Processes of ammonia air-surface exchange in a fertilized corn canopy

Processes of ammonia (NH3) air-surface exchange in fertilized crops include bi-directional flux (emission or deposition) from the soil, surface litter, leaf stomatal cavity, and leaf cuticle. These component fluxes establish the net exchange between the canopy and atmosphere. We conducted an experiment in the summer of 2007 in eastern North Carolina to quantify the net flux of NH3 from a fertilized corn canopy over the course of the growing season. A primary objective was to examine the relative importance of soil vs. foliage exchange pathways with respect to net canopy-scale fluxes. Continuous wet rotating denuder and photoacoustic spectroscopic NH3 measurement methods were configured in a gradient mode to measure canopy-scale fluxes using the modified Bowen-ratio technique. In-canopy source-sink relationships were examined by inverse modeling of NH3 concentration, temperature, and turbulence profiles. Additionally, measurements of NH4+ and H+ in the soil solution, leaf apoplast, and leaf surface water were used in combination with resistance modeling to examine the relationships between net canopy-scale fluxes and soil, stomatal, and cuticular exchange pathways. Measurement and modeling results are presented and the relevance of this work to national NH3 emission inventories and regional air quality modeling is discussed.