Are greenhouse gas emissions from dairy manure management currently underestimated?

Background/Question/Methods

Globally, about 7% of greenhouse gas emissions from agriculture are from livestock manure management and about 33% are from enteric fermentation; however, manure management contributes a greater proportion in increasingly industrialized dairy agroecosystems.  For example, manure management in the U.S. produces 43% of the methane (CH₄) from dairies, and in California that fraction increases to 53%.  Errors in estimates of dairy manure management emissions may have a significant impact on total agriculture emission inventories.  Current emissions inventories are calculated using emissions factors determined from small-scale experiments which have not been compared to field-scale measurements.  We compiled published data on field-scale measurements of greenhouse gas emissions from working and research dairies (38 studies total globally) and compared these to rates predicted by the IPCC Tier 2 approach.  

Results/Conclusions

Anaerobic lagoons were the largest source of CH₄ (1008 ± 528 g CH₄ hd^-1 d^-1), more than  three times that from liquid slurry storage, the next highest at 276 ± 129 g CH₄ hd^-1 d^-1.  Corrals and solid manure piles were large sources of nitrous oxide (N₂O, 4.2 ± 2.3 and 2.9 ± 1.8 g N₂O hd^-1 d^-1, respectively).  Unexpectedly, N₂O emissions from anaerobic lagoons were also large (2.4 ± 1.3 g N₂O hd^-1 d^-1) and barns were an order of magnitude greater (28 ± 17 g N₂O hd^-1 d^-1).  Measured greenhouse gas emissions varied widely due to the heterogeneity of surface conditions and manure composition, and the measurement technique.  The tier 2 calculations underestimated methane emissions for most manure management practices, and underestimated nitrous oxide emissions from anaerobic lagoons and manure piles. Refining these calculations requires: 1) multiple year data sets, 2) within-site comparisons across measurement scales using different measurement techniques, and 3) better metadata (e.g. climate and management parameters) to constrain greenhouse gas emission models.  Our results suggest that targeting greenhouse gas emissions from liquid manure storage systems, by capturing emitted methane or handling more manure in solid form, provides the greatest mitigation opportunity.