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Processes of Ammonia Air-Surface Exchange in a Fertilized Zea Mays Canopy
Citation:
Walker, Johnt, M. Jones, J. Bash, L. Myles, T. Meyers, D. Schwede, J. Herrick, E. Nemitz, AND W. Robarge. Processes of Ammonia Air-Surface Exchange in a Fertilized Zea Mays Canopy. Biogeosciences. Copernicus Publications, Katlenburg-Lindau, Germany, 10(2):981-998, (2013).
Impact/Purpose:
The field experiment described in the accompanying manuscript was conducted in an effort to: 1) develop a better quantitative understanding of the emissions of ammonia from fertilized corn, 2) examine the relative importance of foliage versus soil exchange process with respect to canopy-scale fluxes, and 3) to develop datasets of sufficient temporal and process-level detail to parameterize and validate compensation point resistance-type air-surface exchange models.
Description:
Recent incorporation of coupled soil biogeochemical and bi-directional NH3 air-surface exchange algorithms into regional air quality models holds promise for further reducing uncertainty in estimates of NH3 emissions from fertilized soils. While this advancement represents a significant improvement over previous approaches, the evaluation and improvement of such modeling systems for fertilized crops requires process level field measurements over extended periods of time that capture the range of soil, vegetation, and atmospheric conditions that drive short term (i.e., post fertilization) and total growing season NH3 fluxes. This study examines the processes of NH3 air-surface exchange in a fertilized corn (Zea mays) canopy over the majority of a growing season to characterize the soil emissions after fertilization and investigate soil-canopy interactions. Micrometeorological flux measurements above the canopy, measurements of soil, leaf apoplast and dew/guttation chemistry, and a combination of in-canopy measurements, inverse source/sink, and resistance modeling were employed. Daily mean and median NH3 fluxes over a period of approximately 10 weeks following fertilization yielded cumulative total N losses of 6.5% and 3.8%, respectively, of the 134 kg N ha-1 applied to the soil as urea ammonium nitrate (UAN). We partially attribute this relatively low fractional loss to the use of a fertilizer urease inhibitor. During the first month after fertilization, daily mean emission fluxes were positively correlated with soil temperature and soil volumetric water. Diurnally, peak hourly average fluxes of ≈ 700 ng N m-2 s-1 occurred near mid-day, coincident with the daily peak in friction velocity. 5 to 10 weeks after fertilization, net emission was still observed though mid-day peak fluxes had declined to ≈ 125 ng N m-2 s-1. A key finding of the surface chemistry measurements was the observation of high pH (7.0 – 8.5) in leaf dew/guttation, which reduced the ability of the canopy to recapture soil emissions during wet periods. In-canopy measurements near peak LAI indicated that the concentration of NH3 just above the soil surface was highly positively correlated with soil volumetric water, which likely reflects the inverse relationship between soil moisture content and the soil diffusive resistance. Inverse source/sink and resistance modeling indicated that the canopy recaptured ≈ 73% of soil emissions near peak LAI and that stomatal uptake may account for 15 – 36% of total uptake by foliage during the day. Future process-level NH3 studies in fertilized cropping systems should focus on the temporal dynamics of net emission to the atmosphere from fertilization to peak LAI, and improvement of soil and cuticular resistance parameterizations.
