Simulation of GHG Impact and Calf Production from Converting Degraded Croplands to AMP Grazing in Southeast USA
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Chien, Catherine
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Chien, Catherine. 2021. Simulation of GHG Impact and Calf Production from Converting Degraded Croplands to AMP Grazing in Southeast USA. Master's thesis, Harvard University Division of Continuing Education.Abstract
Within the global food system, beef cattle is the single largest contributor at 8.5% of total anthropogenic greenhouse gas (GHG) emissions globally (Xu et al., 2021). On the other hand, rangelands cover nearly half of the Earth’s land surface and store almost 20% of the global soil organic carbon (SOC) (Conant, 2012), and there is significant potential for increasing SOC by restoring degraded cultivated lands (Sanderson, 2020). Yet, the SOC impacts from beef cattle production are only beginning to be understood.There are emerging studies that adaptive multi-paddock (AMP) grazing could turn cattle from a GHG emitter into a net carbon sink, if done on degraded cropland, thereby restoring degraded cropland while providing a carbon sink and a food source for humans.
My thesis research identified and quantified the amount of suitable degraded cropland that can be converted into AMP cow-calf operations in the U.S. Southeast region, the GHG impact, and the expected calf output. My research objectives were to determine: (i) the GHG impact of converting degraded cropland to AMP cow-calf operations in the U.S Southeast region; (ii) the calf output that can be expected from such conversion; and (iii) the amount of cropland where such conversion would be the most effective for GHG mitigation, and suggest ways to prioritize such conversion.
The three characteristics (soil taxonomy, slope and state) of the five AMP sites studied by Mosier et al. (2020) were used to distinguish cropland in the Southeast region into 10 categories of cropland (Cropland Groups) across four states: Alabama, Kentucky, Mississippi, and Tennessee.
Google Earth Engine (GEE) was used to digitally map each Cropland Group, calculate the SOC levels across the qualifying terrain, and determine degraded cropland (Degraded Cropland)—defined as cropland with less than the lowest 20th percentile SOC within each Cropland Group.
Livestock emissions were estimated using EPA and IPCC coefficients for enteric emissions, manure related emissions (direct and indirect N2O), along with on-farm emissions. SOC sequestration was approximated using the five AMP farm pair sites in Mosier et al. (2020).
The analysis identified 76,902 ha of Degraded Cropland across all 10 Cropland Groups, which could yield 251,935 calves per 612-day production cycle. The base case using EPA coefficients showed that four of the 10 Cropland Groups could be net carbon sinks, whereas in the IPCC scenario, only two of the 10 Cropland Groups were net carbon sinks.
This study has implications for land use planning, food security and climate mitigation, and points to a need for greater understanding in three areas to determine whether AMP grazing could achieve a net carbon sink in a given area of degraded cropland: (1) more accurately estimate livestock emissions through more analyses and direct observations of beef cattle emission factors, such as the co-dependent relationship between forage digestibility and methane conversion factor; (2) more accurately determine SOC sequestration potential and rate through better understanding of SOC sequestration pathways, and (3) fine-tune AMP herd assumptions for local site conditions to maximize chances of achieving a net carbon sink.
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