Optimizing Carbon Sequestration in Croplands: A Synthesis

Abstract

Climate change and ensuring food security for an exponentially growing global human population are the greatest challenges for future agriculture. Improved soil management practices are crucial to tackle these problems by enhancing agro-ecosystem productivity, soil fertility, and carbon sequestration. To meet Paris climate treaty pledges, soil management must address validated approaches for carbon sequestration and stabilization. The present synthesis assesses a range of current and potential future agricultural management practices (AMP) that have an effect on soil organic carbon (SOC) storage and sequestration. Through two strategies—increasing carbon inputs (e.g., enhanced primary production, organic fertilizers) and reducing SOC losses (e.g., reducing soil erosion, managing soil respiration)—AMP can either sequester, up to 714 ± 404 (compost) kg C ha⁻¹ y⁻¹, having no distinct impact (mineral fertilization), or even reduce SOC stocks in the topsoil (bare fallow). Overall, the carbon sequestration potential of the subsoil (>40 cm) requires further investigation. Moreover, climate change, permanent soil sealing, consumer behavior in dietary habits and waste production, as well as the socio-economic constraints of farmers (e.g., information exchange, long-term economic profitability) are important factors for implementing new AMPs. This calls for life-cycle assessments of those practices.

1. Introduction

Terrestrial soils store twice as much carbon as the atmosphere [1,2,3], with an estimated soil carbon storage of 1462 to 1584 Pg in the upper 100 cm of the soil profile [4]. Historical land-use changes—especially the conversion of grasslands and forests to cropland—as well as crop management practices have provoked a significant decrease in soil organic carbon (SOC) stocks, thus leading to enhanced carbon dioxide (CO₂) emissions to the atmosphere [5]. Combatting climate change requires a reduction in atmospheric CO₂ concentrations, which can be achieved by both reducing CO₂ emissions and increasing carbon sinks [3]. The total organic carbon content of soils is an important soil quality indicator that has an impact on different soil functions—including primary productivity and climate regulation—and ecosystem services, and its loss is one of the soil threats addressed in the Soil Thematic Strategy [6,7,8,9].

The 4-per-mille initiative launched by the French Minister of Agriculture at the UN Climate Change Conference (COP21) in Paris (2015) aims to increase SOC stocks by 0.4 percent per year through optimized land and soil management in agricultural soils [10]. SOC stock boosting management measures such as fertilization, diverse crop rotations, cover cropping, and reduced tillage practices have been extensively discussed over the last 20 years and there is agreement that those beneficial practices must be maintained [10,11,12]. Furthermore, long-term monitoring, at least for more than 10 years, is necessary [10,12,13] to detect changes in SOC stocks. One of the challenges to measure these changes is linked to the uncertainties of sampling or analyzing SOC and of determining soil bulk density [14]. SOC stocks are always a result of soil organic matter (SOM) stabilization and decomposition processes in soil. In the past 10 years, a paradigm shift has occurred, indicating that the biotic and abiotic environment is more important for the persistence of SOM than the qualitative information organic matter contains, including its complexity and composition [15,16,17]. However, other publications emphasize the enormous importance of structural features and other qualitative information about SOM [6]. The authors of [18] evaluated 20 different fractionation methods and highlight that fractionation of SOC (in fact SOM) is crucial to understand SOM decomposition and stabilization processes. Hence, SOC can be located in different C pools with distinct turnover times, and changes in land use and agricultural management may have different effects on these C pools [19,20]. Labile pools may accumulate much faster but are also more prone to losses than stable pools. The authors of [21] conclude that (energy) crops with high root biomass and high annual root productivity may help enhance physically protected SOC stocks, even if the total stock does not change. Nonetheless, the authors of [15] suggest to focus on a continuum of organic compounds rather than on differentiating between C pools with different turnover times and on the protection of these compounds. Besides agricultural land-use practices, in temperate regions the main explanatory variables for SOC storage are: Soil texture, soil water regime, biological activity, soil C/N ratio, total SOC content, soil pH, climate, vegetation, as well as land-use history of the sites [20].

Climate change along with changes in the soil water regime, the vegetation, or in soil management may further change SOC stocks. In general, higher temperatures and unchanged precipitation patterns increase soil C turnover, which means that a higher proportion of the soil organic matter stocks is converted or mineralized. In order to maintain or increase the current SOC stock, more organic matter must therefore be added (harvest residues, cover crops, field forage), or processes promoting mineralization (soil cultivation) must be reduced [22,23]. Overall, the intensity and type/mode of management practice influence the soil organic carbon stocks of cropland soils [24,25,26]. Thus, benefits and trade-offs of a certain agricultural management practice should be considered more precisely prior its application. Here, we synthesize the most recent scientific literature regarding the impact of agricultural management practices on cropland SOC stocks. We selected nine management practices that have been extensively studied in long-term agricultural field experiments:

(1)

Mineral fertilization

(2)

Organic amendments

(3)

Crop residues

(4)

Plant cultivation, including cover and deep-rooting crops

(5)

Tillage

(6)

Organic farming

(7)

Irrigation

(8)

Biochar

(9)

Lignocellulosic crops (e.g., agroforestry, bioenergy production) and

(10)

Application of inorganic carbon.

We classified these practices according to their effect on SOC, availability and the technological requirements for the farmers. For each management practice, we identified gaps of knowledge and emphasized their potential trade-offs. Beyond the environmental dimensions, increasing soil organic carbon via agricultural management practices will also influence sociological, economic and ethical aspects of our society. In this review, we synthesize the most recent scientific literature on how carbon sequestration can be optimized in croplands. Overall, this review is structed the following: In Section 2 we briefly explained the scientific foundation of SOC dynamics. In Section 3 we concentrate on current and potential future agricultural management practices leading to a synopsis enquiring if soils an unlimited carbon sink (Section 4) and conclusions (Section 5).

2. Scientific Foundation

2.1. Carbon Saturation Concept

Before going into the details of agricultural management practices and their responses to carbon sequestration, note that the soil mineral size fraction is crucial for SOC stabilization [27], especially fine-grained minerals (<20 µm); this includes clay minerals and oxides/hydroxides that are responsible for stabilizing (long residence time) organic carbon in soils [28,29,30]. They protect organic carbon from leaching through organo-mineral interactions (physico-chemical interactions) or limit the accessibility of organic compounds to the soil microbial community or restrict the oxygen supply for decomposers (physical protection) [29,31]). The mineral fraction < 20 µm is known to determine the theoretical maximum of carbon stored. The assumption is therefore that soils have a finite capacity to store carbon (“carbon saturation capacity”) [32]. In that concept, the organic carbon associated with the coarse-sized particles is neglected because organic matter associated with sand-sized fraction is lost rapidly [32]. Overall, the building up of SOC stocks features a progressive development with marginal increases in the SOC stock once saturation is approached. Consequently, SOC stock increases are not directly proportional to carbon inputs. Furthermore, adding fresh organic material can promote the mineralization of “old” SOC (“priming effect”). This means that soil is not an unlimited sink for carbon [22]. At the same time, the carbon sequestration potential of European cropland could be limited [13,33]. Only depleted soils can temporarily accumulate carbon up to their optimum C content [13]. The soil organic carbon content of the topsoil (0–20 cm) remained constant [33] or declined by 25% [34] when constant agricultural management practices, such as fixed crop rotation, were applied on fields over the last 20 years—this is the “business as usual” variant. As a consequence, achieving the “4-per-mille initiative” goal might be challenging.

