A framework to estimate climate mitigation potential for US cropland using publicly available data

Results and Discussion

Regional Weighted Emission Reduction Coefficients. Among the cropland management practices evaluated, the top five wERCS for the nine regions were similar with different rankings for each region (figure 1). Regardless of ranking, certain practice options typically were best for all regions. For example, among the cover crop options, implementing a legume cover crop was on average 0.29 t CO₂e ha⁻¹ y⁻¹ greater than that for a nonlegume. Converting to NT/ST regardless of initial starting point (e.g., RT or IT) had higher wERCs than converting to RT from IT. On average, converting to NT from IT was 0.25 t CO₂e ha⁻¹ y⁻¹ greater than converting from RT. Conversion to RT (from IT) was the lowest among the tillage options and was 0.55 t CO₂e ha⁻¹ y⁻¹ lower than conversion to NT from RT and 0.80 t CO₂e ha⁻¹ y⁻¹ lower than conversion to NT from IT. On average, replacing synthetic N with dairy or beef feedlot manure had a lower wERC at 0.27 t ha⁻¹ y⁻¹ compared to swine manure at 0.53 t ha⁻¹ y⁻¹.

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Figure 1

Farm resource regions and regional weighted emission reduction coefficients (wERCs) for the six cropland management categories and three management categories for Conservation Reserve Program land. Within each land use, bars are scaled to the minimum and maximum wERC and reported in t CO₂e ha⁻¹ y⁻¹. CCR is conservation crop rotation; IT is intensive tillage; RT is reduced tillage; and NT/ST is no-till/strip till. Map was generated with the leaflet package (Graul 2016) in R (R Core Team 2021).

Averaged across the practices evaluated, the regions in ranking order of wERCs from highest to lowest were Mississippi Portal, Heartland, E. Uplands, S. Seaboard, N. Crescent, Prairie Gateway, Fruitful Rim, N. Great Plains, and Basin and Range. The cropland management practice with the highest wERC was CCR for in the Basin and Range (0.63 t CO₂e ha⁻¹ y⁻¹), converting from IT to NT/ST for Fruitful Rim, Heartland, N. Crescent, N. Great Pains, and Prairie Gateway with an average value of 1.2 t CO₂e ha⁻¹ y⁻¹, and adopting a legume cover crop in the Mississippi Portal, E. Uplands, and S. Seaboard (average = 2.0 t CO₂e ha⁻¹ y⁻¹).

Converting IT or RT to NT/ST were among the top five practices for all regions and were the top two wERCs for the Heartland, N. Crescent, N. Great Plains, and the Prairie Gateway. The Heartland had the greatest wERC for all three options compared to the other regions and was estimated at 0.51, 1.35, and 1.69 t CO₂e ha⁻¹ y⁻¹ for conversion to IT from RT, to NT from RT, and to NT from IT, respectively. The Basin and Range region had the lowest wERC for these practice options, which were 0.20, 0.47, and 0.61 t CO₂e ha⁻¹ y⁻¹ for conversion to IT from RT, to NT from RT, and to NT from IT, respectively. The wERC ranking among regions for the tillage options from greatest to lowest was Heartland, Mississippi Portal, E. Uplands, N. Crescent, S. Seaboard, Prairie Gateway, N. Great Plains, Fruitful Rim, and Basin and Range.

Adding a legume or nonlegume cover crop was in the top five wERCs for all regions except Basin and Range, N. Crescent, N. Great Plains, and Prairie Gateway. Adding legume cover crop was the top wERC for the E. Uplands, Mississippi Portal, and S. Seaboard. The top four regions for cover cropping include Mississippi Portal, E. Uplands, S. Seaboard, and the Heartland with wERCs of 2.91, 1.67, 1.41, and 1.20 t ha⁻¹ y⁻¹, respectively. The Basin and Range had the lowest wERC for cover cropping (0.25 t ha⁻¹ y⁻¹).

In some regions, CCR (i.e., decreasing the time of fallow and when possible adding a perennial crop to the rotation) can be an alternative practice to cover cropping. CCR was among the top five wERCs for four regions (e.g., Basin and Range, Fruitful Rim, N. Great Plains, and Prairie Gateway). Only N. Crescent had neither cover cropping or CCR in the top five wERCs. Adding mulch was in the top five for all regions except Heartland, and stripcropping was in the top five wERCs for three regions. For the manure management options evaluated, E. Uplands and Heartland tended to have higher wERCs than the other regions. Replacing 20% of synthetic N with manure (swine) was among the top five wERCs for Heartland, N. Crescent, and Prairie Gateway.

