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| Titel |
Processes of ammonia air–surface exchange in a fertilized Zea mays canopy |
| VerfasserIn |
J. T. Walker, M. R. Jones, J. O. Bash, L. Myles, T. Meyers, D. Schwede, J. Herrick, E. Nemitz, W. Robarge |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 10, no. 2 ; Nr. 10, no. 2 (2013-02-12), S.981-998 |
| Datensatznummer |
250017514
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| Publikation (Nr.) |
 copernicus.org/bg-10-981-2013.pdf |
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| Zusammenfassung |
| 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 represents
a significant advancement 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 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. Over a period of approximately 10
weeks following fertilization, daily mean and median net canopy-scale fluxes yielded cumulative
total N losses of 8.4% and 6.1%, respectively, of the
134 kg N ha−1 surface applied to the soil as urea ammonium nitrate
(UAN). During the first month after fertilization, daily mean emission
fluxes were positively correlated with soil temperature and soil volumetric
water. Diurnally, maximum hourly average fluxes of ≈ 700 ng N m−2 s−1 occurred near mid-day, coincident with
the daily maximum in friction velocity. Net emission was still observed 5 to 10
weeks after fertilization, although 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 leaf area index (LAI) indicated that the concentration of NH3 just above
the soil surface was highly positively correlated with soil volumetric
water, which likely reflects the influence of soil moisture on resistance to
gaseous diffusion through the soil profile and hydrolysis of remaining urea.
Inverse source/sink and resistance modeling indicated that the canopy
recaptured ≈ 76% of soil emissions near peak LAI. Stomatal
uptake may account for 12–34% of total uptake by foliage during the
day compared to 66–88% deposited to the cuticle. 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. |
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