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Overview

Soils are the largest organic carbon pool on the land surface, and agricultural soils that have been disturbed by tillage and other practices for many years have lost carbon to the atmosphere. This historical loss, however, means that there is substantial opportunity to increase soil organic carbon (SOC) in agricultural soils by adopting practices that reduce soil disturbance and increase carbon from organic matter. These practices include conservation tillage, diverse crop rotations, residue retention and cover crops (Paustian et al., 2016). In recent years, many private sector efforts have begun to explore potential SOC sequestration as a strategy for meeting company and industry targets for climate change mitigation. 

Carbon accumulation in the soil is difficult to measure because it occurs slowly over long time periods and does not follow a linear trend. While initial increase in carbon following a farm management change may be rapid, that rate will slow over time as the soil system begins to approach a new ecosystem equilibrium, or steady state (Paustian et al., 2016). For example, after conversion from conventional tillage to a continuous no tillage system, a field may approach a new equilibrium after 15-20 years with the largest sequestration rates occurring between 5-10 years (West and Post, 2003). Measuring soil carbon sequestration is complex; there are important dynamics occurring underground and out of range of direct observation. For example, studies have shown that SOC increases in the upper layers of soil following adoption of no tillage corresponds with a reduction of SOC in the lower layers. In effect, reduction in tillage reallocates carbon in the soil profile (Blanco-Canqui and Lal, 2008). This dynamic is attributed to how the shift to no-tillage reduces the incorporation of crop residues and root material into soils (Baker et al., 2007). Regardless of the climate mitigation benefit, SOC is an important sustainability indicator as a measure of soil health that supports many of the functions and ecosystem services vital to agricultural production (Lal, 2016). 

Field to Market's goals include recognizing the critical importance of soil carbon both to mitigate climate change and to improve soil health and the resilience of agricultural lands to extreme climate events. To incorporate soil carbon into the Field to Market program, we have adopted two field-level assessment tools that provide farmers with an annual snapshot of their soil's health and assess the potential to increase SOC by adopting conservation practices. SECTION 1: FIELD TO MARKET'S
SOIL CARBON METRIC

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The primary Soil Carbon metric in the Fieldprint Platform is the Soil Conditioning Index (SCI), a conservation planning tool developed by USDA NRCS to provide guidance to users on probable directional change in soil carbon as a result of practice adoption and change. SCI has three main components-soil organic matter (SOM), field operations and erosion. SOM contains approximately 58% carbon, and therefore the SCI provides an indication of whether a soil is gaining or losing carbon. SCI is calculated from the Revised Universal Soil Loss Equation 2 (RUSLE2) and is a unitless, relative and crop-specific measure with an output range of -1 to +1. Very small values (- 0.05 to +0.05) represent index levels where there is little or no confidence that soil organic matter (SOM) is changing in either direction. As the SCI value moves further away from zero, it indicates greater confidence that the soil carbon is changing; therefore, higher values approaching +1.0 indicate greater confidence that SOC is increasing and lowest values, approaching -1.0 indicate greater confidence that soil carbon is decreasing. The advantages of the SCI are that it is relatively simple to use and can be applied with just one year of information about a farm operation. Note that this method only captures the dynamics of soil carbon in the surface layer of the soil. 

Field to Market has also integrated a second-COMET-Planner-as an optional scenario planner to assess how recent or planned changes in practices might impact carbon in their soils (Swan et al., 2020). This feature allows producers and their advisers to quickly and simply estimate the quantity of carbon various conservation practices might sequester in their fields. Together, these tools provide both a high-level assessment of soil health and a starting point for understanding the potential benefits to a producer and farm from engaging with private sector carbon markets before committing to the extensive testing and modeling requirements of market entry. SECTION 2: National Trends in 
Soil Carbon from 1990 to 2015

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Given the complexity of soil organic carbon measurements described above, understanding how the SOC content of agricultural soils in the United States has changed over time requires application of sophisticated simulation models. As a party to the UN Framework Convention on Climate Change, the United States produces annual inventories of all greenhouse gas emissions sources and sinks (U.S. EPA, 2021a), including those from agriculture. To support this reporting, the USDA publishes a quadrennial greenhouse gas inventory, focused on agriculture, forestry and land use change, which contains detailed national- and state-scale modeling of all greenhouse gas sources and sinks for agricultural lands. The most recent USDA Agriculture and Forestry Greenhouse Gas Inventory assesses these changes from 1990 to 2015 (USDA, 2021) and provides the most comprehensive estimate of SOC change on U.S. croplands available. Here we examine the results for major cropping systems grown on mineral soils, which are low in organic matter. It is important to note that organic soils, while small in area in the U.S., are very vulnerable to soil carbon loss when cultivated. 