2.2. Carbon Sequestration

The evaluation of soil management practices for sequestering atmospheric carbon in agricultural topsoils can be misleading because the terms carbon sequestration and carbon storage are often used interchangeably [25]. In order to avoid misunderstandings, we adopted the definitions of carbon sequestration as formulated in [35]: “Process of transferring CO₂ from the atmosphere into the soil of a land unit through a unit plant, plant residues and other organic solids, which are stored or retained in the unit as part of the soil organic matter (humus). The sequestrated SOC process should increase the net SOC storage during and at the end of a study to above the previous pre-treatment baseline.” The carbon sequestration potential of a certain management practice represents the maximum increase or decrease in SOC stock for a certain climate over a specific period of time and a certain soil depth, mainly 0–20 cm soil depth. Overall, a negative carbon sequestration potential represents a net loss of SOC, while a positive carbon sequestration corresponds to an increase of SOC stock. Except for recent management practices (e.g., biochar, agroforestry, bioenergy production, liming, and fertilization with silicate minerals), this review quantifies the carbon sequestration potential of agricultural management practices for 20 years [36].

2.3. Carbon Storage

The increase of carbon storage is defined as the increase of SOC stocks of a given land unit over a certain soil depth and period of time [36]. In the following, soil carbon storage is regulated by the organic carbon input and output, and changes in soil carbon storage do not necessarily account for the net removal of CO₂ from the atmosphere [22]. In other words, carbon storage is defined as a mass change for a certain soil depth over a certain time period, whereas the carbon sequestration is formulated as a rate for a certain soil depth over a certain time period.

3. Cropland Management Practices

3.1. Mineral Nitrogen Fertilization

Overall, the impact of nitrogen (N) fertilization on soil organic carbon (SOC) is characterized by two contrasting trends. On the one hand, N fertilization fuels primary production, thus enhancing the above- and belowground biomass, which in turn can enrich SOC stocks [37]. On the other hand, nitrogen fertilization can stimulate biodegradation of litter and soil organic matter [38]. This reduces SOC stocks [39]. Thus, an optimal nitrogen supply might be crucial for soil carbon sequestration [40]. In a global meta-analysis by [39], N fertilization reduced SOC in agricultural soils by 10%, whereas in another meta-analysis of 340 paired observations, N fertilization enhanced SOC storage by 3.5% [41]. From the dataset compiled by the authors of [41], N fertilization apparently mainly affected the plant organic matter pool, whereas the SOC pool was altered to a lesser degree. Furthermore, they attributed the stimulating effect of N fertilization on SOC storage to the higher growth of aboveground biomass and less to the root biomass. In contrast, the meta-analysis of [39] revealed that N fertilization fueled soil organic matter mineralization. The authors concluded that the earth system models should differentiate between organic carbon inputs from above- and belowground biomass because the impact of N fertilization on SOC storage was interrelated with (above- and belowground) biomass.

Soil organic matter can be stabilized physically through macro- and micro-aggregation or interactions with silt and clay particles [17] or/and be stabilized biochemically through the formation of recalcitrant SOM compounds [27,42]. Apparently, “old” SOM with a decadal or centennial turnover time was rich in nitrogen and soil protein [43]. The authors of [44] also assumed that protein-rich compounds are responsible for long-term soil organic matter stabilization/sorption. However, little experimental evidence is available for interactions between the nitrogen application rate and assimilation of recalcitrant organic carbon. In nitrogen-limited environments, the soil microbial community utilized nitrogen from stable SOM to meet its nutrient requirements for growth and reproduction [45,46]. Confirming the aforementioned, the authors of [47] determined that nitrogen fertilization improved the stable SOC pool (fine fraction < 0.4 mm) by enhancing litter decomposition and simultaneously reducing microbial nutrient mining. Nevertheless, the authors of [48] observed that mineral fertilization tended to increase the particulate organic matter in soils, while organic matter occluded in silt and clay sized fraction was decreased, yielding less stable forms of C.

The average carbon sequestration potential of mineral N fertilization in agricultural topsoils (0–10/30 cm) was—20 ± 210 kg C ha⁻¹ y⁻¹ [25,48,49,50,51] (Figure 1). Nevertheless, mineral fertilization decreased the SOC stock at a rate of −198 ± 29 kg C ha⁻¹ y⁻¹ and the unfertilized control revealed an even higher humus deficit of −457 kg C ha⁻¹ y⁻¹ [48]. Considering the greenhouse gas emissions during fertilizer production, the carbon sequestration potential of mineral fertilization remained neutral [52]. Especially in North America and Europe, mineral N fertilizer is already applied at a near optimum rate. This means a very low likelihood of further enhancing SOC stocks [50].

Similarly to solely mineral nitrogen fertilization, chemical nitrogen, phosphorus, and potassium fertilizers (NPK) increase crop yields [13,53], thus the elevated plant and root biomass increased soil microbial biomass as well as the SOC stock [22,54,55]. In contrast to no-fertilizer conditions, a meta- analysis of 84 long-term trails revealed that NPK, N, P, and K fertilization can enhance the SOC storage of the upper soil layer (0–20 cm) by 10, 5, 5, and 2%, respectively [55]. Furthermore, they found out that N and P fertilization increased the root biomass and root exudates, which in turn enhanced the SOC storage. Moreover, N/P ratio negatively corresponds to soil C storage. Long- term fertilization experiments with different soil types and cropping systems revealed that the impact of mineral NPK fertilizers on SOC sequestration rely on soil parameters, mainly the soil available N content and soil pH [56]. Besides enhanced primary production, P fertilizers can affect soil C sequestration by impacting arbuscular mycorrhizal fungi. In contrast to solely nitrogen fertilizers, the application of NPK reduces the colonization of arbuscular mycorrhizal fungi, thus reducing mycorrhiza-mediated nutrient plant uptake, which in turn negatively affects the soil C sequestration [57,58]. However, the role of arbuscular mycorrhizal fungi in the terrestrial carbon cycling is largely overlooked, especially its impact on the carbon sequestration potential [59].

Overall, the nutrient stoichiometry has shown to impact the microbial biomass C assimilation and re-assimilation of SOM (C:N:P:S 10,000:833:200:143) [60].

3.1.1. Knowledge Gaps

Overall, the soil microbial community plays a major role in controlling terrestrial carbon cycling [16]. The carbon use efficiency, which determines the fate of carbon during decomposition, is strongly linked to nitrogen and phosphorus (P) availability and may be further affected by the accessibility of potassium and micronutrients [61,62]. These connections need to be addressed by further research. Above all, life cycle assessments (LCA) would enable a coherent synopsis of mineral N fertilizers.

3.1.2. Trade-Offs

Fertilization with reactive N increases nitrous oxide (N₂O) emissions from soils. These N₂O losses associated with N fertilization can be reduced by using an appropriate fertilization rate (amount, timing), by enhancing the fertilizer use efficiency as well as by incorporating N-fixing crops in the crop rotation [63,64]. Importantly, the frequent overuse of mineral N fertilization can adversely affect adjacent water bodies and biodiversity (eutrophication) [65].

Overall, CO₂ emissions emitted by fertilizer production exceed the sequestered amount of soil carbon [51]. Globally, 2–3% of the anthropogenic CO₂ emissions are emitted during the production of mineral fertilizers [66], and the net greenhouse gas emissions for mineral fertilization amounted to 2000 kg CO₂-eq ha⁻¹ y⁻¹ [48].

