Greenhouse gas fluxes over managed grasslands in Central Europe

Central European grasslands are characterized by a wide range of different management practices in close geographical proximity. Site-specific management strategies strongly affect the biosphere-atmosphere exchange of the three greenhouse gases (GHG) carbon dioxide (CO₂ ), nitrous oxide (N₂ O), and methane (CH₄ ). The evaluation of environmental impacts at site level is challenging, because most in situ measurements focus on the quantification of CO₂ exchange, while long-term N₂ O and CH₄ flux measurements at ecosystem scale remain scarce. Here, we synthesized ecosystem CO₂ , N₂ O, and CH₄ fluxes from 14 managed grassland sites, quantified by eddy covariance or chamber techniques. We found that grasslands were on average a CO₂ sink (-1,783 to -91 g CO₂  m^-2  year^-1 ), but a N₂ O source (18-638 g CO₂ -eq. m^-2  year^-1 ), and either a CH₄ sink or source (-9 to 488 g CO₂ -eq. m^-2  year^-1 ). The net GHG balance (NGB) of nine sites where measurements of all three GHGs were available was found between -2,761 and -58 g CO₂ -eq. m^-2  year^-1 , with N₂ O and CH₄ emissions offsetting concurrent CO₂ uptake by on average 21 ± 6% across sites. The only positive NGB was found for one site during a restoration year with ploughing. The predictive power of soil parameters for N₂ O and CH₄ fluxes was generally low and varied considerably within years. However, after site-specific data normalization, we identified environmental conditions that indicated enhanced GHG source/sink activity ("sweet spots") and gave a good prediction of normalized overall fluxes across sites. The application of animal slurry to grasslands increased N₂ O and CH₄ emissions. The N₂ O-N emission factor across sites was 1.8 ± 0.5%, but varied considerably at site level among the years (0.1%-8.6%). Although grassland management led to increased N₂ O and CH₄ emissions, the CO₂ sink strength was generally the most dominant component of the annual GHG budget.