ABSTRACT. In recent years there has been interest in the dispersal of maize (Zea mays) pollen from crops, particularly in relation to gene flow and seed quality. We report the results of experiments that measured maize pollen dispersal from a 20 m × 20 m experimental crop. The experiments were done in a commercial farm in France during the summer of 2000. Pollen production was estimated to range from 10 4 to 2×10 6 grains per day per plant. Pollen concentrations and deposition rates decreased rapidly with distance from the crop: concentrations decreased by about a factor of 3 between 3 m and 10 m downwind of the source; deposition rates at 30 m were less than 10% of those at 1 m. Horizontal flux of pollen were estimated from pollen concentration and wind speed profiles using a mass balance approach, and ranged from 5 to 560 grains m -1 s -1 at 3 m from the source. Comparison of deposition rates estimated with the mass balance and direct measurement suggests that only a small proportion of the pollen released from the crop would have been still airborne at distances greater than 30 m downwind. Deposition velocity determined as the ratio of the deposition rate to the airborne concentration at 3 m from the source averaged 0.6 m s -1 , which is twice as large as the settling velocity for maize pollen.

ABSTRACT. The co-existence of genetically modified (GM) crops with conventional crops has become a subject of debate and inquiry. Maize (Zea mays L.) is one of the most cultivated crop plants in the world and there is a need to assess the risks of cross-pollination. Concentration and deposition rate downwind from different-sized maize crops were measured during three flowering seasons, together with micrometeorological conditions in the surrounding environment. Pollen release started once the air vapour pressure deficit (VPD) increases above 0.2 to 0.5 kPa. Moreover, the dynamics of release was correlated with the dynamics of VPD surrounding the tassels. Horizontal deposition appeared to follow a power law over short distance downwind from the source, and the dispersal distance increased with the source canopy height, and the roughness length of the downwind canopy. This work also provides a data set containing both pollen measurements and contrasting weather conditions to validate dispersal models and further investigate maize pollen dispersal processes.

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