Long-term mineral N fertilization increases nitrogen availability in soils, whereby changes in the nutritional status over a long time period can contribute to eutrophication [67].

3.2. Organic Amendments

Organic amendments affect the SOC pool in two ways: (I) Organic fertilization stimulates net primary production; thus, atmospheric carbon is fixed through photosynthesis [68,69,70]; (II) organic amendments are an additional source for the existing SOC pool [71]; (III) similarly to mineral fertilization, organic fertilization may stimulate SOC biodegradation [22]. Note, however, that using external carbon sources may not contribute to climate change mitigation [66,70,72]. Applying organic fertilizers mostly results in a displacement, with high organic carbon concentrations at specific sites but reduced concentrations at donating sites [72]. Overall, the alternative usage of organic material is crucial and net sequestration will occur (a) when organic fertilizers are produced for a particular cropland field or (b) when the carbon of the existing fertilizer would otherwise be lost, e.g., to the atmosphere by burning [70] or with food wastes.

3.2.1. Farmyard Manure

Organic fertilizer such as cattle/pig/poultry manure is rich in organic matter and can therefore enhance the soil organic carbon stock [11,24,25,49,50,51,73]. However, such organic amendments can increase soil pH, which in turn can enhance the solubility of soil organic matter [74]. Furthermore, farmyard manure appears to be rapidly degraded in soil; this enhanced microbial activity (priming effect) may reduce the SOC stock [75]. Beyond enhancing biodegradation, the application of farmyard manure is reported to improve soil structure and water holding capacity in agriculturally managed soils [51].

The average carbon sequestration potential of farmyard manure amounted to 292 ± 132 kg C ha⁻¹ y⁻¹ [11,24,25,49,50,51,73] (Figure 1). In temperate climates, solid manure application of 5 to 10 t ha⁻¹ y⁻¹ resulted in a carbon sequestration rate of 160 kg C ha⁻¹ y⁻¹ [11]. Overall, the impact of farmyard manure on the SOC stock varied among types. The authors of [76] reported that in Denmark, during the first 8 years after implementation, cattle slurry, pig slurry, and anaerobically digested slurry increased the SOC stock of the topsoil (0–25 cm) by 300 kg C ha⁻¹ y⁻¹, 200 kg C ha⁻¹ y⁻¹ and 200 kg C ha⁻¹ y⁻¹, respectively. A recent meta-analysis of 217 datasets showed that farmyard manure can enhance the carbon sequestration potential of agricultural topsoils (0–20/30 cm) by 409 kg C ha⁻¹ y⁻¹ [24]. In contrast to mineral fertilization, organic fertilization enhanced the carbon sequestration potential of Mediterranean soils by 23.5% [52]. An analysis of 80 long-term experimental sites in Europe showed that bovine farmyard manure increased SOC stocks by 33%, while liquid slurry increased the stocks by 17%, a value that was also achieved through mineral fertilizer application (16%) [77]. However, the impact of farmyard manure on SOC declined with time and, after a decade, no SOC enhancement was recorded [52]; in another case, a new equilibrium was reached after 50 years [78].

3.2.2. Compost Application

Compost consists mostly of plant-based material, e.g., biowaste or sewage-sludge, which is decomposed under aerobic conditions in a controlled environment [79]. The composition/quality of the final compost varies according to the source and duration of the maturation phase. Consequently, the quality of composts are inhomogeneous and they may even contain toxic substances such as heavy metals and organic contaminants [80]. In long-term field experiments, fertilization with compost enhanced SOC stocks [25,48,81]. In a comparative long-term study with four different compost types, the authors of [81] pointed out that compost application should follow a regular pattern if SOC storage is to be improved. Overall, the average carbon sequestration potential of compost application was 714 ± 404 kg C ha⁻¹ y⁻¹ [25,48,49,50,51,73] (Figure 1). Additionally, that value varied with the application rate: Rates of 8, 14 and 20 t ha⁻¹ y⁻¹ could potentially sequestered 115, 558 and 1021 kg C ha⁻¹ y⁻¹ [48].

3.2.3. Knowledge Gaps

Overall, the chemical analysis of manure and compost is often restricted to total C content, C/N ratio and pH [71], thus limiting the assessment and synthesis of field studies. Increasingly, methods evaluating the decomposition of organic fertilizers through short-term incubations or biochemical characterization [82,83] are being used to parameterize the quality of organic fertilizers. Those results can be further used to parameterize SOC models and may help in reassessing the impact of organic fertilizers on SOC pools (long-term/recalcitrant soil organic matter). Despite the advantages of these new approaches, studies evaluating the decomposition of organic fertilizers are rarely implemented [22]. Furthermore, an LCA would call for a precise analysis of the benefits and trade-offs of organic fertilizers.

3.2.4. Trade-Offs

Animal husbandry is a dominant source of greenhouse gases, and industrial animal husbandry annually emits 5.6–7.6 Gt CO₂-eq globally [84]. In the future, improved feed digestibility, grazing management, cultivation of legumes, and manure management (designed to reduce losses via volatilization and runoff) could potentially halve N₂O and CH₄ emissions from animal husbandry [84]. Additionally, CO₂ emitted during transportation could outrange the carbon sequestration potential of manure [70]. Thus, organic fertilizers are not always available on site, and emissions caused by transportation over 100 km can exceed the carbon sequestration potential by 30% [51]. Importantly, the process of composting also emits N₂O [51].

3.3. Crop Residues

Besides enhancing SOC stocks, the incorporation of crop residues into agricultural soils improves soil structure, reduces bulk density, reduces evaporation, decreases erosion and improves the infiltration rate in soils [23,85,86,87]. In agricultural management systems, where straw is used for thermal energy production or animal feeding/bedding, stubbles are cut short and the straw is removed. This reduces the overall amount of crop residues [88]. Generally, maize or perennial crops produce abundant residues, whereas root crops such as potatoes generate smaller amounts [11]. Residue C:N quality also influences SOC, and crop residues with a lower C:N ratio (e.g., soybean) promote microbial decomposition. This would enhance the decomposition of soil organic matter [89]. Crop residues with a higher C:N ratio (maize) were typically associated with a SOC build up [89,90]. At similar crop residue management, SOC stocks increased with yields [91]. Nonetheless, due to the application of growth regulators and the selection of crop types with short stems, crop residues might not increase proportionally with crop yield. The authors of [92] did, however, report that crop residues with a low C:N ratio could increase the SOC stocks.

The predominant view in the scientific literature is that incorporating crop residues yields an average carbon sequestration potential of 168 ± 67 kg C ha⁻¹ y⁻¹ in the upper soil layer (0–20/30 cm) [25,49,50,51,52,73] (Figure 1). In a meta-analysis of 39 publications involving long-term field experiments, residue incorporation enhanced SOC stocks by 7% [86]. The authors of [93] reported that a decade after incorporation of straw, the carbon sequestration reaches a new equilibrium, i.e., a stable SOC stock in topsoil. In contrast, ref. [78] observed an increase in SOC stocks in only six out of 25 straw incorporation studies.

3.3.1. Knowledge Gaps

Since the incorporation of crop residues into soils is connected with tillage, this physical disruption of the soil surface might pose a risk for the SOC stock. Future analyses should consider such potential effects [94] and also more closely evaluate the impact of soil-borne diseases on crop health.

