Coarse-scale soil–atmosphere NOx exchange modeling: status and limitations

Gaseous NO and NO2 (collectively termed NOx) are trace atmospheric constituents with important functions in various atmospheric and ecosystem processes. Because nitrification and denitrification in soil are included among major sources of the gases, simulation models for predicting soil-atmosphere NOx exchange should incorporate the strong dependence of these two microbial processes on soil temperature, soil water content, and substrate N availability. We briefly review current understanding of these controls, then describe how various authors have incorporated that knowledge into model parameterization schemes, and finally present some general thoughts regarding how well those schemes work and what might be missing. Existing coarse-scale models have evolved to the point that they are beginning to explicitly address the influences, interactions, and dynamics of all three microscale controllers in formulations suitable for studies at regional to global and seasonal to interannual scales. Perhaps the greatest limitation of the models, as well as their predictor variables, is that they often fail to account for the large pulse of gaseous N oxide emissions commonly observed following wetting of very dry soil. This failure is exacerbated by mounting evidence that similar pulses may occur following sudden removal of other environmental limitations on microbial growth and metabolism. Such pulses ostensibly make a large contribution to soil- atmosphere NOx exchange, especially in semi- arid, subhumid, and seasonally dry tropical regions of the globe where the exchange has been poorly characterized despite being subject to intense anthropogenic disturbance.