Benjamin Loubet

Towards a climate-dependent paradigm of ammonia emission and deposition

Existing descriptions of bi-directional ammonia (NH3) land–atmosphere exchange incorporate temperature and moisture controls, and are beginning to be used in regional chemical transport models. However, such models have typically applied simpler emission factors to upscale the main NH3 emission terms. While this approach has successfully simulated the main spatial patterns on local to global scales, it fails to address the environment- and climate-dependence of emissions. To handle these issues, we outline the basis for a new modelling paradigm where both NH3 emissions and deposition are calculated online according to diurnal, seasonal and spatial differences in meteorology. We show how measurements reveal a strong, but complex pattern of climatic dependence, which is increasingly being characterized using ground-based NH3 monitoring and satellite observations, while advances in process-based modelling are illustrated for agricultural and natural sources, including a global application for seabird colonies. A future architecture for NH3 emission–deposition modelling is proposed that integrates the spatio-temporal interactions, and provides the necessary foundation to assess the consequences of climate change. Based on available measurements, a first empirical estimate suggests that 5°C warming would increase emissions by 42 per cent (28–67%). Together with increased anthropogenic activity, global NH3 emissions may increase from 65 (45–85) Tg N in 2008 to reach 132 (89–179) Tg by 2100.

The mass budget of atmospheric ammonia in woodland within 1 km of livestock buildings

The emissions of ammonia from point sources close to the ground and the rapid deposition to vegetation lead to very large horizontal gradients in both concentration and deposition close to sources. The sources are primarily livestock related and some of the largest terrestrial N inputs occur in the proximity of intensive production facilities. This study quantifies the local fate of livestock NH 3 emissions from a poultry farm using measured NH 3 concentrations and the relationship between canopy resistance (r c ) and ambient NH 3 concentration from intensive flux measurements. The results of the measured concentrations and deposition are compared with those obtained using a dispersion model of the emission, transport and deposition close to point sources. The results of the measurements showed annual mean concentrations in the range 23 μg m -3 to 63 μg m -3 at a distance of 15 m from the source, declining to background concentrations for the region of 1 to 2 μg m -3 at a distance of 276 m and in reasonable agreement with the model. The deposition of NH 3 -N estimated from the measurements, declined from 42 kg N ha -1 at 15 m to 5 kg N ha -1 at 270 m and was smaller than the deposition estimated using the dispersion model by about a factor of two. Annual deposition within 270 m of the source to the woodland amounted to 155 kg N, and represented 3.2% of annual emissions from the poultry unit. The comparison between measurements and the model indicated substantial uncertainty in the deposition budget values, but supports the overall conclusion that local deposition of NH 3 to woodland within 300 m of the source represents a small fraction (3% to 10%) of the local emission source.

HONO Emissions from Soil Bacteria as a Major Source of Atmospheric Reactive Nitrogen

From Soil to Sky Trace gases emitted either through the activity of microbial communities or from abiotic reactions in the soil influence atmospheric chemistry. In laboratory column experiments using several soil types, Oswald et al. (p. 1233) showed that soils from arid regions and farmlands can produce substantial quantities of nitric oxide (NO) and nitrous acid (HONO). Ammonia-oxidizing bacteria are the primary source of HONO at comparable levels to NO, thus serving as an important source of reactive nitrogen to the atmosphere. HONO emissions from soil are comparable to those of NO in arid and arable regions. Abiotic release of nitrous acid (HONO) in equilibrium with soil nitrite (NO2–) was suggested as an important contributor to the missing source of atmospheric HONO and hydroxyl radicals (OH). The role of total soil-derived HONO in the biogeochemical and atmospheric nitrogen cycles, however, has remained unknown. In laboratory experiments, we found that for nonacidic soils from arid and arable areas, reactive nitrogen emitted as HONO is comparable with emissions of nitric oxide (NO). We show that ammonia-oxidizing bacteria can directly release HONO in quantities larger than expected from the acid-base and Henry’s law equilibria of the aqueous phase in soil. This component of the nitrogen cycle constitutes an additional loss term for fixed nitrogen in soils and a source for reactive nitrogen in the atmosphere.

Field measurements of airborne concentration and deposition rate of maize pollen

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 x 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 x 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 and 10 m downwind of the source deposition rates at 30 m were < 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 15 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.

Ammonia Deposition Near Hot Spots: Processes, Models and Monitoring Methods

Atmospheric reduced nitrogen (NHx) mainly originates from hot spots, which can be considered as intensive area or point sources. A large fraction of the emitted NHx may be recaptured by the surrounding vegetation, hence reducing the contribution of these hot spots to long-range transport of NHx. This paper reviews the processes leading to local recapture of NHx near hot spots, as well as existing models and monitoring methods. The existing models range from research models to more operational models that can be coupled with long-range transport model provided the necessary information on emissions is available. Local recapture of NH3 ranges from 2% to 60% within 2 km of a hot-spot and it is sensitive to source height atmospheric stability, wind speed, structure of the surrounding canopies, as well as stomatal absorpton, which mainly depends on green leaf area index and stomatal NH3 compensation point of vegetation, and finally, cuticular deposition, which depends primarily on vegetation wetness. The main uncertainties and limitations on NHx recapture models and monitoring techniques are discussed. Due to the decrease of sulphur and nitrogen oxides emissions under a series of UNECE protocols, reduced nitrogen (NHx), has become the dominant pollutant in Western Europe contributing to acidification of ecosystems (e.g. Vestreng and Storen 2000). At the global scale NHx and NOx emissions are comparable, although large uncertainties exist on NHx emissions (Dentener and Crutzen 1994; Bouwman et al. 1997). Moreover, NHx deposition, with other nitrogen (N) deposition, leads to eutrophication and changes in the biodiversity of semi-natural ecosystems (Van Breemen and van Dijk 1988; Roelofs et al. 1985; Fangmeier et al. 1994; Krupa 2003; EEA 2003). Although atmospheric ammonia (NH3) is not a greenhouse gas (GHG), deposition of NHx may lead to increased GHG emissions (N2O) (Melillo et al. 1989) or reduced consumpton of CH4. Additionally, ammonium sulphate aerosols (NH4)2SO4, contribute to half of the negative radiatve forcing of the atmosphere due to aerosols (Houghton et al. 2001; Adams et al. 2001), as well as contriuting to impacts of secondary aerosol on human health.

