Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 6 9 2009 10.5194/bg-6-1903-2009 http://www.biogeosciences.net/6/1903/2009/ http://www.biogeosciences.net/6/1903/2009/bg-6-1903-2009.html http://www.biogeosciences.net/6/1903/2009/bg-6-1903-2009.pdf 1903 1915 2009-09-23 Ammonia sources and sinks in an intensively managed grassland canopy M. David B. Loubet loubet@grignon.inra.fr P. Cellier M. Mattsson J. K. Schjoerring E. Nemitz R. Roche M. Riedo M. A. Sutton Inst. National de la Recherche Agronomique, UMR Environnement et Grandes Cultures, 78850 Thiverval-Grignon, France Plant and Soil Science Laboratory, University of Copenhagen, Faculty of Life Sciences, Thorvaldsensvej 40, 1871 Frederiksberg C, Copenhagen, Denmark Centre for Ecology and Hydrology (Edinburgh Research Station), Bush Estate, Penicuik, Midlothian, EH26 0QB, UK Inst. fur Agrarokologie, Bundesforschungsanstalt fur Landwirtschaft (FAL), Bundesallee 50, 38116 Braunschweig, Germany now at: Section for Economy and Technology, Halmstad University, Halmstad, 30118, Sweden Grasslands represent canopies with a complex structure where sources and sinks of ammonia (NH₃) may coexist at the plant level. Moreover, management practices such as mowing, hay production and grazing may change the composition of the sward and hence the source-sink relationship at the canopy level as well as the interaction with the atmosphere. There is therefore a need to understand the exchange of ammonia between grasslands and the atmosphere better, especially regarding the location and magnitude of sources and sinks. <br><br> Fluxes of atmospheric NH₃ within a grassland canopy were assessed in the field and under controlled conditions using a dynamic chamber technique (cuvette). These cuvette measurements were combined with extraction techniques to estimate the ammonium (NH₄⁺) concentration and the pH of a given part of the plant or soil, leading to an estimated ammonia compensation point (<I>C_p</I>). The combination of the cuvette and the extraction techniques was used to identify the potential sources and sinks of NH₃ within the different compartments of the grassland: the soil, the litter or senescent "litter leaves", and the functioning "green leaves". A set of six field experiments and six laboratory experiments were performed in which the different compartments were either added or removed from the cuvettes. <br><br> The results show that the cuvette measurements agree with the extraction technique in ranking the strength of compartment sources. It suggests that in the studied grassland the green leaves were mostly a sink for NH₃ with a compensation point around 0.1–0.4 μg m^&minus;3 and an NH₃ flux of 6 to 7 ng m^&minus;2 s^&minus;1. Cutting of the grass did not increase the NH₃ fluxes of the green leaves. The litter was found to be the largest source of NH₃ in the canopy, with a <I>C_p</I> of up to 1000 μg m^&minus;3 NH₃ and an NH₃ flux up to 90 ng m^&minus;2 s^&minus;1. The litter was found to be a much smaller NH₃ source when dried (<I>C_p</I>=160 μg m^&minus;3 and <I>F</I>_NH3=35 ng m^&minus;2 s^&minus;1 NH₃). Moreover emissions from the litter were found to vary with the relative humidity of the air. 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