- Lead Authors:
- Alexander N. Hristov, The Pennsylvania State University
- Jane M. E. Johnson, USDA Agricultural Research Service
- Contributing Authors:
- Charles W. Rice, Kansas State University
- Molly E. Brown, University of Maryland
- Richard T. Conant, Colorado State University
- Stephen J. Del Grosso, USDA Agricultural Research Service
- Noel P. Gurwick, U.S. Agency for International Development
- C. Alan Rotz, USDA Agricultural Research Service
- Upendra M. Sainju, USDA Agricultural Research Service
- R. Howard Skinner, USDA Agricultural Research Service
- Tristram O. West, DOE Office of Science
- Benjamin R. K. Runkle, University of Arkansas
- Henry Janzen, Agriculture and Agri-Food Canada
- Sasha C. Reed, U.S. Geological Survey
- Nancy Cavallaro, USDA National Institute of Food and Agriculture
- Gyami Shrestha, U.S. Carbon Cycle Science Program and University Corporation for Atmospheric Research
<b>Hristov</b>, A. N., J. M. F. <b>Johnson</b>, C. W. Rice, M. E. Brown, R. T. Conant, S. J. Del Grosso, N. P. Gurwick, C. A. Rotz, U. M. Sainju, R. H. Skinner, T. O. West, B. R. K. Runkle, H. Janzen, S. C. Reed, N. Cavallaro, and G. Shrestha, 2018: Chapter 5: Agriculture. In Second State of the Carbon Cycle Report (SOCCR2): A Sustained Assessment Report [Cavallaro, N., G. Shrestha, R. Birdsey, M. A. Mayes, R. G. Najjar, S. C. Reed, P. Romero-Lankao, and Z. Zhu (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 229-263, https://doi.org/10.7930/SOCCR2.2018.Ch5.
Agriculture
SUPPORTING EVIDENCE
KEY FINDINGS
Key Finding 1
Agricultural greenhouse gas (GHG) emissions in 2015 totaled 567 teragrams (Tg) of carbon dioxide equivalent (CO2e) in the United States and 60 Tg CO2e in Canada, not including land-use change; for Mexico, total agricultural GHG emissions were 80 Tg CO2e in 2014 (not including land-use change) (high confidence). The major agricultural non-CO2 emission sources were nitrous oxide (N2O) from cropped and grazed soils and enteric methane (CH4) from livestock (very high confidence, very likely).
Description of evidence base
Bottom-up estimates of GHG emissions are from U.S. EPA (2018), ECCC (2017), and FAOSTAT (2017) data for the United States, Canada, and Mexico, respectively. These estimates include rice cultivation, field burning of agricultural residues, fertilization and liming, enteric fermentation, and manure management, but they do not include land-use change. The major components of agricultural non-CO2 emissions have been consistent in numerous reports including those listed above for the emissions estimates part of this Key Finding.
Major uncertainties
Uncertainty exists in any measurement or projection of GHG emissions. Emissions from enteric fermentation are relatively well studied and predictable, but there is larger uncertainty regarding manure CH4 and N2O emissions. Considerable uncertainty exists in soil carbon accumulation and quantities as well as in terms of emissions from soils under different conditions and management practices. There are large uncertainties in GHG emissions from agricultural cropping systems due to high spatial and temporal variability, measurement methods, cropping systems, management practices, and variations in soil and climatic conditions among regions.
Assessment of confidence based on evidence and agreement, including short description of nature of evidence and level of agreement
There is very high certainty that N2O and CH4 are the major agricultural non-CO2 emission sources. There is high confidence in the numerical estimates.
Summary sentence or paragraph that integrates the above information
For Key Finding 1, enteric CH4 emissions are predictable, but GHG emissions from manure applications or management and agricultural soil and cropping systems are less certain.
Key Finding 2
Agricultural regional carbon budgets and net emissions are directly affected by human decision making. Trends in food production and agricultural management, and thus carbon budgets, can fluctuate significantly with changes in global markets, diets, consumer demand, regional policies, and incentives (very high confidence).
Description of evidence base
Key Finding 2 and the supporting text document the changes resulting from shifts in policy as summarized by Nelson et al. (2009).
Major uncertainties
Major uncertainties related to this Key Finding are the extent and direction of direct and indirect changes in emissions. A change in agricultural management, prompted by many possible social, economic, and policy drivers, often affects both onsite emissions (e.g., soil carbon, N2O, and CH4 emissions) and offsite emissions occurring upstream and downstream (e.g., in energy used for inputs to production and indirect land-use change; Nelson et al., 2009).
