The greenhouse gas balance of European grasslands

The greenhouse gas ( GHG ) balance of European grasslands ( EU ‐28 plus Norway and Switzerland), including CO 2 , CH 4 and N 2 O, is estimated using the new process‐based biogeochemical model ORCHIDEE ‐ GM over the period 1961–2010. The model includes the following: (1) a mechanistic representation of the spatial distribution of management practice; (2) management intensity, going from intensively to extensively managed; (3) gridded simulation of the carbon balance at ecosystem and farm scale; and (4) gridded simulation of N 2 O and CH 4 emissions by fertilized grassland soils and livestock. The external drivers of the model are changing animal numbers, nitrogen fertilization and deposition, land‐use change, and variable CO 2 and climate. The carbon balance of European grassland ( NBP ) is estimated to be a net sink of 15 ± 7 g C m −2  year −1 during 1961–2010, equivalent to a 50‐year continental cumulative soil carbon sequestration of 1.0 ± 0.4 Pg C. At the farm scale, which includes both ecosystem CO 2 fluxes and CO 2 emissions from the digestion of harvested forage, the net C balance is roughly halved, down to a small sink, or nearly neutral flux of 8 g C m −2  year −1 . Adding CH 4 and N 2 O emissions to net ecosystem exchange to define the ecosystem‐scale GHG balance, we found that grasslands remain a net GHG sink of 19 ± 10 g C‐ CO 2 equiv. m −2  year −1 , because the CO 2 sink offsets N 2 O and grazing animal CH 4 emissions. However, when considering the farm scale, the GHG balance ( NGB ) becomes a net GHG source of −50 g C‐ CO 2 equiv. m −2  year −1 . ORCHIDEE ‐ GM simulated an increase in European grassland NBP during the last five decades. This enhanced NBP reflects the combination of a positive trend of net primary production due to CO 2 , climate and nitrogen fertilization and the diminishing requirement for grass forage due to the Europe‐wide reduction in livestock numbers.