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The main hurdle in instrumentalizing agricultural soils to sequester atmospheric carbon is the development of methods to measure soil carbon stocks which are robust, scalable, and widely applicable. Our objective is to develop an approach that can help overcome these hurdles. In this paper, we present the Wageningen Soil Carbon STOck pRotocol (SoilCASTOR). SoilCASTOR uses a novel approach fusing satellite data, direct proximal sensing-based soil measurements, and machine learning to yield soil carbon stock estimates. The method has been tested and applied in the USA on fields with agricultural land use. Results show that the estimates are precise and repeatable and that the approach could be rapidly scalable. The precision of farm C stocks is below 5% enabling detection of soil organic carbon changes desired for the 4 per 1000 initiative. The assessment can be done robustly with as few as 0.5 sample per hectare for farms varying from 20 to 150 hectares. These findings could enable the structural implementation of carbon farming. [ABSTRACT FROM AUTHOR]

The EU Mission Board for Soil Health and Food proposed a series of quantitative targets for European soils to become healthier. Among them, current soil organic carbon (SOC) concentration losses in croplands (0.5% yr−1 on average at 20 cm depth) should be reversed to an increase of 0.1–0.4% yr−1 by 2030. Quantitative targets are used by policy makers to incentivize the implementation of agricultural practices that increase SOC stocks. However, there are different approaches to calculate them. In this paper, we analyzed the effect of exogenous organic matter (EOM) inputs on the evolution of SOC stocks, with a particular focus on the new European targets and the different approaches to calculate them. First, we illustrated through two case-study experiments the different targets set when the SOC stock increase is calculated considering as reference: 1) the SOC stock level at the onset of the experiment and 2) the SOC stock trend in a baseline, i.e., a control treatment without EOM addition. Then, we used 11 long-term experiments (LTEs) with EOM addition in European croplands to estimate the amount of carbon (C) input needed to reach the 0.1 and 0.4% SOC stock increase targets proposed by the Mission Board for Soil Health and Food, calculated with two different approaches. We found that, to reach a 0.1 and 0.4% increase target relative to the onset of the experiment, 2.51 and 2.61 Mg C ha−1 yr−1 of additional C input were necessary, respectively. Reaching a 0.1 and 0.4% increase target relative to the baseline required 1.38 and 1.77 Mg C ha−1 yr−1 of additional input, respectively. Depending on the calculation method used, the estimated amounts of additional C input required to reach each quantitative target were significantly different from each other. Furthermore, the quality of C input as represented by the C retention rate of the additional organic material (EOM and crop residue), had a significant effect on the variation of SOC stocks. Our work highlights the necessity to take into consideration the additional C input required to increase SOC stocks, especially for soils with decreasing SOC stocks, when targets are set independently of the baseline.

Increasing soil organic carbon (SOC) stocks is a promising way to mitigate the increase in atmospheric CO2 concentration. Based on a simple ratio between CO2 anthropogenic emissions and SOC stocks worldwide, it has been suggested that a 0.4% (4 per 1000) yearly increase in SOC stocks could compensate for current anthropogenic CO2 emissions. Here, we used a reverse RothC modelling approach to estimate the amount of C inputs to soils required to sustain current SOC stocks and to increase them by 4‰ per year over a period of 30 years. We assessed the feasibility of this aspirational target first by comparing the required C input with net primary productivity (NPP) flowing to the soil, and second by considering the SOC saturation concept. Calculations were performed for mainland France, at a 1 km grid cell resolution. Results showed that a 30%–40% increase in C inputs to soil would be needed to obtain a 4‰ increase per year over a 30‐year period. 88.4% of cropland areas were considered unsaturated in terms of mineral‐associated SOC, but characterized by a below target C balance, that is, less NPP available than required to reach the 4‰ aspirational target. Conversely, 90.4% of unimproved grasslands were characterized by an above target C balance, that is, enough NPP to reach the 4‰ objective, but 59.1% were also saturated. The situation of improved grasslands and forests was more evenly distributed among the four categories (saturated vs. unsaturated and above vs below target C balance). Future data from soil monitoring networks should enable to validate these results. Overall, our results suggest that, for mainland France, priorities should be (1) to increase NPP returns in cropland soils that are unsaturated and have a below target carbon balance and (2) to preserve SOC stocks in other land uses. [ABSTRACT FROM AUTHOR]