Assessment of confidence based on evidence and agreement, including short description of nature of evidence and level of agreement
The confidence that agricultural regional carbon budgets and net emissions are directly affected by human decision making is very high.
Summary sentence or paragraph that integrates the above information
For Key Finding 2, human decisions and policy very likely will affect food production and agricultural management. Management choices strongly influence emissions and soil carbon stocks.
Key Finding 3
Most cropland carbon stocks are in the soil, and cropland management practices can increase or decrease soil carbon stocks. Integration of practices that can increase soil carbon stocks include maintaining land cover with vegetation (especially deep-rooted perennials and cover crops), protecting the soil from erosion (using reduced or no tillage), and improving nutrient management. The magnitude and longevity of management-related carbon stock changes have strong environmental and regional differences, and they are subject to subsequent changes in management practices (high confidence, likely).
Description of evidence base
Most of this carbon pool exists within soils, with less than 5% residing in cropland vegetation, a finding consistent with previous reports such as the First State of the Carbon Cycle Report (CCSP 2007) and USDA (2016). The U.S. Department of Agriculture’s Natural Resources Conservation Service has established 15 standard soil health conservation practices, which have the potential to increase soil carbon and coincidently reduce atmospheric CO2 (Chambers et al., 2016). Evidence indicates that adoption of no tillage may increase carbon storage, especially in the soil surface, compared to conventional tillage (Chambers et al., 2016; Paustian et al., 2016; Sperow 2016), although soil heterogeneity and slow rates of change make the conclusive measurement of short-term changes difficult. It may not be appropriate to assume that adopting no tillage will sequester carbon over the long term or mitigate GHG emissions (e.g., Baker et al., 2007; Luo et al., 2010; Powlson et al., 2014; Ugarte et al., 2014). Practices that convert lands from perennial systems, such as converting retired lands or other lands to row crops, will release stored carbon back to the atmosphere (Gelfand et al., 2011; Huang et al., 2002). Conversely, management practices with the potential to release stored carbon are the inadequate return of crop residues (Blanco-Canqui and Lal 2009) and aggressive tillage (Conant et al., 2007). Conservation practices improve soil aeration, aggregate stability, and nutrient reserves, while modulating temperature and water and increasing microbial activity and diversity. As a result, soil is more resilient to climate variability and more productive (Lal 2015; Lehman et al., 2015).
Major uncertainties
Major uncertainties are related to individual practices such as no-tillage management, in particular the magnitude and longevity of changes to soil carbon stocks. Meta-analyses by Luo et al. (2010) and Ugarte et al. (2014) suggest that other factors contributing to variability in soil organic carbon sequestration include climatic and soil properties interacting with management factors (e.g., cropping frequency, crop rotation diversity, nitrogen, and drainage), along with impacts on rooting depth and above- and belowground biomass. Future shifts in management can reverse gains.
Assessment of confidence based on evidence and agreement, including short description of nature of evidence and level of agreement
Confidence that conservation practices have the potential to increase soil carbon stocks is high.
Estimated likelihood of impact or consequence, including short description of basis of estimate
Implementation of conservation practices on croplands is likely to increase soil carbon stocks. Adopting conservation practices also provides co-benefits such as erosion control.
Summary sentence or paragraph that integrates the above information
For Key Finding 3, implementing conservation practices has strong undisputed co-benefits, including reducing erosion, and may increase soil carbon stocks over time, provided that the practices are continued. Cessation of conservation with reversion to degrading practices will result in a loss of carbon stocks and reduction of co-benefits.
Key Finding 4
North America’s growing population can achieve benefits such as reduced GHG emissions, lowered net global warming potential, increased water and air quality, reduced CH4 flux in flooded or relatively anoxic systems, and increased food availability by optimizing nitrogen fertilizer management to sustain crop yields and reduce nitrogen losses to air and water (high confidence, likely).
Description of evidence base
Agricultural soil management (i.e., synthetic nitrogen fertilizer) is a major source of GHG fluxes in North America (FAOSTAT 2017). Matching nitrogen fertilizer needs to crop needs reduces the risk of loss to air and water (Robertson and Grace 2004; Wang et al., 2011). Nitrogen fertilizer additions generally lead to increased CH4 emissions and decreased CH4 oxidation from soils, particularly under anoxic conditions or flooded soil systems such as rice (Liu and Greaver 2009).
Major uncertainties
Large uncertainties in GHG emissions from agricultural systems exist due to high spatial and temporal variability, measurement methods, cropping systems, management practices, and variations in soil and climatic conditions among regions (Parkin and Venterea 2010).
Assessment of confidence based on evidence and agreement, including short description of nature of evidence and level of agreement
There is high confidence that matching crop needs to nitrogen fertilizer applications can reduce fertilizer-induced GHG emissions.
