The greenhouse gas balance of European grasslands

Jinfeng Chang (1, 2) , Philippe Ciais (1, 3) , Nicolas Viovy (1, 4) , Nicolas Vuichard (1, 4) , Benjamin Sultan , Jean-François Soussana (5)

1 LSCE - Laboratoire des Sciences du Climat et de l'Environnement [Gif-sur-Yvette]
2 LOCEAN - Laboratoire d'Océanographie et du Climat : Expérimentations et Approches Numériques
3 ICOS-ATC - ICOS-ATC
4 MOSAIC - Modélisation des Surfaces et Interfaces Continentales
5 UREP - Unité de recherche sur l'Ecosystème Prairial

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.

Mots clés: greenhouse gases balance; European grassland; livestock; ORCHIDEE; grassland management

Domaines: Climatologie; Géophysique [physics.geo-ph]

Jinfeng Chang, Philippe Ciais, Nicolas Viovy, Nicolas Vuichard, Benjamin Sultan, et al.. The greenhouse gas balance of European grasslands. Global Change Biology, 2015, 21 (10), pp.3748-3761. ⟨10.1111/gcb.12998⟩. ⟨hal-01196330⟩