For the CRP land use, conversion of cropland to herbaceous (e.g., conservation cover) or woody cover (e.g., tree establishment) and riparian restoration provides relatively high CO₂e reduction potential compared to cropland management practices. The available hectares for CRP are fewer, but fewer hectares implementing these practices can have an equal or greater impact on CO₂e reduction compared to cropland management practices implemented on more hectares. Similar to cover crop adoption, inclusion of legumes in the herbaceous cover has a wERC that is two to three times greater than nonlegume cover. Conversion of nonirrigated cropland to a woodlot has an average wERC of 18.5 t ha⁻¹ y⁻¹. These practices are not exclusive to CRP, which has certain requirements for land to be enrolled (USDA FSA 2019). In general, land most suitable for this conversion includes low producing annual cropland that are highly erodible with major limitations such as water availability or degraded grasslands that historically were wooded.

Existing Practice Adoption: Carbon Dioxide Equivalents Reduction Potentials from Previous Shifts in Crop Management Practices. Based on 2017 hectares in conservation tillage, CCR, cover crops, stripcropping, mulching, manure application, and CRP, the national, total CO₂e reduction potential was estimated at 134.2 million tonnes (Mt) y⁻¹, ranging from a low of 1.8 Mt y⁻¹ in the Basin and Range to 42.7 Mt y⁻¹ in the Heartland. Tillage, CRP, and CCR made up 94% of the total reductions (figure 2).

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Figure 2

Existing (2017) carbon dioxide equivalents (CO₂e) reduction potentials from previous shifts in cropland management for each region: (a) summed across practices and reported in t CO₂e y⁻¹ and (b) the proportion of each practice for each region with values in the bars reported in Mt CO₂e y⁻¹. CCR is conservation crop rotation and CRP is conservation reserve program.

Adoption of 42.2 Mha of NT and 39.5 Mha of RT was estimated to reduce CO₂e by 64.8 Mt y⁻¹ and ranged from a low of 0.8 Mt CO₂e y⁻¹ in the Basin and Range to 28.9 Mt CO₂e y⁻¹ in the Heartland. The 9.3 Mha of CRP land was estimated to reduce CO₂e by 45.6 Mt y⁻¹ and ranged from 0.5 Mt CO₂e y⁻¹ in the Basin and Range to 14.2 Mt CO₂e y⁻¹ in the S. Seaboard. CRP contributed to nearly 75% of the total estimated CO₂e reduction potential in the Mississippi Portal and S. Seaboard.

Conservation crop rotation on 28.1 Mha nationally was estimated to reduce CO₂e by 16.3 Mt CO₂e y⁻¹ and ranged from 0.4 Mt CO₂e y⁻¹ in the Basin and Range to 4.6 Mt CO₂e y⁻¹ in the Heartland. Cover crop hectares were 6.2 M in 2017, equating to a CO₂e reduction potential of 4.6 Mt CO₂e y⁻¹ and ranged from 16,000 t CO₂e y⁻¹ in the Basin and Range to 1.8 Mt CO₂e y⁻¹ in the Heartland.

Business-as-Usual and Accelerated Scenarios for Carbon Dioxide Equivalents Reduction Projections—National. Under the 10-year BAU scenario, if conservation practice adoption continues at current annual growth rates, a total of 15.9, 104.4, 45.5, and 8.4 Mha is expected to be in cover cropping, conservation tillage, CCR, and manure application practices within 10 years, which represents new adoption levels of 9.7, 31.0, 17.4, and 1.2 Mha since 2017, respectively (tables S4 through S7). Nationally, the projected annual growth rate for the conservation management practices (i.e., cropland and CRP) was 6.2 Mha y⁻¹ of cropland, with the highest growth in conservation tillage (3.1 Mha y⁻¹), CCR (1.7 Mha y⁻¹), and cover crops (0.97 Mha y⁻¹); whereas the growth rates for the other four practices were substantially lower at <0.14 Mha y⁻¹ (figure 3). These adoption rates translate to a total national CO₂e reduction potential of an additional 48.7 Mt CO₂e y⁻¹ (figure 4) with conservation tillage (20.4 Mt CO₂e y⁻¹), CCR (10.1 Mt CO₂e y⁻¹), cover crop (8.8 Mt CO₂e y⁻¹), and CRP (8.1 Mt CO₂e y⁻¹) comprising 98% of the national total (figure 3). Except for CRP, where conversion to woodlots dominated in some regions (e.g., S. Seaboard), the number of hectares adopting a given practice in large part drives the CO₂e reduction potential and in descending order was conservation tillage, CCR, cover crop, CRP, manure, mulching, and stripcropping.

