Advances in understanding, models and parameterizations of biosphere-atmosphere ammonia exchange

[摘要] Atmospheric ammonia (NH₃) dominates global emissions of total reactivenitrogen (N_r), while emissions from agricultural productionsystems contribute about two-thirds of global NH₃ emissions; theremaining third emanates from oceans, natural vegetation, humans, wildanimals and biomass burning. On land, NH₃ emitted from the varioussources eventually returns to the biosphere by dry deposition to sink areas,predominantly semi-natural vegetation, and by wet and dry deposition asammonium (NH₄⁺) to all surfaces. However, the land/atmosphereexchange of gaseous NH₃ is in fact bi-directional over unfertilized aswell as fertilized ecosystems, with periods and areas of emission anddeposition alternating in time (diurnal, seasonal) and space (patchworklandscapes). The exchange is controlled by a range of environmental factors,including meteorology, surface layer turbulence, thermodynamics, air andsurface heterogeneous-phase chemistry, canopy geometry, plant developmentstage, 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 therange of processes controlling NH₃ emission and uptake in the differentparts of the soil-canopy-atmosphere continuum, with NH₃ emissionpotentials defined at the substrate and leaf levels by different[NH₄⁺] / [H⁺] ratios (Γ).Surface/atmosphere exchange models for NH₃ are necessary to compute thetemporal and spatial patterns of emissions and deposition at the soil, plant,field, landscape, regional and global scales, in order to assess the multipleenvironmental impacts of airborne and deposited NH₃ and NH₄⁺.Models of soil/vegetation/atmosphere NH₃ exchange are reviewed from thesubstrate and leaf scales to the global scale. They range from simplesteady-state, "big leaf" canopy resistance models, to dynamic, multi-layer,multi-process, multi-chemical species schemes. Their level of complexitydepends on their purpose, the spatial scale at which they are applied, thecurrent level of parameterization, and the availability of the input datathey require. State-of-the-art solutions for determining the emission/sinkΓ potentials through the soil/canopy system include coupled,interactive chemical transport models (CTM) and soil/ecosystem modelling atthe regional scale. However, it remains a matter for debate to what extentrealistic options for future regional and global models should be based onprocess-based mechanistic versus empirical and regression-type models.Further discussion is needed on the extent and timescale by which newapproaches can be used, such as integration with ecosystem models andsatellite observations.

[学科分类] 地球化学与岩石