The USDA uses a simulation model called DayCent to estimate soil carbon using detailed data on land management, weather conditions, soil characteristics and land use history. The modeling is conducted for 400,000 National Resources Inventory (NRI) survey points that represent a statistical sampling of land use and management practices on all non-federal lands in the United States. DayCent models plant-soil nutrient cycling by simulating key processes occurring in the soil, including plant growth, senescence, decomposition of dead plant matter and other organic matter and nitrogen mineralization (DelGrosso et al., 2001b; a). Because the simulations run for multiple years, they are accounting for whole crop rotations so results are available for entire cropping systems rather than just individual crops. The analysis defined 10 major cropping systems based on five-year rotations as determined by the NRI survey data. Six of the cropping systems contain results for the commodity crops considered in this report. 

 

• Row crops: At least three of five years in corn, soybean and/or sorghum;
• Small grains: At least three of five years in barley, wheat and/or oats;
• Low residue crops: At least three of five years in cotton, potatoes, sugar beets, dry beans, onions and/or tomatoes;
• Hay (legume): Five continuous years in legume hay;
• Flooded rice: At least three of five years in flooded rice production;
• Other: agricultural lands that did not have three of five years in any of the other definitions. Contains a mix of crops and diverse rotations.

We also include in our discussion the soil carbon change in land enrolled in the Conservation Reserve Program (CRP). This land has been identified as environmentally sensitive and set aside from active crop production, typically planted to grasslands or other perennial vegetation. SECTION 3: Soil Carbon Trends by Cropping System The results presented here are taken from the USDA analysis (USDA, 2021) and represent the change in SOC stock in one year from all lands in the U.S. in a particular cropping system (as defined above). The results are displayed in units of million metric tons of carbon dioxide equivalent (MMT CO₂e). We display carbon sequestration (gain) in soils as a positive stock change and carbon emissions (loss) from soils as a negative stock change. 

Overall, soils actively managed under the six cropping systems considered here have increased soil carbon stock throughout the last 25 years (Figure 2.2.1). The amount of carbon gained fluctuates over time with both the area in production for each of the cropping systems and changes in management practices and weather. Lands that are left fallow are also included and illustrate the importance of living plants to maintaining and increasing soil carbon. Overall, the amount of carbon gained has varied and the most recent two years of analysis available-2010 and 2015-indicate losses of soil carbon from small grains and low residue crops, and relatively steady gains in the other cropping systems.

Row Crops

USDA modeling of soil carbon stock change for row crop systems considers croplands that have been in production of corn, soybean and/or sorghum in at least three years of a five-year period. This definition captures most lands in corn and sorghum production. Row crop rotations typically contain some high residue crops. These lands have consistently added carbon to the soil over the past 25 years. The increase in soil carbon can be attributed both to increases in the acreage used for production of these crops, as well as shifts toward reduced and no tillage that have occurred since 1990 (Figure 2.2.2).

Small Grains

Small grain systems are defined as lands that are in production of wheat, barley and/or oats in at least three years of a five-year period. Land area dedicated to small grain systems has been declining over the past 25 years. In addition, these crops are prevalent in western regions of the country that may be water limited, and wheat is frequently grown in a wheat-fallow rotation small grain systems, which produce lower amounts of crop residues, were responsible for a modest amount of soil carbon gain from 1990 to 2005; however, in 2010 and 2015, these lands have seen reduction in soil carbon stock, representing an emission of carbon dioxide from the soil (Figure 2.2.3).