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Figure 3

National carbon dioxide equivalents (CO₂e) reduction potentials (Mt CO₂e y⁻¹) by management practice under three different scenarios. X-intercepts are the CO₂e reduction potentials based on existing (2017) adoption levels, y-values are CO²e reduction potentials associated with new practice adoption for the BAU scenario (hatched circles) or accelerated growth rates (solid circles). Points along each arrow show the progression of increased reduction potential as practice adoption is increased from existing to BAU to accelerated growth for each management category. The size of each circle is scaled according to the required growth rate (Mha y⁻¹) necessary to achieve the scenario adoption levels. Inset axes and units are the same, but the scales have been changed to accommodate the low reduction potentials of stripcropping and mulching.

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Figure 4

Carbon dioxide equivalents (CO₂e) reduction potentials under the business-as-usual scenario from projected shifts in cropland management for each region: (a) summed across practices and reported in t CO₂e y⁻¹ and (b) the proportion of each practice for each region with values in the bars reported in Mt CO₂e y⁻¹. CCR is conservation crop rotation and CRP is conservation reserve program.

Projecting the accelerated growth rates for the conservation management practices evaluated translated to new practice adoption on 13.1 Mha y⁻¹ of cropland nationally, with an increase in adoption ranging from 7,000 ha y⁻¹ for stripcropping to 4.9 Mha y⁻¹ for tillage. Other annual increases included approximately 2.7 Mha y⁻¹ in cover crops, 0.54 Mha y⁻¹ in CRP, 0.24 Mha y⁻¹ in mulching, and 1.3 Mha y⁻¹ in manure amendments (figure 3). These accelerated growth rates translate into CO₂e reduction potential of an additional 118.5 Mt CO₂e y⁻¹ (figure 5) with the tillage (39.8 Mt CO₂e y⁻¹), CRP (27.7 Mt CO₂e y⁻¹), cover crop (25.4 Mt CO₂e y⁻¹), and CCR (20.2 Mt CO₂e y⁻¹) comprising 95% of the national total (figure 3).

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Figure 5

Carbon dioxide equivalents (CO₂e) reduction potentials under the accelerated scenario from projected shifts in cropland management for each region: (a) summed across practices and reported in t CO₂e y⁻¹ and (b) the proportion of each practice for each region with values in the bars reported in Mt CO₂e y⁻¹. CCR is conservation crop rotation and CRP is conservation reserve program.

Business-as-Usual Scenarios for Carbon Dioxide Equivalents Reduction Projections—Regional. Under the BAU scenario, the Heartland constituted 41% or 19.8 Mt CO₂e y⁻¹ of the total national CO₂e reduction potential of 48.7 Mt CO₂e y⁻¹ (figure 4), reflecting the region’s relatively large number of hectares and relatively high ERCs for most practices. The next four regions in descending order were the Prairie Gateway, N. Great Plains, S. Seaboard, and Mississippi Portal that combined made up 45% of the national total CO₂e reduction potential.

For six of the regions, conservation tillage had the highest contribution to the CO₂e reduction potential except for the E. Uplands (CRP), Mississippi Portal (cover crop), and S. Seaboard (CRP) (figure 4). The regional contribution of conservation tillage averaged 37.5% and ranged from 13.3% for the S. Seaboard to 53.2% in the Heartland. To achieve the CO₂e reduction potential for this scenario, conservation tillage must increase nationally by 3.1 Mha y⁻¹ and ranges regionally between 47,131 ha y⁻¹ in E. Uplands to 1.3 Mha y⁻¹ in the Heartland (table S4). Over the 10-year scenario, IT hectares were reduced by 22.7 Mha, with 16.6 Mha converting to RT and 14.4 Mha converting to NT/ST.