Aims: Increasing soil organic carbon (SOC) stocks is discussed as negative emission technology with the potential to remove relevant amounts of carbon from the atmosphere. At the same time, climate change-driven losses of SOC to the atmosphere might impede such goals. Methods: In this study, we used an ensemble of different SOC models and climate projections to project SOC stocks in German croplands up to 2099 under different climate change scenarios. We then estimated the required increase in organic carbon (OC) input to preserve or increase SOC stocks. Results: Projected SOC stocks of German croplands are estimated to decline under current OC input levels and management, both with and without climate change. Depending on the climate scenario, we estimated that the OC input to the soil in 2099 needs to be between 51% (+ 1.3 Mg ha− 1) and 93% (+ 2.3 Mg ha− 1) higher than today to preserve current SOC stock levels. A SOC stock increase of 34.4% (4‰ a− 1) would even require an OC input increase of between 221% (+ 5.5 Mg ha− 1) and 283% (+ 7.1 Mg ha− 1). Conclusions: Our study highlights that under climate change increasing SOC stocks is considerable challenging since projected SOC losses have to be compensated first before SOC built up is possible. This would require unrealistically high OC input increases with drastic changes in agricultural management. [ABSTRACT FROM AUTHOR]

• The role of scientists in the design of a multistakeholder partnership, the "4 per 1000″ initiative, is analysed. • The involvement of soil scientists leads to keep discussions at a technical level while the initial ambition was political. • The technical focus reinforces the credibility of the initiative but leads some civil society organisations to stay outside. • The case study leads to support the idea of accountability as an integral part of scientific engagement in the policy process. The proliferation of multistakeholder initiatives is deemed to be a key vehicle for the implementation of international agreements for sustainable development. This is the case with the "4 per 1000: Soils for Food Security and Climate" initiative (4PM), launched in March 2015 by French Minister Stéphane Le Foll, whose aim is to increase soil organic carbon (SOC) stocks worldwide, in order to simultaneously address climate change mitigation, adaptation, and food security issues. This paper explores the knowledge dynamics at play and the role scientists played in the early development of the initiative. It shows that the strong involvement of soil scientists in the design of 4 PM, as well as in the enrolment of other stakeholders, has had a dual consequence. On the one hand, it has reinforced the initiative's credibility by keeping most discussions at a mere technical level, considering soil organic carbon sequestration as the main (if not the sole) proxy for climate change mitigation, adaptation and food security. On the other hand, this technical approach has led the initiative to be hardly able to enrol representatives from peasant and familial agricultures, who have criticized the absence of clear definitions of the type of agricultural models the 4 PM intends to promote. We show that this has had detrimental consequences on the results of the 4 PM initiative, and we discuss the implications both for the framing of voluntary multistakeholder initiatives and for the engagement of scientists in such settings, notably in terms of their accountability. [ABSTRACT FROM AUTHOR]