Seasonal variability of apoplastic NH4+ and pH in an intensively managed grassland

The stomatal compensation point of ammonia (χs) is a major factor controlling the exchange of atmospheric ammonia (NH3) with vegetation. It is known to depend on the supply of nitrogen and to vary among plant species, but its seasonal variation has not yet been reported for grassland. In this study, we present the temporal variation of apoplastic NH4+ concentration ([NH4+]apo) and pH (pHapo) measured in leaves of Lolium perenne L. in a grassland, through two periods of cutting / fertilisation, followed by a livestock grazing period. The total free NH4+ concentration measured in foliage ([NH4+]fol), and soil mineral NH4+ and NO3− concentration are also presented. The value of [NH4+]apo varied from less than 0.01 mM to a maximum of 0.5 mM occurring just after fertilisation, whereas the apoplastic pH ranged from pH 6 to 6.5 for most of the time and increased up to pH 7.8, 9 days after the second fertilisation, when grazing started. [NH4+]fol varied between 20 and 50 μg N-NH4+ g−1 f.w. The compensation point at 20°C, ranged from 0.02 μg NH3 m−3 between the fertilisations to 10 μg NH3 m−3 just after the second fertilisation. The reasons for these seasonal changes are discussed, with respect to plant metabolism and the concentration of ammonium and nitrate in the soil.

Investigation of the interaction between sources and sinks of atmospheric ammonia in an upland landscape using a simplified dispersion‐exchange model

Exchange of atmospheric ammonia (NH 3 ) with vegetation is characterized by the juxtaposition of sources and sinks at a landscape level. Such situations lead to a large fraction of the landscape being exposed to local advection effects that if not accounted for, introduce errors in standard micrometeorological measurements of NH 3 exchange with the surface. In this study, a simplified dispersion - exchange model for NH 3 (Flux Interpretation by Dispersion and Exchange over Short Range, FIDES) is evaluated and used to assess the advection fluxes at 260 m downwind of an isolated pasture, grazed with sheep, using the measurements of a classical three-point NH 3 gradient system located on adjacent moorland. The method consists of fitting the measured and modeled concentration profile by adjusting at the same time the emission strength of the local source and the exchange rate of NH 3 to the moorland area downwind. A local dispersion and surface exchange model such as FIDES has proved to be a valuable tool to estimate advection corrections, given sound estimates of background NH 3 concentrations, source location, and standard meteorological parameters. According to the model results the advection fluxes at the moorland measurement site, at 1.0 m height and 260 m downwind of the grazed pasture, were positive. For 80% of the situations they ranged between 30% and 60% of the vertical fluxes. In stable conditions the advection fluxes were large and more sensitive to the surface exchange parameters. These results demonstrate that if not accounted for, advection fluxes may lead to a severe underestimate of the NH3 deposition to seminatural ecosystems, such as moorland, in the vicinity of ground level agricultural sources.

Fluxes of NH3 and CO2 over upland moorland in the vicinity of agricultural land

Intensive field measurements of NH3 and CO2 exchange were made over a wet heathland in the vicinity ( 10%). Corrections were applied using a local-scale dispersion-exchange model. The analysis highlights how advection modifies the classical one-dimensional inferential resistance approach. It is concluded that ecosystems in the vicinity of agricultural land receive more dry deposition than would be estimated using NH3 concentration monitoring and standard inferential models. In the present study, this effect represented an overall increase in total NH3 deposition of 32%.

Relationship between ammonia stomatal compensation point and nitrogen metabolism in arable crops: Current status of knowledge and potential modelling approaches

The ammonia stomatal compensation point of plants is determined by leaf temperature, ammonium concentration ([NH4+]apo) and pH of the apoplastic solution. The later two depend on the adjacent cells metabolism and on leaf inputs and outputs through the xylem and phloem. Until now only empirical models have been designed to model the ammonia stomatal compensation point, except the model of Riedo et al. (2002. Coupling soil-plant-atmosphere exchange of ammonia with ecosystem functioning in grasslands. Ecological Modelling 158, 83-110), which represents the exchanges between the plants nitrogen pools. The first step to model the ammonia stomatal compensation point is to adequately model [NH4+]apo. This [NH4+]apo has been studied experimentally, but there are currently no process-based quantitative models describing its relation to plant metabolism and environmental conditions. This study summarizes the processes involved in determining the ammonia stomatal compensation point at the leaf scale and qualitatively evaluates the ability of existing whole plant N and C models to include a model for [NH4+]apo.

Nitrogen flows and fate in rural landscapes

Nature of the problem Th e transfer of nitrogen by either farm management activities or natural processes (through the atmosphere and the hydrological net• work) can feed into the N cascade and lead to indirect and unexpected reactive nitrogen emissions. Th is transfer can lead to large N deposition rates and impacts to sensitive ecosystems. It can also promote further N • 2 O emission in areas where conditions are more favourable for denitrifi cation. In rural landscapes, the relevant scale is the scale where N is managed by farm activities and where environmental measures are • applied.