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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.

Abstract. A major international experiment on ammonia (NH3) biosphere-atmosphere exchange was conducted over intensively managed grassland at Braunschweig, Germany. The experimental strategy was developed to allow an integrated analysis of different features of NH3 exchange including: a) quantification of nearby emissions and advection effects, b) estimation of net NH3 fluxes with the canopy by a range of micrometeorological measurements, c) analysis of the sources and sinks of NH3 within the plant canopy, including soils and bioassay measurements, d) comparison of the effects of grassland management options on NH3 fluxes and e) assessment of the interactions of NH3 fluxes with aerosol exchange processes. Additional technical objectives included the inter-comparison of different estimates of sensible and latent heat fluxes, as well as continuous-gradient and Relaxed Eddy Accumulation (REA) systems for NH3 fluxes. The prior analysis established the spatial and temporal design of the experiment, allowing significant synergy between these objectives. The measurements were made at 7 measurement locations, thereby quantifying horizontal and vertical profiles, and covered three phases: a) tall grass canopy prior to cutting (7 days), b) short grass after cutting (7 days) and c) re-growing sward following fertilization with ammonium nitrate (10 days). The sequential management treatments allowed comparison of sources-sinks, advection and aerosol interactions under a wide range of NH3 fluxes. This paper describes the experimental strategy and reports the grassland management history, soils, environmental conditions and air chemistry during the experiment, finally summarizing how the results are coordinated in the accompanying series of papers.

Croplands mainly act as net sources of the greenhouse gases carbon dioxide (CO₂) and nitrous oxide (N₂O), as well as nitrogen oxide (NO), a precursor of troposheric ozone. We determined the carbon (C) and nitrogen (N) balance of a four-year crop rotation, including maize, wheat, barley and mustard, to provide a base for exploring mitigation options of net emissions. The crop rotation had a positive net ecosystem production (NEP) of 4.4 +/- 0.7 Mg C ha(-1) y(-1) but represented a net source of carbon with a net biome production (NBP) of -1.3 +/- 1.1 Mg C ha(-1) y(-1). The nitrogen balance of the rotation was correlated with the carbon balance and resulted in net loss (-24 +/- 28 kg N ha(-1) y(-1)). The main nitrogen losses were nitrate leaching (-11.7 +/- 1.0 kg N ha(-1) y(-1)) and ammonia volatilization (-9 kg N ha(-1) y(-1)). Dry and wet depositions were 6.7 +/- 3.0 and 5.9 +/- 0.1 kg N ha(-1) y(-1), respectively. Fluxes of nitrous (N₂O) and nitric (NO) oxides did not contribute significantly to the N budget (N₂O: -1.8 +/- 0.04; NO: -0.7 +/- 0.04 kg N ha(-1) y(-1)) but N₂O fluxes equaled 16% of the total greenhouse gas balance. The link between the carbon and nitrogen balances are discussed. Longer term experiments would be necessary to capture the trends in the carbon and nitrogen budgets within the variability of agricultural ecosystems

Under the experimental conditions, N yield variations were closely associated with post-anthesis N uptake in diseased but also in healthy crops. Understanding the respective roles of N uptake and N remobilization in the case of diseased and healthy crops holds the promise of better modelling of variations in N yield, and thus in GPC.