Cover crop ranged from 1.0% of the regional total for the Basin and Range to 37.3% in the Mississippi Portal, where it ranked as the top practice for CO₂e reduction potential. Other regions where cover crop was greater than 10% of the regional total were the Heartland (27.0%), E. Uplands (12.1%), S. Seaboard (10.2%), and N. Crescent (10.0%). To achieve the CO₂e reduction potential in each region requires an average compounded annual growth rate (CAGR) of 7.5% and ranged from 1.4% or 5,346 ha y⁻¹ in the Fruitful Rim to 15.2% CAGR or 78,394 ha y⁻¹ in the Mississippi Portal (table S4).

CCR was the second highest contributing practice in five of the nine regions (excluding E. Uplands, Heartland, Mississippi Portal, and S. Seaboard). The average regional contribution to CO₂e reduction potential was 25.8% and ranged from 11.7% in the S. Seaboard to 40.0% in the Prairie Gateway. Nationally, CCR must grow at 1.7 Mha y⁻¹ and range from about 31,000 ha y⁻¹ in the Basin and Range to 459,000 ha y⁻¹ in the Heartland to achieve the reported CO₂e reduction potentials.

Accelerated Scenario for Carbon Dioxide Equivalents Reduction Projections—Regional. Regional patterns for the accelerated scenario were similar to those observed for the BAU scenario, with the Heartland accounting for 42.0 Mt y⁻¹ or 35.4% of the 118.5 Mt y⁻¹ national CO₂e reduction potential (figure 5). The next four regions in descending order were the S. Seaboard, Mississippi Portal, N. Great Plains, and Prairie Gateway, and collectively they made up 49.1% of total national reduction potential.

Under the accelerated scenario, conservation tillage made up the largest proportion of CO₂e reduction potential for all regions except E. Uplands, Mississippi Portal, and S. Seaboard, and ranged from 0.5 Mt CO₂e y⁻¹ in Basin and Range to 16.7 Mt CO₂e y⁻¹ in Heartland (figure 5). To achieve the CO₂e reduction potential, hectares implementing conservation tillage must increase at an average regional growth rate of 541,867 ha y⁻¹. Over the 10-year scenario, there will be a decrease of 27.6 Mha of IT, an increase of 38.7 Mha in NT/ST, and an increase of 10.0 Mha in RT. The greatest IT reduction is in the Heartland at 0.84 Mha y⁻¹ and after 10 years will have 26.6 Mha in NT/ST (addition of 12.5 Mha) and 11.8 Mha in RT (addition of 3.6 Mha).

Cover crop was among the top three practices for CO₂e reduction potential for four of the regions and comprised 14.5% of the total reduction potential on average (figure 5). It was the top practice for the Mississippi Portal at 35% (4.9 Mt CO₂e y⁻¹). Cover crop also comprised over 20% of total reduction potential for E. Uplands at 25% (0.6 Mt CO₂e y⁻¹), and Heartland at 37% (15.6 Mt CO₂e y⁻¹). To achieve the CO₂e reduction potential of 25.4 Mt CO₂e y⁻¹ nationally, an average regional CAGR of 13.8% is required and ranges from 2.8% in Fruitful Rim to 25% CAGR in the Heartland and Mississippi Portal (table S3) with the largest net gains in cover crop hectares in the Heartland (16.0 Mha). After 10 years, a total of 26.8 Mha of new cover crops are projected under this scenario with an overall total (new plus existing) of 33.0 Mha.

CCR was the second greatest contributing practice for CO₂e reduction potential in Basin and Range at 24.6% or 0.4 Mt CO₂e y⁻¹, E. Uplands at 22.2% (0.7 Mt CO₂e y⁻¹), and Prairie Gateway at 32.4% (4.6 Mt CO₂e y⁻¹). In the Basin and Range, CCR must continue to grow at 62,310 ha y⁻¹, and up to 918,652 ha y⁻¹ in the Heartland (table S6).

Among the regions, the Heartland has the greatest CO₂e reduction potential for all of the practices except for stripcropping, where the N. Crescent has the greatest potential at 26,297 t CO₂e y⁻¹, and for CRP, where the S. Seaboard is greatest at 11.4 Mt CO₂e y⁻¹.