Estimated likelihood of impact or consequence, including short description of basis of estimate
Avoiding excessive nitrogen fertilizer applications likely will reduce GHG emissions and provide co-benefits such as air and water quality protections.
Summary sentence or paragraph that integrates the above information
For Key Finding 4, nitrogen fertilizer is needed to support grain production. In general, there is high confidence that improving nitrogen management to avoid excess applications can reduce GHG emissions and provide co-benefits. However, considerable uncertainty still exists regarding absolute GHG fluxes.
Key Finding 5
Various strategies are available to mitigate livestock enteric and manure CH4 emissions. Promising and readily applicable technologies can reduce enteric CH4 emissions from ruminants by 20% to 30%. Other mitigation technologies can reduce manure CH4 emissions by 30% to 50%, on average, and in some cases as much as 80%. Methane mitigation strategies have to be evaluated on a production-system scale to account for emission tradeoffs and co-benefits such as improved feed efficiency or productivity in livestock (high confidence, likely).
Description of evidence base
Non-CO2 GHG mitigation strategies for livestock have been summarized in several comprehensive reviews (Montes et al., 2013; Hristov et al., 2013b; Herrero et al., 2016).
Major uncertainties
Uncertainty exists in any measurement or projection of GHG emissions. Uncertainties of GHG mitigation options are related to 1) uncertainties in projecting emissions, 2) uncertainties in projecting mitigation potential, and 3) uncertainties in the extent of the adoption of mitigation options. The uncertainty of farm-scale projections is related to the uncertainty in projecting emissions from individual sources (Chianese et al., 2009). The IPCC (2006) suggested a ±20% uncertainty in projecting both enteric and manure management CH4 emissions. Through the use of process-based models representing common management strategies for the United States, the uncertainty for projecting enteric emissions may be reduced to ±10%, but uncertainty for manure management likely remains around ±20% (Chianese et al., 2009). Considering these uncertainties along with those of other agricultural emission sources, total GHG emissions can be determined with an uncertainty of ±10% to ±15%. As process-level models improve, verified with accurate measurements, this uncertainty can be reduced.
Assessment of confidence based on evidence and agreement, including short description of nature of evidence and level of agreement
There is high confidence that mitigation technologies can reduce livestock enteric and manure emissions. These technologies include practices related to reducing emissions from enteric fermentation (i.e., cattle) and manure management (i.e., cattle and swine) as discussed by Hristov et al. (2013b) and Herrero et al. (2016). Other potential CH4 mitigation strategies include manure solids separation, aeration, acidification, biofiltration, composting, and anaerobic digestion (Montes et al., 2013).
Summary sentence or paragraph that integrates the above information
For Key Finding 5, effective enteric fermentation and manure emissions mitigation options are available or are expected to be available in the near future. Impact will depend on cost-effectiveness and adoption rate.
Key Finding 6
Projected climate change likely will increase CH4 emissions from livestock manure management locations, but it will have a lesser impact on enteric CH4 emissions (high confidence). Potential effects of climate change on agricultural soil carbon stocks are difficult to assess because they will vary according to the nature of the change, onsite ecosystem characteristics, production system, and management type (high confidence).
Description of evidence base
A recent analysis for the northeastern United States (Hristov et al., 2017a) estimated potential climate change effects on livestock GHG emissions.
Major uncertainties
Uncertainties include projecting climate change, its effect on animal feed intake (which determines enteric CH4 emissions), animals’ ability to adapt to climate change, and uncertainties regarding trends in animal productivity. The effect of increased temperature on manure GHG emissions is more predictable.
Assessment of confidence based on evidence and agreement, including short description of nature of evidence and level of agreement
There is high confidence that projected temperature increases are expected to decrease dry matter intake by dairy cows due to heat stress (Hristov et al., 2017a), while CH4 emissions from manure decomposition are expected to increase (Rotz et al., 2016). Climate change effects on soil carbon sequestration balances and interactions with temperature are difficult to predict because temperature may regionally improve or degrade growing conditions, thereby shifting associated biomass inputs (Zhao et al., 2017; Tubiello et al., 2007). Likewise, increased atmospheric CO2 will increase soil CO2 respiration and mineralization as much as carbon inputs, resulting in little net change in soil carbon pools (Dieleman et al., 2012; Todd-Brown et al., 2014; van Groenigen et al., 2014).
Summary sentence or paragraph that integrates the above information
For Key Finding 6, projected climate changes likely will not significantly affect enteric CH4 emissions from livestock, but increased temperature is expected to increase manure GHG emissions.
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