American bison (Bison bison L.) have recovered from the brink of extinction over the past century. The current bison population of about 500,000 is likely to further increase, emphasizing the importance of understanding their environmental impacts. Bison reintroduction creates multiple environmental benefits to grassland ecosystems, but their impacts on greenhouse gas emissions are poorly understood. Bison are thought to have produced some 2 Tg year-1 of the estimated 9-15 Tg year-1 of pre-industrial enteric methane emissions, but few contemporary measurements have been made due to their mobile grazing habits and safety issues associated with direct measurements. Here we use the eddy covariance technique to obtain the first in situ methane flux measurements from a bison herd. We further coupled the eddy covariance methane flux measurements with a flux footprint analysis and bison location measurements from game cameras to obtain estimates of methane flux on a per-animal basis. Measurements were made in an enclosed pasture near Gallatin Gateway, Montana, USA during daytime periods in winter, and we discuss results in the context of opportunities for full annual ruminant greenhouse gas flux measurements.
Results/Conclusions
Methane emissions from the study area were negligible in the absence of bison (mean ± standard deviation = 0.0024 ± 0.042 μmol m-2 s-1) and were significantly greater than zero, 0.048 ± 0.082 μmol m-2 s-1 with a positively skewed distribution, when bison were present. From the eddy covariance flux footprint / game camera analysis we calculated a mean (median) methane flux of 38 μmol s-1 (22 μmol s-1) per animal, or 52 ± 14 g CH4 day-1(31 g CH4 day-1), less than half of measured emission rates for range cattle. Emission estimates are subject to spatial uncertainty in bison location measurements and the flux footprint, and we characterize this uncertainty by performing a sensitivity analysis of flux footprint and bison locations and aggregation, the latter using a two-dimensional Tikhonov Regularization approach. Following our measurements and uncertainty analysis there is no evidence that bison methane emissions exceed those from cattle. We note however that our measurements were made during daytime periods in winter and annual measurements are ultimately necessary to determine the greenhouse gas burden of bison grazing systems. Eddy covariance is a promising technique for measuring ruminant methane emissions in conventional and alternate grazing systems and can be used to compare them going forward.