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This paper presents and evaluates an inverse model for estimating ammonia emission from agricultural land. The method is based on an analytical model derived from the advection-diffusion equation, assuming power law profiles for wind speed and diffusivity. A three-dimensional model and a two-dimensional model are evaluated. The hypotheses of flux-driven or concentration-driven emissions are also tested. The model is evaluated against three datasets covering a range of ammonia fluxes, field geometry/size and measurement techniques. The sensitivity and the uncertainty of the method is also evaluated with a MonteCarlo approach, as well as based on existing datasets. Finally, the capability of the method to work with time-integrated concentrations (e.g. using diffusive concentration samplers) is also evaluated. The inverse model gives estimations of the ammonia emissions within a few per cent of the measurements. Moreover, the method is mainly sensitive to the concentration, the friction velocity and the thermal stratification of the atmosphere. The two-dimensional approaches give similar results to the three-dimensional one, provided the field is large enough. The concentration-driven hypothesis is similar to the flux-driven hypothesis for a fetch greater than approximately 20 m. The results are discussed in comparison with the previous approaches: the Theoretical Profile Shape (TPS or Zinst approach) and the backward Lagrangian Stochastic model (BLS).

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