Considerable advances have been made in the assessment and mapping of soil organic carbon stocks, but rates of change in carbon stocks remain to be quantified for many soils and ecosystems. We sampled 145 sandy soils (mostly Psamments) under permanent cultivation and forest. We used aerial imagery to determine the period of cultivation and to calculate changes in soil organic carbon stocks. Topsoil organic carbon stocks, including the A and O horizons, were highest in soils under forest which were never cultivated (36 Mg C ha−1) and lowest in soils under red pine with prior cultivation (31 Mg C ha−1). Average soil organic carbon stocks of the A horizons of cultivated soils were 33 Mg C ha−1. To meet the 4 per 1000 international initiative, these soils need to achieve a soil organic carbon sequestration rate of 0.1 Mg C ha−1 yr−1. A mean rate of change of −0.16 Mg C ha−1 year−1 was found. The A horizon thickness increased under cultivation, but soil organic carbon concentrations decreased leading to reduced soil organic carbon stocks. The decline in soil organic carbon stock could be explained by an increased rate of organic matter decomposition due to tillage, irrigation, nitrogen applications, and lower clay and silt contents. After about 70 years of afforestation with red pine, soil organic carbon stocks increased. The O horizon accrued organic carbon at a rate of +0.19 Mg ha−1 yr−1, but soil organic carbon stocks were lower in the A horizon compared to cultivated soils. Afforestation of abandoned cultivated fields maintained soil organic carbon in the A horizon and gained organic carbon in the O horizon, but the soil organic carbon stocks were below soils under forest which were never cultivated. • Cultivated sandy soils have thicker A horizons compared to forested soils. • Topsoil (A & O horizons) stocks were highest under never cultivated forests. • Temporal analyses revealed declining soil organic carbon stocks under cultivation. • 4 per 1000 carbon sequestration goal was met through afforestation.. [ABSTRACT FROM AUTHOR]

Soil organic carbon (SOC) was re-analysed 10 years after application of source-segregated green waste compost at a 1 ha previously-developed UK site to compare with the increases suggested by the 4 per mille initiative proposed at COP21 in Paris. Compost prepared to PAS100 standard had been incorporated once at rates of 250, 500 and 750 t·ha−1 in 2007 in clay subsoil prior to planting of perennial energy crops. Our results show statistically significant differential increases in SOC, total nutrients N and P, or contaminants Zn, Pb, Cu, As and B, remain from the compost application after a decade. For the 500 or 750 t·ha−1 compost rates the SOC increments in the upper 10–15 cm were 0.85% or 1.6% over the 4.9% developed from a baseline of 1.8% in control areas by a decade of natural regeneration. Calculation of the elemental loadings from the compost analyses compared to the present-day levels suggests SOC declines after application at an average annual rate approaching 10%, compared to 5% for the nutrients or contaminants, roughly equivalent to half-lives of 5 or 10 years respectively. The study demonstrates the long-term soil organic matter (SOM) additions, fertility benefits and technical feasibility of a one-off, high-rate application of waste-derived compost to improve urban soils, compared to the potential trade-off of adding to PTE loadings. This longevity of SOC addition, previously unrecognised in brownfield soils, may be inferred for other areas where further cultivation is precluded, as is typical after landscape restoration or under perennial energy crops for the production of biomass. This unprecedented result has wider implications for marginal land use for bioenergy and the opportunities therein for SOC management using anthropogenic organic wastes to mitigate greenhouse gas emissions. Unlabelled Image • SOC from green waste compost addition to brownfield soils declines at 7–9% pa. • Mineralisation of N from compost is 4–5% pa under brownfield energy crops. • Pb and Zn levels from compost decline in soil at similar rates to major nutrients. • One-off high-rate compost applications for restoration provide >10-year benefits. • SOC added from >500 t·ha−1 compost averages +0.4% per annum after a decade. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Net primary production and carbon inputs govern carbon storage in cropland. • Carbon storage in cropland depends on the trajectory of regional agro-food system. • Three historical periods of carbon storage in cropland were identified. • Currently carbon storage does not offset the emissions of the agricultural sector. • Mitigation of C emissions requires a reconfiguration of agro-food system metabolism. Abstract Following the COP 21, it has been widely recognized that agricultural soils may contribute toward mitigating or enhancing climate change depending on their behavior as a sink or a source of carbon (C) to the atmosphere. Here, we studied the case of soil organic carbon (SOC) stocks in French cropland at the regional scale from 1852 to 2014 by coupling the GRAFS approach (Generalized Representation of Agro-Food System) with the AMG model (after its authors names: Andriulo, Mary, Guérif). Our results emphasize the role of crop net primary production (NPP) as the main driver of changes in SOC stocks through the input of crop residues. Three distinct phases of NPP were identified: (1) from 1852 to 1950, a slow rise in NPP occurred, which induced a moderate increase of cropland SOC pools (1–2‰ yr−1); (2) from 1950 to 1980, NPP increased considerably, resulting in a much higher rate of SOC accumulation (2–4‰ yr−1); (3) from 1980 to 2014, divergent trajectories were followed according to regions with either a slowdown in NPP growth with moderate increase in SOC stocks (0–2‰ yr−1), or a reversed trend toward declining NPP induced by a reduction of fodder production, replaced by imported feed which led to negative change of cropland SOC stocks. We highlight that these evolutions of carbon dynamics in cropland were closely linked to changes in the French social political context, from the second Empire to the Cold War and the advent of a neo-liberal era. As a whole, for France, we found a rate of C sequestration in the top 30 cm of cropland soils of about 1.7 Mt C yr−1 for the last two decades, less than one fifth of the current fossil-fuel-based CO 2 emission rate of the French agricultural sector which exceeds 8 Mt C yr−1. Our study showed that in developed countries like France, where a sharp increase in cropland SOC stocks occurred owing to the modernization of agriculture, this should be considered as a "fossil-fuel-powered carbon sink", which does only partly compensate for the direct emissions through mechanization and fertilizer production that induced it. [ABSTRACT FROM AUTHOR]

