 1.82 between N 2 O and CO 2 .While the C sequestration in soils ceases once the new steady state of C input and mineralisation is reached, N 2 O may continue to be emitted.Thus, in this example reduced tillage would result in negative emissions if calculated from a mid-term perspective (30 years), but would result in additional emissions from a longer-term perspective.Although this example is a simplification regarding the time-dependency of the GWP, it illustrates that it is critical to consider timescales and leakage when evaluating a measure's climate change mitigation potential.