Full accounting of the greenhouse gas (CO 2, N 2O, CH 4) budget of nine European grassland sites

• Published in: Agriculture, ecosystems & environment ecosystems & environment, 2007-06, Vol.121 (1), p.121-134
• Main Authors: Soussana, J.F., Allard, V., Pilegaard, K., Ambus, P., Amman, C., Campbell, C., Ceschia, E., Clifton-Brown, J., Czobel, S., Domingues, R., Flechard, C., Fuhrer, J., Hensen, A., Horvath, L., Jones, M., Kasper, G., Martin, C., Nagy, Z., Neftel, A., Raschi, A., Baronti, S., Rees, R.M., Skiba, U., Stefani, P., Manca, G., Sutton, M., Tuba, Z., Valentini, R.
• Format: Article
• Language: English
• Subjects: biogeochemical cycles; carbon dioxide; Carbon sequestration; cattle; climatic factors; digestion; Ecology, environment; fertilizer rates; gas emissions; gas exchange; geographical variation; grassland soils; grasslands; grazing management; greenhouse gases; Life Sciences; Livestock; Methane; mowing; net ecosystem exchange; Nitrogen cycle; nitrogen fertilizers; Nitrous oxide; soil microorganisms; source-sink relationships
• ISSN: 0167-8809; 1873-2305; 0167-8809
• DOI: 10.1016/j.agee.2006.12.022

The full greenhouse gas balance of nine contrasted grassland sites covering a major climatic gradient over Europe was measured during two complete years. The sites include a wide range of management regimes (rotational grazing, continuous grazing and mowing), the three main types of managed grasslands across Europe (sown, intensive permanent and semi-natural grassland) and contrasted nitrogen fertilizer supplies. At all sites, the net ecosystem exchange (NEE) of CO 2 was assessed using the eddy covariance technique. N 2O emissions were monitored using various techniques (GC-cuvette systems, automated chambers and tunable diode laser) and CH 4 emissions resulting from enteric fermentation of the grazing cattle were measured in situ at four sites using the SF 6 tracer method. Averaged over the two measurement years, net ecosystem exchange (NEE) results show that the nine grassland plots displayed a net sink for atmospheric CO 2 of −240 ± 70 g C m −2 year −1 (mean ± confidence interval at p > 0.95). Because of organic C exports (from cut and removed herbage) being usually greater than C imports (from manure spreading), the average C storage (net biome productivity, NBP) in the grassland plots was estimated at −104 ± 73 g C m −2 year −1, that is 43% of the atmospheric CO 2 sink. On average of the 2 years, the grassland plots displayed annual N 2O and CH 4 (from enteric fermentation by grazing cattle) emissions, in CO 2-C equivalents, of 14 ± 4.7 and 32 ± 6.8 g CO 2-C equiv. m −2 year −1, respectively. Hence, when expressed in CO 2-C equivalents, emissions of N 2O and CH 4 resulted in a 19% offset of the NEE sink activity. An attributed GHG balance has been calculated by subtracting from the NBP: (i) N 2O and CH 4 emissions occurring within the grassland plot and (ii) off-site emissions of CO 2 and CH 4 as a result of the digestion and enteric fermentation by cattle of the cut herbage. On average of the nine sites, the attributed GHG balance was not significantly different from zero (−85 ± 77 g CO 2-C equiv. m −2 year −1). The net exchanges by the grassland ecosystems of CO 2 and of GHG were highly correlated with the difference in carbon used by grazing versus cutting, indicating that cut grasslands have a greater on-site sink activity than grazed grasslands. However, the net biome productivity was significantly correlated to the total C used by grazing and cutting, indicating that, on average, net carbon storage declines with herbage utilisation for herbivore