We evaluated the "4 per 1000" initiative for increasing soil organic carbon (SOC) by analysing rates of SOC increase in treatments in 16 long-term experiments in southeast United Kingdom. The initiative sets a goal for SOC stock to increase by 4& per year in the 0-40 cm soil depth, continued over 20 years. Our experiments, on three soil types, provided 114 treatment comparisons over 7-157 years. Treatments included organic additions (incorporated by inversion ploughing), N fertilizers, introducing pasture leys into continuous arable systems, and converting arable land to woodland. In 65% of cases, SOC increases occurred at >7% per year in the 0-23 cm depth, approximately equivalent to 4& per year in the 0-40 cm depth. In the two longest running experiments (>150 years), annual farmyard manure (FYM) applications at 35 t fresh material per hectare (equivalent to approx. 3.2 t organic C/ha/year) gave SOC increases of 18% and 43% per year in the 23 cm depth during the first 20 years. Increases exceeding 7% per year continued for 40-60 years. In other experiments, with FYM applied at lower rates or not every year, there were increases of 3%-8% per year over several decades. Other treatments gave increases between zero and 19& per year over various periods. We conclude that there are severe limitations to achieving the "4 per 1000" goal in practical agriculture over large areas. The reasons include (1) farmers not having the necessary resources (e.g. insufficient manure); (2) some, though not all, practices favouring SOC already widely adopted; (3) practices uneconomic for farmers--potentially overcome by changes in regulations or subsidies; (4) practices undesirable for global food security. We suggest it is more realistic to promote practices for increasing SOC based on improving soil quality and functioning as small increases can have disproportionately large beneficial impacts, though not necessarily translating into increased crop yield. [ABSTRACT FROM AUTHOR]

Soil organic carbon is a sink for mitigating increased atmospheric carbon. The international initiative “4 per 1000” aims at implementing practical actions on increasing soil carbon storage in soils under agriculture. This requires a fundamental understanding of the soil carbon changes across the globe. Several studies have suggested that the global soil organic carbon stocks (SOCS) have decreased due to global warming and land cover change, while others reported SOCS may increase under climate change and improved soil management. To better understand how a changing climate, land cover, and agricultural activities influence SOCS across large extents and long periods, the spatial and temporal variations of SOCS were estimated using a modified space-for-time substitution method over a 150-year period in the state of Wisconsin, USA. We used legacy soil datasets and environmental factors collected and estimated at different times across the state (169,639 km2) coupled with a machine-learning algorithm. The legacy soil datasets were collected from 1980 to 2002 from 550 soil profiles and harmonized to 0.30 m depth. The environmental factors consisted of 100-m soil property maps, 1-km annual temperature and precipitation maps, 250-m remote-sensing (i.e., Landsat)-derived yearly land cover maps and a 30-m digital elevation model. The model performance was moderate but can provide insights on understanding the impacts of different factors on SOCS changes across a large spatial and temporal extent. SOCS at the 0−0.30 m decreased at a rate of 0.1 ton ha−1 year−1 between 1850 and 1938 and increased at 0.2 ton ha−1 year−1 between 1980 and 2002. The spatial variation in SOCS at 0−0.30 m was mainly affected by land cover and soil types with the largest SOCS found in forest and wetland and Spodosols. The loss between 1850 and 1980 was most likely due to land cover change while the increase between 1980 and 2002 was due to best soil management practices (e.g., decreased erosion, reduced tillage, crop rotation and use of legume and cover crops).