Discussion. Improved management of agricultural soils can mitigate emissions of CO₂ and non-CO₂ trace gases like N₂O and CH₄, and also draw CO₂ out of the atmosphere (Paustian et al. 2016, 2019a; Griscom et al. 2017; Amelung et al. 2020; Bossio et al. 2020). The accelerated adoption of soil management practices that increase C storage via soil organic matter on the nearly 162 Mha of cropland in the United States can provide readily available, cost-effective climate mitigation benefits (Paustian et al. 2016; Fargione et al. 2018; Sperow 2020).

Conservation practices including those evaluated here result in multiple environmental and productivity benefits that extend beyond increasing the soil C pool (Palm et al. 2014). Among these benefits are improved water quality and availability, aeration, nutrient provision, resistance to erosion, and increased biodiversity, which can result in reduced inputs by improving water and nutrient use and improved resistance to drought or other stresses. Thus, climate mitigation potential is but one of many environmental benefits that are directly and indirectly tied to the practices we consider in this paper.

We recognize the high uncertainty surrounding biogeochemical models upon which we base our estimates and interpretations (Eve et al. 2014). Additional analyses would be beneficial to understand the uncertainty and sensitivity of the calculated ERCs on our scaled estimates of mitigation potential; however, this work was beyond the scope of this project. Estimating data-driven future conservation practices adoption rates would help inform policy and land managers, regardless of the end goal. Thus, our framework that applies previous growth rates to project future adoption levels and then applies these targets to estimate climate mitigation potential is relevant and timely.

Establishing Future Targets Based on Historical Adoption Levels. Current adoption and recent conservation trends can provide an appropriate regional framework for what levels of adoption may be achievable over the next 10 years to inform climate actions by showing the level of increased adoption needed to achieve policy and environmental goals. By setting regionally specific limits of “high” adoption (i.e., accelerated growth scenario), we aimed to be aggressive while recognizing potential limitations based on climate, cropping system, and infrastructure (Bradford et al. 2019).

Conservation tillage has been promoted by NRCS and conservationists for the past several decades and as of 2017, has been implemented on 81.6 Mha, accounting for nearly 75% of the reduction potential, excluding CRP. This successful campaign reflects the capability of achieving widespread adoption of a given practice and sets a model for continued expansion as well as opportunities to layer additional conservation practices (e.g., cover cropping and nutrient management). Despite rapid adoption of NT and RT, which corresponded with the implementation of conservation compliance and new herbicide-tolerant crop varieties (reducing the need to till to manage weeds), adoption has leveled off (or even dropped) in some crops, suggesting that we may be at an adoption plateau. For example, according to data collected from the USDA’s Agricultural Resource Management Survey (ARMS), no-till wheat and soybean (Glycine max) adoption increased between 2002 and 2009, but adoption slowed for wheat between 2009 and 2017, and decreased in soybeans between 2006 and 2012 (Claassen et al. 2018). Thus, our estimates for the BAU scenario may overestimate future adoption levels if adoption is reaching a plateau for all cropping systems nationally. Estimates for the accelerated scenario were not dependent upon a linear response but rather by bringing all counties to the 95th percentile target within each region. Our accelerated scenario models a transition to almost no IT with 96% of available hectares or 109 Mha under conservation tillage, with 74% of conservation tillage in NT (compared to 52% NT in BAU). Under this scenario, CO₂e was reduced by 104.6 Mt y⁻¹, which is similar to estimates generated by Lal et al. (2003), who estimated a reduction of 117.5 Mt CO₂e y⁻¹ associated with 114 Mha of conservation tillage.

In contrast to conservation tillage, cover cropping has a relatively low level of adoption, with less than 5% of available hectares under cover crops nationally. A compound annual growth rate was used to reflect the rapid growth of this practice between the last two agricultural censuses (Wallander et al. 2021). Extending these rates over the following 10 years suggests that nearly 15.9 Mha will be using cover crops by 2027. Nationally, this equates to only 12% of available ha in cover cropping, but key regions such as the Heartland, Mississippi Portal, and the S. Seaboard will have 17% to 25% of available ha in cover crops. Our more ambitious goal of achieving 33 Mha in cover crops in 10 years is lower than the 2025 goal of 40.5 Mha suggested by Hamilton et al. (2017), but still requires increasing cover crop adoption to more than double the current growth rates.

