Abstract
Abstract. Atmospheric ammonia (NH3) dominates global emissions of total reactive nitrogen (Nr), while emissions from agricultural production systems contribute about two-thirds of global NH3 emissions; the remaining third emanates from oceans, natural vegetation, humans, wild animals and biomass burning. On land, NH3 emitted from the various sources eventually returns to the biosphere by dry deposition to sink areas, predominantly semi-natural vegetation, and by wet and dry deposition as ammonium (NH4+) to all surfaces. However, the land/atmosphere exchange of gaseous NH3 is in fact bi-directional over unfertilized as well as fertilized ecosystems, with periods and areas of emission and deposition alternating in time (diurnal, seasonal) and space (patchwork landscapes). The exchange is controlled by a range of environmental factors, including meteorology, surface layer turbulence, thermodynamics, air and surface heterogeneous-phase chemistry, canopy geometry, plant development stage, leaf age, organic matter decomposition, soil microbial turnover, and, in agricultural systems, by fertilizer application rate, fertilizer type, soil type, crop type, and agricultural management practices. We review the range of processes controlling NH3 emission and uptake in the different parts of the soil-canopy-atmosphere continuum, with NH3 emission potentials defined at the substrate and leaf levels by different [NH4+] / [H+] ratios (Γ). Surface/atmosphere exchange models for NH3 are necessary to compute the temporal and spatial patterns of emissions and deposition at the soil, plant, field, landscape, regional and global scales, in order to assess the multiple environmental impacts of airborne and deposited NH3 and NH4+. Models of soil/vegetation/atmosphere NH3 exchange are reviewed from the substrate and leaf scales to the global scale. They range from simple steady-state, "big leaf" canopy resistance models, to dynamic, multi-layer, multi-process, multi-chemical species schemes. Their level of complexity depends on their purpose, the spatial scale at which they are applied, the current level of parameterization, and the availability of the input data they require. State-of-the-art solutions for determining the emission/sink Γ potentials through the soil/canopy system include coupled, interactive chemical transport models (CTM) and soil/ecosystem modelling at the regional scale. However, it remains a matter for debate to what extent realistic options for future regional and global models should be based on process-based mechanistic versus empirical and regression-type models. Further discussion is needed on the extent and timescale by which new approaches can be used, such as integration with ecosystem models and satellite observations.
Highlights
1.1 Ammonia in the environmentAmmonia (NH3) emission from the biosphere to the atmosphere is one of the many unintended consequences of reactive nitrogen (Nr) creation from inert dinitrogen gas (N2) through symbiotic biological nitrogen fixation (BNF) and the Haber–Bosch process, and of the agricultural usage of the fixed Nr for crop and meat production (Sutton et al, 2011)
The present paper focuses on bi-directional NH3 exchange over vegetation and soils in both-natural vegetation and agricultural systems, as well as uni-directional exchange fluxes from land-applied mineral N fertilizers and manures
Most of the fundamental process understanding was gained during the 1980s and 1990s, while many advances in modelling logically followed from the late 1990s onwards, spurred by the canopy compensation point concept of Sutton et al (1995b, 1998a)
Summary
Ammonia (NH3) emission from the biosphere to the atmosphere is one of the many unintended consequences of reactive nitrogen (Nr) creation from inert dinitrogen gas (N2) through symbiotic biological nitrogen fixation (BNF) and the Haber–Bosch process, and of the agricultural usage of the fixed Nr for crop and meat production (Sutton et al, 2011). NH3 emission is one of the main precursors of the nitrogen cascade (Galloway et al, 2003), whereby the N atom of the NH3 molecule may potentially participate in a number of environmental impacts through a series of pathways and chemical and (micro-) biological transformations in the biosphere. The global biological and industrial N2 fixation is of the order of 140 Tg N yr−1 (Galloway et al, 2003), of which NH3 emissions represent a loss of approximately one-third. The environmental impacts of NH3 are expected to become more pronounced in many regions of the world where increases in NH3 emissions are expected to occur during the 21st century, as a result of agricultural intensification and the manifold effects of climatic change on N cycling
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