A review is made of the current state of agriculture, emphasising issues of soil erosion and dependence on fossil fuels, in regard to achieving food security for a relentlessly enlarging global population. Soil has been described as "the fragile, living skin of the Earth", and yet both its aliveness and fragility have all too often been ignored in the expansion of agriculture across the face of the globe. Since it is a pivotal component in a global nexus of soil-water-air-energy, how we treat the soil can impact massively on climate change - with either beneficial or detrimental consequences, depending on whether the soil is preserved or degraded. Regenerative agriculture has at its core the intention to improve the health of soil or to restore highly degraded soil, which symbiotically enhances the quality of water, vegetation and land-productivity. By using methods of regenerative agriculture, it is possible not only to increase the amount of soil organic carbon (SOC) in existing soils, but to build new soil. This has the effect of drawing down carbon from the atmosphere, while simultaneously improving soil structure and soil health, soil fertility and crop yields, water retention and aquifer recharge - thus ameliorating both flooding and drought, and also the erosion of further soil, since runoff is reduced. Since food production on a more local scale is found to preserve the soil and its quality, urban food production should be seen as a significant potential contributor to regenerative agriculture in the future, so long as the methods employed are themselves 'regenerative'. If localisation is to become a dominant strategy for dealing with a vastly reduced use of fossil fuels, and preserving soil quality - with increased food production in towns and cities - it will be necessary to incorporate integrated ('systems') design approaches such as permaculture and the circular economy (which minimise and repurpose 'waste') within the existing urban infrastructure. In addition to growing food in urban space, such actions as draught-proofing and thermally insulating existing building stock, and living/working on a more local scale, would serve well to cut our overall energy consumption. In order to curb our use of fossil fuels, methods for reducing overall energy use must be considered at least equally important to expanding low-carbon energy production. In synopsis, it is clear that only by moving from the current linear, 'take, make, dispose (waste-creation)' model for resource-consumption, to the systemic, circular alternative of 'reduce, reuse, recycle, regenerate', are we likely to meet demands for future generations. [ABSTRACT FROM AUTHOR]

Crop residues contribute to the maintenance of soil organic carbon (SOC) stores, a key component of soil fertility and soil-based climate change mitigation strategies, such as the ‘4 per 1000’ initiative. Residues are also in demand in sectors coupled to crop production, such as the supply chain of livestock and bioenergy production. Ongoing debate revolves around balancing these competing uses, but science-based assessments of the long-term sustainability of residue exploitation are rare. This work uses biophysical simulation modelling to explore the likely response of SOC to different management strategies, using the land area of North Rhine-Westphalia (Germany) as a case study. Four strategies are tested: zero, one third and 100% removal of cereal residues, plus an approach proposed by the State farm advisory service. Simulations are carried out for the period 1971–2050 and 19 crop rotations coincident with land use throughout the study area. Uncertainty associated with the modelled SOC changes is explored by sampling values of relevant parameters for SOC turnover and running an ensemble of model configurations. Simulated SOC is used to trace time-dependent response functions following a change in residue management under different soil textures, initial SOC levels and crop rotations. Results highlight a general exponential decrease in SOC, with relative changes in 2050 distributed between +10% and −40% with respect to a reference period. SOC loss can be buffered or offset by returning all crop residues to the soil. Under such management, an SOC increase can be achieved on clayey soils characterized by a low initial SOC. Under moderate crop residue removal, positive SOC trends are limited to a few crop rotations. In this context, 4 per 1000 increase rate in SOC appears largely out of reach through residue management, calling for additional measures to meet the targets of land-based mitigation of anthropogenic emissions.