3.3.2. Trade-Offs

Incorporating crop residues can promote N mineralization, potentially enhancing N₂O and CH₄ emissions [23,70,86]. However, these greenhouse gas emissions can be reduced by implementing suitable tillage practices [95]. Moreover, conflicting goals—retention of crop residues for increasing SOC stock versus using residues for energy production—must be considered. In the future, the increasing economic viability of bioenergy use (due to rising oil prices etc.) may reduce the availability of crop residues [96].

3.4. Plant Cultivation

3.4.1. Crop Species

Terrestrial vegetation is a crucial element in the global carbon cycle because plants assimilate more than 10% of the atmospheric carbon emissions through photosynthesis [97]. Carbon assimilated by plants is either integrated into the biomass, released as root exudates, or respired back as CO₂ [98]. These translocational processes depend on the genetic variability among crop species and genotypes [99]. On average, crops assimilated 4.5 Mg C ha⁻¹ y⁻¹, ranging from 1.7 Mg C ha⁻¹ y⁻¹ for barley (Hordeum vulgare) to 5.2 Mg C ha⁻¹ y⁻¹ for maize (Zea mays) [69]. These considerable differences reflect different internal C metabolisms [69]. In contrast to C3 crops such as barley, maize assimilates atmospheric C more efficiently through both the C4 photosynthetic pathway and a larger leaf area [100]. Overall, most (61%) of the assimilated carbon was transported to the shoots, 20% to roots and 7% were transferred to the soil [69]. Maize and ryegrass (Lolium perenne) had the greatest allocation to soil (1.0 Mg C ha⁻¹ y⁻¹ or 19% of the total assimilation), whereas wheat (Triticum aestivum) allocated 0.8 Mg C ha⁻¹ y⁻¹ (23% of the total assimilation) [69]. Overall, root growth and rhizodeposition were the main sources of SOC [101,102,103]. Globally, the belowground biomass could assimilate 24.7 ± 5.7 Pg C y⁻¹. Accordingly, the belowground net primary production makes up 46% of the global terrestrial net primary production [101]. On average, the dry root biomass was 177 ± 43 g m⁻² and 101 ± 26 g m⁻² for cereals (including barley and wheat) and cover crops, respectively [104]. However, these values were measured only in 30 cm soil depth. The subsoil carbon sequestration potential with deep-rooting crops is covered in section “Cultivation of deep-rooting crops” below.

3.4.2. Crop Rotation

The vegetal cover of agricultural soils and the manner of its management impacts SOC storage. The input of organic matter through plant biomass predominates in the topsoil and declines with soil depth [105]. In contrast to single cropping systems (monoculture with cereals or maize), diverse crop rotations with various main crops and/or perennial crops/forages and/or cover crops yielded distinctly higher SOC stocks [94,106]. In agricultural fields, crop diversity can be increased on a temporal (crop rotation, catch crops) and spatial scale (several plant species at the same time, cover crop mixture). Crop rotational diversity, the use of organic fertilizers/amendments and/or perennial cropping systems have the potential to accumulate more SOC than conventional (single) cropping systems [11,25]. Beyond its beneficial effects on SOC stocks, a diverse crop rotation can also increase soil microbial diversity, soil aggregate stability or even enhance organic carbon in the subsoil via deep-rooting crops [99,107,108]. Since roots have an up to 2.3 times higher retention of carbon than the aboveground biomass, deep-rooting crops are decisive for the SOC storage [101,102]. Again, this effect predominates in the topsoil and declines with soil depth [105]. The following sections discuss two suitable methods for sequestering carbon in a crop rotation—the cultivation of deep-rooting crops and planting catch crops—in further detail.

Overall, diversified crop rotational systems can enhance the carbon sequestration potential of agricultural topsoils (0–20/30 cm) by 216 ± 117 kg C ha⁻¹ y⁻¹ when compared with single cropping systems [25,94,109,110,111]. Compared to cereal-dominating cropping systems, incorporating legumes in the rotation in Sweden after 35 years enhanced the carbon sequestration potential by 360 and 590 kg C ha⁻¹ y⁻¹ in the topsoil (0–20 cm) at sites with clay and loam texture, respectively [112]. Moreover, in the loamy soils, changes of the SOC stock were detected along the whole soil profile (0–60 cm), whereas for the clay soils the subsoil SOC (>20 cm) remained unaffected [112]. The addition of soybean (Glycine max L.) into a corn monoculture increased the overall carbon sequestration potential of the topsoil by 200 ± 120 kg C ha⁻¹ y⁻¹. In a global meta-analysis, incorporating catch crops increased the SOC stock of the agricultural topsoil (0–22 cm) by 350 ± 80 kg C ha⁻¹ y⁻¹ [94]. In Denmark, SOC stocks of the topsoil (0–25 cm) increased with green manure by 400 kg C ha⁻¹ y⁻¹ [76].

Cultivation of Deep-Rooting Crops

Deep-rooting crop species and varieties can transfer carbon into the subsurface through root exudates (e.g., sugars, amino acids, and other organic acids), where a high carbon sequestration potential exists [113], especially if organic substances are protected in organo-mineral aggregates [3]. Plant species with deep-rooting systems are alfalfa (Medicago sativa), sunflower (Helianthus annuus), or perennial crops such as grass, grass-clover, and legume- and alfalfa-grass mixtures [114,115]. For instance, in Sweden, the implementation of grass-clover into the crop rotation increased SOC contents by 8% in 20 years [116].

Overall, the cultivation of deep-rooting crops can sequester 374 ± 117 kg C ha⁻¹ y⁻¹ [50,109,112,117] (Figure 1). Considering the whole soil profile (0–90 cm), deep-rooting crops (alfalfa) enhanced the SOC stock at a rate of 380 kg C ha⁻¹ y⁻¹, whereas in the topsoil (0–30 cm) that value was 240 kg C ha⁻¹ y⁻¹ [117]. A meta-analysis revealed that carbon sequestration via N-fixing crops is limited to the first 20 years; thereafter, N₂O emissions exceed the ability of such crops to mitigate CO₂ emissions [118].

Another advantage is that deep-rooting crops can use resources such as water and nutrients from the subsurface horizon. This helps prevent nutrients, especially N, from leaching, making plants more resilient to drought [90,119]. Finally, deep-rooting crops enhance deep infiltration and improve the soil pore connectivity [51]. The biopores created by such crops can enhance the expansion of the subsequent crops. However, plant breeding has focused on yield increases for decades [120]. Consequently, both optimized fertilization and genetic selection have resulted in crops with a high aboveground biomass and a limited root growth.

Catch Crops

Catch crops are sown after harvest of the main crop (such as cereals) or undersown in/with main crops. This creates a permanent vegetal cover of the arable land and an additional period of carbon assimilation [121]. Catch crops therefore prevent soil erosion, weeds and subsoil autumn losses of nitrate and other nutrients [94,122,123,124]. Different grass and legume varieties, rye or several cruciferous species are suitable catch crops [125,126,127,128]. They are used as fodder crops for ruminants or are sown as a soil improvement, so-called green manure. If catch crops remain on the field, the additional plant organic matter increases SOC stocks. Accordingly, implementing catch crops in crop rotations can yield a positive SOC balance.