Although not originally designed to address C sequestration, CRP has been shown to sequester C from 0.15 to more than 4.0 t CO₂e ha⁻¹ y⁻¹ depending on previous land management history and length of time enrolled in the program (Collins et al. 2012; Li et al. 2017). At the maximum historical enrollment of 14.9 Mha, we estimated a national average of 4.9 t CO₂e ha⁻¹ y⁻¹. The overall importance of this program and its impacts on C sequestration has prompted a new US$10M initiative to measure and monitor these rates nationally (USDA FSA 2021). By investing in this monitoring program, models and conservation planning resources will be improved to better quantify climate benefits and other ecosystem services.

What Determines the Mitigation Potential of Practices in Different Regions? The mitigation potential of a particular practice in each region depends on the ERC associated with the practice change in the region (which also depends on biophysical and climatic factors), the amount of cropland in the region, and the current level and expected rate of adoption of the practice. Said differently, we saw that a change in practice adoption in a given region made a larger relative contribution to mitigation potential, all else equal, if (1) its land base in agricultural cropland is large or (2) the potential emissions reductions per hectare due to the practice change (wERCs) were large due to regional heterogeneity in soils or climate. As such, the Heartland consistently leads the nation in CO₂e reduction potential for the three scenarios. With over 28% of the cropland and relatively high wERCs for practices, the Heartland constituted 32%, 41%, and 35% of the CO₂e reduction potential in existing, BAU, and accelerated scenarios. Although the S. Seaboard comprises less than 5% of US cropland, it ranked highly in CO₂e reduction potential relative to other regions due to the high wERCs associated with planting trees on CRP land, the role of maintaining and increasing NT adoption, and soil and climatic factors (e.g., when compared with the Basin and Range region, which has a similar amount of cropland, but lower wERCs for the same practice change).

Conservation Crop Rotations and Cover Crops Could Play Larger Roles in Future Mitigation Potential. While reducing tillage on cropland remains a key practice to reduce emissions, we predict that CCR and cover crops could play a relatively larger role in potential emissions reductions over the next 10 years. For four regions, the wERC is greater for CCR than either of the two cover crop practice options. These four regions are in the drier western United States (Basin and Range, N. Great Plains, and Prairie Gateway) or colder regions of the Northeast (N. Crescent). In the southern plains of New Mexico, cover crops may result in lower yield and profitability as compared to fallow (Acharya et al. 2019) but still reduce erosion and increased soil C. CCR in eastern Colorado on the other hand showed a significant increase in soil C and yield (Sherrod et al. 2003). Claassen et al. (2018) estimate that approximately 28% of corn area, 12% of soybean area, and 19% of wheat area was planted as part of a CCR in recent years. In our accelerated scenario, where adoption rates need to be increased, it may be practical and/or beneficial to prioritize CCR over cover crops depending upon management and policy goals.

Additionally, there are several limitations with respect to estimating the emissions reduction potential of CCR. First, we used data on hectares with conservation practice payments for CCR as a proxy for adoption of CCR, which results in our analysis likely underestimating the reduction potential of the practice since this is only a subset of total adoption. For example, in the case of cover crops, only about a third of cover cropped hectares in 2018 were estimated to be receiving a payment from a state or federal program (Wallander et al. 2021). COMET-Planner estimates assume scenarios of decreasing fallow frequencies and/or adding perennial crops to rotations, which likely model higher wERCs for CCR than those associated with the practice as implemented and thus overestimate reduction potential. In contrast, the NRCS practice standard for CCR in many cases/regions can be achieved through modifying the crop rotation to include double cropping, cover cropping, legumes, and/or higher residue crops, but without incorporating perennials or reducing fallow. Clearly, acquiring more accurate practice estimates on US cropland can help alleviate these uncertainties and improve overall climate benefit estimates.

Remaining Potential. Regardless of regional limitations to cover crop adoption, there is a large land base available for cover cropping providing large potential for CO₂e reductions. After accounting for adoption under the accelerated scenario, there are 97.6 Mha remaining, which is three times the total projected to be in cover crops under the accelerated scenario. Full adoption of this practice alone could translate to as much as 83 Mt CO₂e y⁻¹ of additional reduction potential. Remaining hectares ranged from 2.4 Mha in E. Uplands to 23.6 Mha in Heartland.