The effects of climate variations on crop yield have been widely studied. However, the effects of soil on crop-climate responses are often ignored in crop yield prediction. We investigated the effects of soil texture and soil organic carbon concentration (SOC) on the yield responses of seven major crops (corn, winter wheat, soybean, cotton, barley, oats, rice) to growing season precipitation and temperature between 1958 and 2019 across the conterminous US. We also evaluated the effects of irrigation and groundwater depth on crop-climate responses. Crop yields were most sensitive to precipitation and temperature variability in coarse-textured soils and less responsive to these weather parameters in medium- and fine- textured soils. Increasing SOC concentration (> 2%) contributed to crop yields being less sensitive to precipitation – due to increased water retention, and less responsive to temperature – presumably due to increased buffering capacity against increased water lost through evapotranspiration. Irrigation and an intermediate depth to groundwater increase the resilience of crops to precipitation and temperature changes and these effects were also dependent on soil texture and SOC. To enhance food security for a rapidly growing global population under a changing climate, best management practices should be adopted that improve soil structure and carbon stocks that can increase soil available water storage ("Green Water") and nutrient retention and promote energy conservation. The spatial-temporal variations of soil texture, SOC, and depth to groundwater should be considered in agricultural and ecosystem modeling to more accurately capture crop yield response to climate variations. [Display omitted] • Effects of soil texture and organic carbon on crop-climate responses evaluated. • Crop are more sensitive to precipitation and temperature in coarse-textured soils. • Increasing soil organic carbon reduces crop sensitivity to climate variability. • Irrigation and intermediate groundwater depth increases crop resilience to climate. • Effects of soil texture and carbon should be included in modeling and policy-making. [ABSTRACT FROM AUTHOR]

Soil organic carbon is a sink for mitigating increased atmospheric carbon. The international initiative "4 per 1000" aims at implementing practical actions on increasing soil carbon storage in soils under agriculture. This requires a fundamental understanding of the soil carbon changes across the globe. Several studies have suggested that the global soil organic carbon stocks (SOCS) have decreased due to global warming and land cover change, while others reported SOCS may increase under climate change and improved soil management. To better understand how a changing climate, land cover, and agricultural activities influence SOCS across large extents and long periods, the spatial and temporal variations of SOCS were estimated using a modified space-for-time substitution method over a 150-year period in the state of Wisconsin, USA. We used legacy soil datasets and environmental factors collected and estimated at different times across the state (169,639 km2) coupled with a machine-learning algorithm. The legacy soil datasets were collected from 1980 to 2002 from 550 soil profiles and harmonized to 0.30 m depth. The environmental factors consisted of 100-m soil property maps, 1-km annual temperature and precipitation maps, 250-m remote-sensing (i.e., Landsat)-derived yearly land cover maps and a 30-m digital elevation model. The model performance was moderate but can provide insights on understanding the impacts of different factors on SOCS changes across a large spatial and temporal extent. SOCS at the 0–0.30 m decreased at a rate of 0.1 ton ha−1 year−1 between 1850 and 1938 and increased at 0.2 ton ha−1 year−1 between 1980 and 2002. The spatial variation in SOCS at 0–0.30 m was mainly affected by land cover and soil types with the largest SOCS found in forest and wetland and Spodosols. The loss between 1850 and 1980 was most likely due to land cover change while the increase between 1980 and 2002 was due to best soil management practices (e.g., decreased erosion, reduced tillage, crop rotation and use of legume and cover crops). [ABSTRACT FROM AUTHOR]