On average, the carbon sequestration potential of an annual cultivation of catch crops amounted to 403 ± 142 kg C ha⁻¹ y⁻¹ in agricultural topsoils (0–25/30 cm) [76,94,129,130,131] (Figure 1). In Denmark, SOC stocks of the topsoil (0–25 cm) were increased by 210 kg C ha⁻¹ y⁻¹ after introducing catch crops into the rotation [76]. A meta-analysis of 131 studies across the globe reported a mean carbon sequestration rate of 560 kg C ha⁻¹ y⁻¹ [131]. Furthermore, cover cropping under permanent crops (vineyards or orchards) led to a carbon sequestration rate of 550 kg C ha⁻¹ y⁻¹ [129].

3.4.3. Knowledge Gaps

Crop species and genotypes with a distinct root system (high root biomass) can promote soil carbon storage of the subsoil as well as the nutrient and water acquisition in deep soil layers. Overall, other benefits and costs of deep-rooting crops remain largely unclear. Especially the conflicting goals of enhanced inputs via root biomass (rhizodeposition) versus the priming effects require further assessment [22].

Furthermore, secondary plant metabolites may affect soil organic matter degradation. For instance, ref. [132] showed that polyphenol-rich plant litter can inhibit bacterial mineralization of both carbon- and nitrogen-containing compounds as well as enhance nitrogen recycling by mycorrhizae and SOM. Nonetheless, studies evaluating the effect of secondary plant metabolites are rare and further research would be an asset.

Due to the mostly shallow sampling approach (0–30 cm soil depth), the effect of deep-rooting (cover) crops below the plough layer might be largely underestimated [112,116,133]. Additionally, long-term effects of deep-rooting crops on SOC stocks are important, but the extent is largely unknown due to short-term experiments (<10 years). Another aspect that might influence crop rotational decisions is soil-borne diseases. Such diseases are probably influenced by plant (root)-microbiome interactions. Extensive research on the root- associated microbial community could improve the resistance breeding against soil-borne pathogens [134]. Overall, future research should address crop rotations and the combinations of crops or genotypes with different rooting depths.

3.4.4. Trade-Offs

Catch crops are a source of soil greenhouse gas emissions (N₂O, CO₂) [125,135]. Despite their carbon sequestration potential, nitrogen-fixing catch crops can become a net source of greenhouse gas emissions, especially N₂O, over decades [118].

Despite its advantages, catch crop cultivation is not always possible. Temporally, it is hardly achievable prior to fall-seeded crops or after late-harvested crops such as potatoes, maize, and sugar beet. Water scarcity in autumn can also restrict the planting/growing of catch crops, whereas too low winter temperatures and sustained frost periods can prevent cover crops from being destroyed. This then entails weed control in the main crop due to cover crop reestablishment in the following spring. At the same time, new studies show that winter freezing cover crop species have a water consumption similar to that of bare soil and thus have no significant impact on the water balance [11]. Due to water scarcity, costs, and the lack of experience, the cultivation of catch crops (sown between main crops or sown underneath crops) is still restricted globally [70]. Importantly, the evaluation of 16 long-term experimental sites across Europe showed that bare soil impairs the SOC stock most strongly: Without any addition of organic matter (crop residues, organic fertilizers) the SOC decreases markedly after 10 years [13].

3.5. Tillage

Conventional tillage such as ploughing mechanically destroys soil aggregates on the soil surface, exposing formerly protected SOM to decomposition by microorganisms [136]. It also promotes soil erosion, reducing SOC stocks [137,138,139]. Fields cultivated with no- or reduced tillage practices proved to have higher SOC contents in the topsoil (0–10 cm) than under conventional tillage such as moldboard ploughing [52,140,141]. However, with increasing soil depth (>10 cm) no impact of tillage practices on SOC storage could be found [141]. The tillage-induced SOC losses were associated with soil erosion [140]. Moreover, minimizing mechanical disturbances enhances soil health by improving aggregate stability, thereby reducing erosion [142,143]. In Mediterranean climates, topsoil SOC (0–10 cm) was more abundant in fields managed with no-tillage than under moldboard ploughing (ploughing depth of 30/35 cm) [141]. However, with increasing depth (>10 cm) no impact of tillage practices on SOC storage was detected [141]. In a further study on Mediterranean soils, no-tillage practices proved to increase SOC stocks (5–40 cm) by 7% compared to minimum tillage practices [52].

Reduced/no-tillage practices have been controversially discussed as a climate mitigation option [73,144,145] due to their high uncertainties—even when adjusted for equivalent soil mass [146]. Overall, the effects of reduced and no-tillage practices on carbon sequestration are minimal [73,144,145] and relatively insignificant when the whole soil profile (0–60 cm) is considered [12,25,73,144,145,147]. Nevertheless, in the topsoil (0–20/30 cm) no-tillage practices had a carbon sequestration potential of 343 ± 167 kg C ha⁻¹ y⁻¹ [25,51,111,141,147,148], and reduced tillage practices can sequester 324 ± 138 kg C ha⁻¹ y⁻¹ in the upper soil layer (0–20/30 cm) [25,49,51] (Figure 1). However, note that the uncertainties regarding no-tillage practices as a climate mitigation tool are high. In a global meta-analysis, the 95% confidence interval for a sandy soil in warm climates ranged from −100 to 460 kg C ha⁻¹ y⁻¹ [146]. The carbon sequestration potential of Mediterranean topsoils (0–30 cm) ranged between −60 kg C ha⁻¹ y⁻¹ and 400 kg C ha⁻² y⁻¹ for conventionally tilled and no-tilled soils, respectively [141]. Furthermore, in a global database of 311 long-term experiments, converting conventional tillage to no-till practices enhanced the carbon sequestration potential by 460 ± 380 kg C ha⁻¹ y⁻¹ in the topsoil layer 0–15 cm [147]. Overall, the carbon sequestration potential increased rapidly during the first ten years (+0.75 Mg ha⁻¹ y⁻¹), whereas after 15–20 years a new equilibrium was reached [111,141]. The authors of [149] reported that especially intensively tilled Chernozems are susceptible to SOM losses under climate change.

The direct mechanical effect on soil organic matter mineralization is questionable [22]. The authors of [150] suggested that only a part of the SOC stock differences are induced by the conversion from conventional tillage to no-tillage. The effects of no-tillage practices on SOC are more likely caused by the concomitantly altered carbon inputs, such as continuous vegetative cover and living roots, rather than by the reduced soil aggregate disruption after implementing no-tillage itself [150,151]. Several studies also related the absence of tillage to changes to certain crops. For example, the inclusion of temporary grassland with clover in the crop rotation is a typical practice when no-tillage practices are applied [151]. In contrast, for the transition of the grassland period to cereal production the soil is often tilled.

Despite its beneficial effects on SOC stocks and soil microbial activity, no-tillage practices are not very popular [152]. Conservation agriculture practices, such as no- or minimum tillage, cover cropping and diverse crop rotations, are applied on only 12.5% of the total global agricultural cropland [153].

3.5.1. Knowledge Gaps

Reduced/minimum tillage is not well defined but usually implies a reduction of tillage depth and/or frequency. Many studies do not differentiate between different types of tillage (e.g., chisel ploughing, moldboard ploughing, or disc harrowing) or do not account for tillage depth (e.g., conventional tillage and various forms of reduced tillage). This complicates assessing those factors, especially when tillage intensity varies from year to year [147]. Future studies would benefit from evaluating the impact of tillage on SOC stocks in different soil layers, especially including the subsoil [22,151]. Importantly, the calculation of SOC stocks based on fixed soil depths can overestimate the carbon sequestration potential of no-tillage practices. Using the equivalent soil mass approach is strongly recommended to overcome such problems [154].