Low Residue Crops

Cotton, potatoes and sugar beets are included in the USDA modeling category of Low Residue crops due to their plant characteristics and harvest practices leaving little residue on the soil after harvest. Harvesting root crops like potatoes and sugar beets requires a greater amount of soil disturbance. Together, low crop residue and necessary soil disturbance contribute to the soils in these cropping systems typically emitting carbon rather than gaining carbon. Their overall acreage and contribution to the total soil carbon storage on croplands is small and has typically gained or lost less than 1 MMT CO₂e in the years considered. The exception is a greater loss of soil carbon occurring in the most recent analysis year of 2015 (Figure 2.2.4).

Legume Hay

Perennial hay crops have greater potential to increase carbon in the soil as they require less disturbance of the soil in most years. Alfalfa is the most common legume hay grown in the United States, and the combination of reduced disturbance and nitrogen fixation contribute to lands growing alfalfa consistently gaining soil carbon stock throughout the time period analyzed here (Figure 2.2.5).

Rice

Rice systems are considered separately in the USDA modeling analysis as the crop is typically grown on flooded fields and the biogeochemical cycles that determine the carbon and nitrogen balance in the soil operate differently in the oxygen-deficient flooded environment. In the United States, the acreage in rice production is small, so the contribution to overall soil carbon stock is small. Rice has consistently demonstrated a gain in soil carbon stock over the past 25 years (Figure 2.2.6).

Other Crops

The "other crops" category refers to cropping systems that did not fall under the more specific categories considered above and typically represent more complex rotations. These lands also demonstrate consistent increase in soil carbon stock over the study period (Figure 2.2.7).

Soil Carbon Trends for Lands in the Conservation Reserve Program

Lands in the Conservation Reserve Program (CRP) are removed from active crop production for a period of time, and are included here as these lands were previously, and are likely to be again, in active production of crops. Setting aside land in a perennial grassland can increase the carbon in the soil and improve the overall soil health. These lands consistently provide a sink for soil carbon throughout the study period, with fluctuations in the carbon stock change determined by the extent and location of land set aside in any given period (Figure 2.2.8). SECTION 4: Soil Carbon Summary

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Overall, soils actively managed under the cropping systems considered in the Field to Market program have increased soil carbon stock during the last 25 years. The greatest soil carbon gain was observed in 2005 and in later years less carbon gain and some increases in carbon losses is observed. These findings are consistent with the analysis of Soil Conservation in Part 1 of this report, which indicate that reductions in reducing soil erosion have largely plateaued as total acres in reduced and no tillage practices has stayed steady. The adoption of conservation tillage is the most significant factor influencing the soil carbon gains observed here, with additional contributions from manure management and including perennial hay in rotations (USDA, 2021). 

While cover crops are included in the USDA analysis, they do not correspond to a significant increase in soil carbon. This is due to a limitation in the data available for the modeling, which does not include details on cover crop termination practice and, as a result, the models assume termination using tillage (USDA, 2021). Better information is needed from surveys on the methods of termination, such as through herbicide application or mechanical rolling, that do not involve soil disturbance. Under those conditions, cover crops are associated with increasing soil carbon (USDA, 2021). While cover crop acreage is currently relatively small, it is increasing and this detail will become an important consideration for assessing trends in soil carbon over time. 

Other conservation practices that increase soil carbon sequestration on a farm are not included in the USDA analysis. For example, conservation practices that convert small areas of sensitive and low productivity cropland within a crop field to grasslands are also increasingly part of the toolkit available to farmers. These include grassed waterways, buffer strips at the edge of fields and prairie strips, as well as using economic and spatial analysis to identify where land can be taken out of production without negatively impacting the profitability of a farm operation. These practices have multiple environmental benefits, including soil carbon storage, erosion control and creating habitat to support diverse ecosystems. 

Over the past decade there has been increasing awareness of the importance of soil organic carbon for agricultural productivity, soil health and climate mitigation. Public and private sector efforts to improve climate outcomes hold promise to accelerate adoption of agronomic practices that improve soil health and store soil carbon. The development of incentives and market programs for these benefits hold promise to accelerate adoption of agricultural management practices through financial and technical assistance to farmers. Future soil carbon sequestration in croplands will depend both on the adoption of SOC sequestering practices as well as on changes in weather conditions from ongoing climate change. Continuing to track trends over time is important for understanding agriculture's potential to contribute to climate mitigation and meeting domestic and international goals and commitments.