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Abstract. The exchange of ammonia between crop canopies and the atmosphere depends on a range of plant parameters and climatic conditions. However, little is known about effects of management factors. We have here investigated the stomatal ammonia compensation point in response to cutting and fertilization of a grass sward dominated by Lolium perenne. Tall grass had a very low NH 3 compensation point (around 1 nmol mol −1 ), reflecting the fact that leaf nitrogen (N) concentration was very low. During re-growth after cutting, leaf tissue concentrations of NO concentration of the newly emerging leaves to increase dramatically. The NH 3 compensation point peaked between 15 and 25 nmol mol −1 the day after the fertiliser was applied and thereafter decreased over the following 10 days until reaching the same level as before fertilisation. Ammonium concentrations in leaf apoplast, bulk tissue and litter were positively correlated (P=0.001) throughout the experimental period. Bulk tissue NH

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Abstract. Grasslands represent canopies with a complex structure where sources and sinks of ammonia (NH 3 ) 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.Fluxes of atmospheric NH 3 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 (NH + 4 ) concentration and the pH of a given part of the plant or soil, leading to an estimated ammonia compensation point (C p ). The combination of the cuvette and the extraction techniques was used to identify the potential sources and sinks of NH 3 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.The results show that the cuvette measurements agree with the extraction technique in ranking the strength of compartCorrespondence to: B. Loubet (loubet@grignon.inra.fr) ment sources. It suggests that in the studied grassland the green leaves were mostly a sink for NH 3 with a compensation point around 0.1-0.4 µg m −3 and an NH 3 flux of 6 to 7 ng m −2 s −1 . Cutting of the grass did not increase the NH 3 fluxes of the green leaves. The litter was found to be the largest source of NH 3 in the canopy, with a C p of up to 1000 µg m −3 NH 3 and an NH 3 flux up to 90 ng m −2 s −1 . The litter was found to be a much smaller NH 3 source when dried (C p =160 µg m −3 and F NH3 =35 ng m −2 s −1 NH 3 ). Moreover emissions from the litter were found to vary with the relative humidity of the air. The soil was a strong source of NH 3 in the period immediately after cutting (C p =320 µg m −3 and F NH3 =60 ng m −2 s −1 ), which was nevertheless always smaller than the litter source. The soil NH 3 emissions lasted, however, for less than one day, and were not observed with sieved soil. They could not be solely explained by xylem sap flow extruding NH + 4 . These results indicate that future research on grassland-ammonia relationships should focus on the post-mowing period and the role of litter in interaction with meteorological conditions.

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