Global warming potential of French grassland-based dairy livestock systems under climate change.

Despite the increasing interest in assessing the greenhouse gas (GHG) budget of livestock production systems, little is known about the possible impacts of climate change on the future contribution of such systems to global warming. The aim of this study was to assess the global warming potential (GWP) of differently managed grassland-based dairy systems, based either on permanent or on sown grasslands, under climate change at two sites: Avignon (sub-arid/arid) and Mirecourt (sub-humid/humid), representative of French contrasting climates. We compared the near-past conditions (1970-1999) and projections for 2020-2049 (near future) and 2070-2099 (far future), which correspond to the SRES A2 storyline projected by the ARPEGE climate model and downscaled with quantile-quantile regionalization method. The pasture simulation model (PaSim) simulated on-site GHG emissions. Off-site emissions were assessed according to the 2006 IPCC guidelines and attributed to the corresponding grassland field under the assumption that harvested herbage is fully eaten by stalled cattle. The attributed GWP (GWP) of each system was calculated by subtracting from the net C storage the NO and CH emissions occurring within the grassland plot and off-site emissions resulting from farm effluents (i.e. solid and liquid manure and slurry) and the digestion and enteric fermentation by cattle of the cut herbage. Climate change was not expected to significantly modify the GWP of systems, on average, but general trends were observed. Systems based on permanent grasslands presented the largest increase in GWP in the far future, due to faster soil organic matter (SOM) decomposition under climate change and the additional GHG fluxes induced by increased forage production and digestion by dairy-stalled cattle. GWP increase was more evident in extensively managed grassland systems conducted in humid environments (Mirecourt), with twofold higher GWP. On the contrary, GWP reduction is expected to be met with systems based on sown grasslands where SOM decomposition acceleration is compensated by enhanced net primary production, especially under humid conditions (Mirecourt) and for irrigated systems (with a 13% reduction of GWP expressed per livestock unit day, LSU d). Although the expected reduction of the net C storage (down to 68% at Mirecourt in far future), systems based on extensive permanent grasslands will continue to be the least detrimental to global warming, with an GWP of 1.2 t CO-C eq. ha year and of 3.4 kg CO-C eq. LSU day year in the far future. [ABSTRACT FROM AUTHOR]