Agriculture is a significant source of greenhouse gas (GHG) emissions as well as a potential C sink to offset emissions from other sectors of the economy.  Hence policies to encourage mitigation of GHGs though better land use and management practices are being pursued in the US and internationally.  Accurate and practical methods to quantify emissions and emission reductions are key to guiding policy- and decision-making.  Requirements and attributes of viable quantification systems are discussed, considering applications at both national and regional scales and at the project level.  Examples of systems operational in the US, at the national level and at the farm-scale, are given.

Results/Conclusions

National/state-level estimates for agricultural sources employ ‘bottom-up’ inventory methods, using empirical and/or process-based models.  Direct measurements of C stock changes and gas emissions play a key role in estimating bias and uncertainty in model-based estimates.  For soil-based emissions, the relative scarcity of field measurements are the limiting factor in reducing uncertainty in inventory estimates.  Top-down atmospheric-based emission estimates can help constrain bottom-up inventories but cannot provide specific source attribution.  Results from the US national inventory system, employing the Century and DayCent process models and empirically based uncertainty estimates, are given.  Project-level estimation of emission reductions imposes additional requirements with respect to determining additionality, leakage and permanence.  Ex ante estimates of baseline conditions and project scenarios typically required model-based systems, while ex post estimates rely more heavily on direct measurements.  Several approaches for project-level accounting have been developed/proposed and there is an increasing need for more uniform standards for voluntary as well as mandatory emission reduction programs.  Application of the COMET-VR system for estimating field-level soil GHG emissions under the US 1605B program is demonstrated.