Since reduced tillage practices can cause an accumulation of N, P, and K in the upper soil layer (0–20 cm) [155], the combination of reduced tillage and reduced fertilization should be examined in detail.

3.5.2. Trade-Offs

Contrary to conventional tillage, reduced tillage practices use more herbicides [156] but less fossil fuels [157]. An additional side effect of reduced tillage is that the soil may become more anaerobic; this promotes denitrification, which in turn may enhance the production of N₂O [78,158]. At the same time, no-tillage practices often result in lower crop yields than conventional tillage practices [88,158]. Tillage increases the mineralization rate of soil organic matter. As a consequence, CO₂ is released and SOC stocks are reduced [22].

3.6. Organic Farming

In organic farming systems, SOC stocks can be improved through (I) usage of organic fertilizers, (II) diversified crop rotation with legumes, as well as (III) cultivation of cover crops [159,160,161]. On average, organic farming practices sequestered 287 ± 102 kg C ha⁻¹ y⁻¹ [51,76,159,161,162,163] more carbon than conventional farming practices (Figure 1). In organic farming systems, organic fertilization with cattle manure and green manure (alfalfa) sequestered 160 kg C ha⁻¹ y⁻¹ and 180 kg C ha⁻¹ y⁻¹, respectively [159]. A global meta-analysis of 68 datasets from 32 peer-reviewed publications showed that organic fertilization is the most important driver in increasing SOC stocks in organic farming systems [160]. Moreover, a conversion of all available cropland in the European Union into organic farming would sequester a total of 30 Mt carbon at once—equivalent to one quarter of the current agricultural emissions [163]. Allocating SOC increases to organic farming practices is challenging because all the aforementioned beneficial farming practices (particularly cover crops, legumes in the crop rotation and organic fertilizers) can be implemented in conventional farming as well. For instance, if organic fertilizers are applied on conventionally managed cropland at similar rates as in organic farming systems, then the SOC stocks were at similar levels in both farming systems [160].

On average, organic fertilizers, cover crops and diversified crop rotations are more frequently used in organic farming than under conventional management practices. In contrast, ploughing—a management practice prone to reduce the soil organic stock—is frequently used in organic farming systems to suppress weeds [161]. Beyond the carbon sequestration potential, organic farming enhances biodiversity and reduces the risk of eutrophication and water pollution [161]. Organic management practices are also characterized by low artificial external inputs (omission of chemically synthesized products such as pesticides, herbicides, or easily soluble mineral fertilizers). Such practices therefore consume less primary energy than conventional farming systems [164]. Compared with conventional farming practices, plant cultivation in organic farms produces lower emissions per hectare [165]. Since organic farming may result in lower yields [166], the emissions of organic farming per crop yield (product) are still under debate [165].

3.6.1. Knowledge Gaps

Towards a more sustainable development, future research in organic farming should include crop yields in the evaluation of greenhouse gas emissions. In animal production, LCA should be used to compare GHG emissions and increased SOC over the production chain.

The overall goal of the European Green Deal is to make the European Union the first climate-neutral continent by 2050. In order to achieve this ambitious aim, one of the targets is that 25% of the EU’s agricultural land should be farmed in an organic manner [167]. Future research needs to address the impact of organic farming on SOC sequestration by further closing the current gaps of knowledge.

3.6.2. Trade-Offs

In mixed farming systems, CH₄ emissions are enhanced by ruminants through enteric fermentation. These emissions, however, could be compensated by increasing the SOC stocks in grasslands used for feeding the animals [161]. Due to the omission of herbicides, organic farming systems use tillage practices to control weeds. This, in turn, could negatively affect the carbon sequestration potential. Importantly, however, a full analysis of greenhouse gas balances (including fuel needed for management practices) showed that organic farming systems have a higher carbon sequestration potential than conventional farming practices [162]. In organic farming systems, organic fertilization, the incorporation of crop residues into agricultural soils and the cultivation of cover crops may increase N₂O emissions, but the cultivation of deep-rooting crops can reduce NO₃ leaching [90].

3.7. Irrigation/Water Table Management

Irrigation increases plant-available water in soils. This practice helps increase net primary production under dry conditions. The enhanced plant growth increases organic carbon inputs into agricultural soils, but the accelerated C and N mineralization might reduce the benefits of irrigation for SOC [168,169]. In a global meta-analysis the carbon sequestration potential of irrigation varied between 50 and 100 kg C ha⁻¹ y⁻¹ [170]. However, a field study revealed that irrigation reduced SOC stocks in 25–30 cm soil depth at a rate of −144 kg C ha⁻¹ y⁻¹ [49]. On average, the carbon sequestration potential of irrigation amounted to −13 ± 78 kg C ha⁻¹ y⁻¹ [49,51,170] (Figure 1).

3.7.1. Knowledge Gaps

Overall, the effects of irrigation and changes in the water table on SOC stocks and processing should be assessed further. Here, the newly invented subsoil soil irrigation system—a device installed at 25 cm soil depth—may provide special advantages (greater water use efficiency) over traditional irrigation systems [22].

3.7.2. Trade-Offs

Most of the negative impacts of irrigation could be mitigated (pumping cost) or avoided (leaching and runoff) when irrigation water management aimed at improving water use efficiency (or water productivity) are carried out. Deficit irrigation strategies of precise irrigation methods can sustain crop productivity while limiting the negative externalities, thus improving the capability of this AMP to help increasing SOC stock. Overall, water use efficiency, defined as the relationship between plant productivity (biomass or crop yield) and water use [171], is the evaluation criteria for sustainable irrigation. The impact of irrigation on SOC sequestration can be negligible or negative, because the carbon costs of pumping water can exceed the increased SOC stock benefits [51]. Beyond environmental side effects, consideration should be given to economic impacts such as costs for irrigation water and fuel to pump the water [51]. In regions with water scarcity, irrigation inevitably increases potential conflicts for water between agriculture, industry and household uses [172].

Furthermore, irrigation can boost denitrification and N₂O emissions from soils [173] and may exacerbate nitrate leaching as well as the runoff of agrochemicals (e.g., herbicides, pesticides, and insecticides).

3.8. Biochar

Biochar is the product of thermally processing organic materials (plant- or animal-based) at temperatures above 350 °C and under restricted oxygen supply (“pyrolysis”) [174]. The porous, fine-grained and carbon-rich biochar is mostly recalcitrant [175]. The long residence time of biochar in soils is related to its condensed aromatic nature [176]. Considering its high recalcitrance, biochar offers a long-lasting sink for carbon in soils. Only a small proportion of biochar is labile, and its application to soil can induce “priming”, in which microbial decomposition of soil organic matter is promoted over a short period of time [177]. Contradictory evidence concerning the “priming” effect is also available, with positive [178,179], neutral [180] and negative [181] effects on SOC stocks. Besides such short-term effects, biochar application reportedly improves SOC stocks in agricultural fields [182,183] through enhanced primary production [184], increased recalcitrant fractions of SOC [185,186] and enlarged SOC pools in the subsoil [187,188]. It may also enhance aggregate stability, increase soil water retention, reduce soil erosion and enhance the activity of soil biota [189,190,191]. Besides improved soil quality, biochar application proved to immobilize copper and cadmium in laboratory experiments [192].

