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The greenhouse gas balance of European grasslands

Chang, Jinfeng, Ciais, Philippe, Viovy, Nicolas, Vuichard, Nicolas, Sultan, Benjamin, Soussana, Jean‐François
Global change biology 2015 v.21 no.10 pp. 3748-3761
carbon, carbon dioxide, carbon sequestration, climate, ecosystems, farms, forage grasses, grassland soils, grasslands, grazing, greenhouse gas emissions, greenhouse gases, land use change, livestock, methane, models, net ecosystem exchange, nitrogen fertilizers, nitrous oxide, primary productivity, Norway, Switzerland
The greenhouse gas (GHG) balance of European grasslands (EU‐28 plus Norway and Switzerland), including CO₂, CH₄ and N₂O, 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 N₂O and CH₄ 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 CO₂ 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 CO₂ fluxes and CO₂ 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 CH₄ and N₂O 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‐CO₂ equiv. m⁻² year⁻¹, because the CO₂ sink offsets N₂O and grazing animal CH₄ emissions. However, when considering the farm scale, the GHG balance (NGB) becomes a net GHG source of −50 g C‐CO₂ 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 CO₂, climate and nitrogen fertilization and the diminishing requirement for grass forage due to the Europe‐wide reduction in livestock numbers.