For conservation tillage, there are 4.7 Mha of IT that could be converted to RT or NT and 18.3 Mha of RT that could be converted to NT and opportunities differ based on resource region. For example, in the Fruitful Rim, 59% of hectares remain under IT. The Mississippi Portal and N. Crescent have 40% and 34% of hectares under IT, respectively. Crops in these regions are characterized by grain crops and may be regions to prioritize over the Fruitful Rim since these cropping systems are more amenable to conservation tillage in comparison to specialty crops common to the Fruitful Rim. Conservation tillage adoption is currently lower in many vegetable and specialty crops, particularly organic farms; however, reduced tillage practices have been shown to be successful when combined with cover crops to help promote weed suppression (Pieper et al. 2015; Kornecki and Price 2019). Therefore, further expansion in these regions may be achievable as alternative methods of weed suppression are further refined.

Future Research and Directions. There is a continuous need for model enhancement and validation to improve estimates and reduce uncertainties. For example, recent evidence has called into question the impact of NT management on soil C sequestration when the entire soil profile is considered (Powlson et al. 2014; Stewart et al. 2017), and global efforts are underway to address this issue (Paustian et al. 2019b; Smith et al. 2020). Additionally, model parametrization that allows for estimation of C saturation/equilibrium limit of soils (Six et al. 2002; Stewart et al. 2007) will help extend our ability to project beyond the current 20-year timeframe.

Acquisition of inventory data at multiple spatial, temporal, and commodity levels also is necessary to address the unique characteristics of agricultural systems. The simultaneous use of cover crops and NT on the same hectares may provide synergistic climate benefits (Olson et al. 2014; McClelland et al. 2020; McNunn et al. 2020) but national inventory data are not collected in a consistent or frequent enough manner to capture these changes. Currently, the Ag Census occurs every five years and is not designed to track multiple practices on the same land or by commodity. Other surveys, such as the ARMS or the USDA Conservation Effects Assessment Project, collect much more detail on field- or farm-level adoption of agricultural practices (including conservation practices), but have smaller sample sizes and, in the case of ARMS, targets the sample to be representative of management practices for a particular commodity each year.

In this paper, we assume practices are adopted county-wide but specific crops may limit that assumption and many farmers may vary practice use year-to-year, such as with cover crops or tillage. Many farms in corn (Zea mays L.) and soybean rotations, for example, use alternating tillage in which soybean is NT and tillage occurs before corn. The USDA ERS has documented temporal variation and joint adoption of practices through surveys (Claassen et al. 2018; Wallander et al. 2021), but the spatial resolution is not at the county level, such as the Ag Census. Finally, survey questions that ask farmers to report practices (such as tillage or cover crop use), rely on farmer self-identification of practice categories, which may not always align with practice definitions as modeled or written into a federal practice standard. USDA NASS regularly requests input from stakeholders on improving the census questionnaire; thus, these limitations may be addressed in future federal surveys to provide more specific information about the conservation practices used, how they fit into rotations, and how they are stacked.

New remotely sensed data sources are becoming increasingly useful for tracking practice adoption and potentially can augment survey data, though additional algorithms and models to tie remotely sensed imagery to activities on the ground are needed. A good example of this is the Operational Tillage Information System, or OpTIS, that can detect the presence or absence of a cover crop (Hagen et al. 2020) but is not yet equipped to distinguish between different cover crop species or estimate biomass, which are important for estimating GHG emissions and water quality benefits. Similarly, remotely sensed estimates of tillage practices rely on observed residue cover, which can be, but is not always, a good proxy for soil disturbance or GHG impacts.

The BAU scenario cannot be reached without continued investment in current programs, whereas achieving accelerated targets will require additional investments to overcome economic, social, and policy barriers (Prokopy et al. 2019; Ranjan et al. 2019). To achieve targets set by the US economy-wide commitment under the Paris Climate Agreement, the agricultural sector may have to utilize an aggressive, multipronged approach to reduce emissions far beyond the maximal 40% contribution estimated here. Potential approaches include (1) increased adoption levels of the cropland practices beyond those reported in this paper; (2) additional uptake of practices not included in this study on crop and grazinglands (e.g., precision agriculture, nutrient management, silvopasture, and prescribed grazing) and within the livestock subsector (e.g., feed additives and improved manure handling) (Patra 2016; Harrison et al. 2021); and (3) investing in the research and development of novel approaches such as new crop and microbial genetics, electric tractors, etc. (Northrup et al. 2021). Reaching net C neutrality or beyond would likely require addressing the myriad of barriers associated with adoption, developing nascent market opportunities, supporting the social processes involved in transitioning production systems, and providing substantial incentives.