Various articles estimate the average carbon sequestration potential of biochar application to be 1.6 ± 5.14 Mg C ha⁻¹ y⁻¹ [3,193,194,195]. Generally, the recalcitrance of biochar is the most decisive factor for evaluating its carbon sequestration potential, but stability assessments are still ongoing [196].

3.8.1. Knowledge Gaps

Overall, the assessment of recalcitrance has certain weaknesses. Besides the need to biochemically evaluate biochar recalcitrance, this calls for long-term strategic research that includes environmental and management factors [190]. Moreover, further research is needed to assess the sorption potential of biochar for organic pollutants (e.g., pesticides) or heavy metals/trace elements [197].

Beyond agronomic utilization, no EU legislation addresses biochar application on agricultural fields [187]. Until now, biochar application as a fertilizer on agricultural fields is regulated country-specifically. From an environmental point of view, LCA analyses of biochar fertilization would be beneficial.

3.8.2. Trade-Offs

On a large scale, biochar application is not feasible due to its production limits. Economically, the monetary costs of biochar exceed its economic advantages [193].

3.9. Lignocellulosic Crops

3.9.1. Agroforestry

Agroforestry combines woody perennials, such as trees and shrubs, with agricultural crop or grassland. Generally, agroforestry fulfils multiple functions simultaneously, including ecological (such as increased SOC stocks, improved soil fertility) and socioeconomic benefits (e.g., higher crop productivity and the provision of crops, fodder, or timber) [72,198,199]. Recent studies highlight a mean carbon sequestration rate of agroforestry systems of 725 ± 100 kg C ha⁻¹ y⁻¹ in (sub) tropical and temperate regions [72,198,200,201]. In order to fulfil the 4 per mille target on the global scale, agroforestry should be expanded to a proportion of 6 % of the agricultural land [202].

3.9.2. Bioenergy Production

Generally, bioenergy production intends to replace non-renewable fossil energy-based products by renewable plant-based products. Due to their high primary production, lignocellulosic crops, such as Miscanthus, poplar tree (Populus spp.) and switchgrass (Panicum virgatum), can capture large amounts of atmospheric CO₂ through photosynthesis in their above- and belowground biomass throughout the vegetation period [21,63,203]. After 6 years, the carbon sequestration potential of lignocellulosic crops ranged between 1050 and 710 kg C ha⁻¹ y⁻¹ (topsoil; 0–10 cm) for woody (popular and willow trees) and herbaceous species (switchgrass and Miscanthus), respectively [204].

3.9.3. Knowledge Gaps

Due to the lack of published data and field studies, the potential carbon sequestration of agroforestry and bioenergy production is still under debate.

3.9.4. Trade-Offs

The main challenges of agroforestry are the lack of long-term research studies, the lack of markets for timber, the higher work load and the limited awareness of advantages of agroforestry [70,198]. Due to the high investment costs and the high working time requirement, the success of agroforestry in Europe will mainly rely on the object funding provided by the EU through the European Agricultural Fund for Rural Development.

Overall, the cultivation of crops for bioenergy production further increases the demand for agricultural land [205]. Moreover, fertilization could increase anthropogenic N₂O emissions, thus reducing the climatic benefits of lignocellulosic crops [63]. Additionally, the authors of [206] observed that harvesting Miscanthus giganteus exported large amounts of potassium in autumn; potassium replacement by (mineral) fertilizers and the CO₂ emissions connected with fertilizer production should be considered.

3.10. Application of Inorganic Carbon

3.10.1. Carbonate Minerals (Liming)

Liming can improve SOC by enhancing primary production (improved nutrient availability) or ameliorating the soil structure (bridging effects of carbonates). In calcium carbonate-rich soils, free calcium can bind with organic matter and form complex aggregates; this physically protects organic matter from microbial decomposition. Liming, however, can also stimulate soil microbial activity when applied to bring pH into a range of plant optimum, thus reducing SOC stocks [207].

Since liming is often combined with other fertilizers, its effect on SOC is challenging to estimate [207]. Various studies have found that liming either increased [208], decreased [209] or had no effect [207] on SOC stocks. In Brazil, liming (dolomitic lime) enhanced the SOC stock in the topsoil (0–20 cm) of a dystrophic red latosol (Oxisol), with an mean carbon sequestration rate of 300 kg C ha⁻¹ y⁻¹ over 15 years [208]. Nonetheless, published data on how liming affects SOC stocks remain scarce, and a global meta-analysis yielded no clear assertion [207].

3.10.2. Silicate Minerals

Carbon can be fixed permanently as pedogenic carbonates through chemical weathering of silicate minerals [210]:

CaSiO₃ + 2 CO₂ + H₂O -> Ca²⁺ + 2 HCO₃⁻ + SiO₂

In an experimental field trial in a New Hampshire forest, applying 3.5 t/ha calcium silicate minerals (wollastonite) doubled the net removal rate of bicarbonate in the runoff of the catchment [211]. Thus, silicate minerals (wollastonite) proved to be an sink for carbon, and a global application could potentially sequester roughly 2% of global fossil fuel emissions [66].

3.10.3. Knowledge Gaps

Overall, the extraction of inorganic carbon is often associated with environmental problems. This calls for rigorous LCAs. The lack of published data and field studies makes assessments of its carbon sequestration potential inconclusive. Finally, little is known about how the application of inorganic carbon affects SOC processing/stabilization or soil aggregation and structure.

3.10.4. Trade-Offs

The production, transportation and application of inorganic carbon requires energy. This entails CO₂ emissions. Note also hat if demand for inorganic carbon increases, the enhanced production rates may adversely affect the environment or human health (e.g., dust inhalation) at extraction sites [70].

On one hand, liming can reduce the availability of essential nutrients (e.g., phosphorous, zinc, and manganese), while on the other hand it can lessen the availability of potential toxic elements such as cadmium [212]. Nevertheless, by stimulating the soil microbial community, liming is associated with CO₂ emissions [70].

After applying silicate minerals, the increased HCO₃^- in runoff waters may enhance ocean alkalinity and promote the growth of calcareous organisms [210]. From a financial perspective, the high investment costs of silicate minerals make its application uneconomical [66].

4. Synopsis—Are Soils an Unlimited Carbon Sink?

Agricultural management practices can sequester organic carbon in agricultural topsoil by enhancing primary production (with e.g., mineral and organic fertilization, irrigation), by applying additional organic carbon (manure, compost, incorporation of crop residues, biochar or cover crops), enriching subsoil organic carbon (deep-rooting crops), improving aggregate stability (no-tillage), replacing non-renewable fossil fuels (bioenergy production) or integrating woody biomass (agroforestry). All these processes allow a net removal of CO₂ from the atmosphere. Beyond the role of agricultural management practices, further constraints directly or indirectly affect the carbon sequestration potential of cropland soils. These constraints are summarized in the following section.

4.1. Bare Fallow—A Threat for SOC Stocks?

As opposed to sustainable agricultural management practices, such as conservation agriculture, the soil in conventional cropping systems is often left bare after harvest. Accordingly, such fields produce no noticeable amount of biomass over much of the year [213]. Except for the atmospheric deposition or weeds, the carbon inputs in periods of bare fallow are close to zero [214]. Above all, regaining the carbon losses from bare fallow periods is mostly not achievable, and agricultural management systems with bare fallows are mostly associated with a net loss of carbon [50,214,215,216].

Under current agricultural management practices, Swiss agricultural topsoils (0–20 cm) are losing SOC at a rate of 290 kg C ha⁻¹ y⁻¹ [215]. Among such practices, bare fallow resulted in the overall highest SOC losses at a rate of −531 ± 198 kg C ha⁻¹ y⁻¹ in the topsoil (0–20 cm) over 20/50/100 years relative to the start of the experiment [49,50,148,214,216,217] (Figure 1). Carbon is mostly depleted from the topsoil, whereas in subsoil (25–100 cm) it declined at a rate of −145 ± 5 kg C ha⁻¹ y⁻¹ [216].

4.2. Climate Change

In the absence of profound knowledge on how SOC cycling responds to a changing global climate, the estimates for the future SOC sequestration potential involve a considerable element of uncertainty [218]. According to [218], the following aspects must be addressed further: (I) Temperature sensitivity of soil organic matter [219,220], (II) equilibrium between organic carbon input and output under accelerated soil decomposition and enhanced primary production [136], and (III) interactions of global warming and changes in land use [221,222] and atmospheric composition [66,223] (e.g., rising CO₂ levels). In a current earth system model (Community Earth System Model—CESM, validated with LMER model selection), temperature rise accelerates SOC losses, and under a temperature rise of 1 degree, 30 ± 30 Pg carbon could be potentially lost within a year [224]. This positive carbon-climate feedback could potentially quicken climate change [224]. Furthermore, future climate conditions can initiate a sustained trend reversal, and ARMOSA model for future climate scenarios (Representative Concentration Pathway (RCP) 6.0) forecasted SOC depletion rates ranging from 160 to 88 kg ha⁻¹ y⁻¹ [148]. Moreover, under conditions of climate change the frequency and intensity of droughts and storm events are expected to increase in the near future. This is crucial since high intensity rainfall on dry soils have shown to multiply organic carbon exports from agricultural land to streams many times over [225].

Moreover, anthropogenic climate change and desertification of agricultural land will reduce the available land to sequester atmospheric carbon and to produce food. Prolonged droughts and high air temperatures may also be challenging for animal husbandry, especially in water-scarce regions. This will make dietary changes towards more plant-based diets more important in the near future.

4.3. Soil Sealing

Globally, land consumption threatens soils and their functions [226]. Due to the massive removal of topsoil and minimal accumulation of belowground C underneath sealed surfaces via root growth, urban soil sealing restricts soil C storage and is associated with substantial losses in C storage [227]. Urban sprawl/expansions are consuming agricultural land. In Europe, about 9% of the surface area is sealed, and artificial surfaces are increasing at an annual rate of 0.75% [228]. In the period 2012–2018, the annual land take in the European Union consumed 440 km² of agricultural land [8]. The loss of cropland with a high theoretical production potential is generating particular pressure on soils, ecosystem services and on future food security [229]. In 2006, the European Commission considered surface sealing as one of the major soil threats [229]. Despite being a pressing threat, no EU legislation currently protects soils from urban sprawl [230,231]. In Germany, for instance, a federal guideline is in place to deal with soil protection, whereas in Austria, soil protection is regulated by the regional authorities. In the future, the European Commission plans to harmonize those national legislations and regulations with the Soil Thematic Strategy [8]. As part of the European Green Deal, the update of this Soil Thematic Strategy aims to achieve land degradation neutrality by 2030, and by 2050 no net land take should occur in the EU [8].

4.4. Consumer Behaviour

The current global food sector emits 14 ± 3.4 Gt CO₂eq y⁻¹ [232]. Global trade may shift environmental pressures to other countries; thus, agricultural food production should increasingly meet local consumer demand. In Austria, for example, the agricultural self-sufficiency regarding vegetables, fruits, wheat, dairy products and beef lies at 54, 59, 87, 128, and 141%, respectively [233]. Goods over 100% have to be exported, whereas products below 100% have to be imported. Due to an exponentially growing world population and an increasing per capita incomes, the pressure on food production is expected to increase by 50–90% in 2050 [234]. In a comparative analysis of food production systems, the highest greenhouse gas emissions are caused by a high demand for meat, whereas plant-based diets (vegan) produce the lowest emissions [235]. Therefore, a synergistic combination of dietary changes towards more plant-based diets, improved technologies and management, and reduced food loss and waste would help reduce the negative impact on ecosystems [234]. A reasonable option to counteract overproduction and food waste as well as to recycle organic carbon would be to compost food waste and/or organic wastes and apply them on agricultural fields. Food waste could also be digested in biogas plants and the digestates then used as an organic fertilizer.

4.5. Socio-Economic Limitations and Practical Considerations of Farmers

Given the low trading price of carbon and the high contribution margin of carbon- rich materials, farmers are unlikely to implement agricultural management practices to achieve the “4p1000” goal [26,66]. According to [26], the main motivation for farmers to change agricultural management practices is clear benefits, yield increases and long-term economic profitability. Accordingly, information exchange, financial incentives and the provision of appropriate infrastructure could encourage farmers to convert towards sustainable farming practices [236,237,238]. For instance, biochar technology is largely unknown to farmers, and the high transportation costs and high demand for organic residues make the application of biochar uneconomical for farmers [193]. The trading of CO₂ certificates by various providers is a questionable practice that should be carefully assessed in terms of benefits for farmers [11]. Finally, novel management practices often pose financial risks for farmers due to the high investment costs. In the future, object funding/subsidies (agroforestry) and/or securing the placing of customer products appear to be essential for the economic viability of sustainable farming practices [72,237,239].

5. Conclusions

Through two strategies—increasing carbon inputs and reducing soil organic carbon losses—agricultural management practices can improve carbon sequestration in soils. By enhancing primary production (fertilization, liming) and/or integrating additional organic carbon to soils (additional biomass: Catch crops, agroforestry, deep rooting crops; external carbon sources: e.g., compost, recalcitrant biochar), carbon can be added to agricultural cropland. Farmers can reduce the organic carbon losses from soils by minimizing the deliberate removal of crops (retention of crop residues), by reducing soil erosion (cover crops, reduced tillage) or by managing CO₂ from mineralization (reduced/no-tillage). Despite their advantages, the application of organic fertilizers (compost, farmyard manure) or biochar is limited by their local availability, and transportation causes additional CO₂ emissions. Importantly, CO₂ produced during manufacturing (e.g., mineral fertilizer) or pumping (e.g., irrigation) can outrange their beneficial value for carbon sequestration or the extraction of minerals and can cause severe environmental problems at the extraction sites (e.g., inorganic carbon, liming). Some practices can reduce nutrient availability (liming), change soil pH (biochar), promote nitrate leaching (irrigation) or cause problems in weed regulation (e.g., reduced/no-tillage). Greenhouse gas emissions are also associated with animal husbandry (CH₄, N₂O) or linked to soil processes such as improved mineralization of soil organic matter (CO₂) or denitrification (N₂O). Moreover, the carbon sequestration potential of relatively new management practices, such as the application of inorganic carbon or biochar and agroforestry, is not conclusive because published data on long-term field experiments are lacking. Based on the literature, bare fallows are responsible for the greatest SOC losses through SOM decomposition. Farmers are being requested to apply as many beneficial SOC-preserving practices as possible, but the overall benefit for SOC sequestration remains to be verified by an LCA under the respective soil and climate conditions. Summa summarum, the preservation of SOC is crucial because SOC is essential for soil fertility, soil health and ecosystem services, including primary production and climate regulation.