Estimation of In-Canopy Ammonia Sources and Sinks in a Fertilized Zea mays Field

Abstract

An analytical model was developed to describe in-canopy vertical distribution of ammonia (NH 3 ) sources and sinks and vertical fluxes in a fertilized agricultural setting using measured in-canopy mean NH 3 concentration and wind speed profiles. This model was applied to quantify in-canopy air-surface exchange rates and above-canopy NH 3 fluxes in a fertilized corn (Zea mays) field. Modeled air-canopy NH 3 fluxes agreed well with independent above-canopy flux estimates. Based on the model results, the urea fertilized soil surface was a consistent source of NH 3 one month following the fertilizer application, whereas the vegetation canopy was typically a net NH 3 sink with the lower portion of the canopy being a constant sink. The model results suggested that the canopy was a sink for some 70% of the estimated soil NH 3 emissions. A logical conclusion is that parametrization of within-canopy processes in air quality models are necessary to explore the impact of agricultural field level management practices on regional air quality. Moreover, there are agronomic and environmental benefits to timing liquid fertilizer applications as close to canopy closure as possible. Finally, given the large withincanopy mean NH 3 concentration gradients in such agricultural settings, a discussion about the suitability of the proposed model is also presented.

Role of ambient ammonia in particulate ammonium formation at a rural site in the North China Plain

“…The morning peak of NH 3 was also observed elsewhere and could have resulted from emissions from fertilised soils and plant stomata, evaporation of dew, human sources, and mixing down of ammonia from the residual layer (Trebs et al, 2004;Norman et al, 2009;Bash et al, 2010;Ellis et al, 2011). Figure 3b reveals that the relative humidity (90-89 %) and temperature (21.5-22.1 • C) remained relatively constant before 06:00 but increased later in the morning.…”

Fluxes of total reactive atmospheric nitrogen (ΣN_r) using eddy covariance above arable land

“…Also, canopy recapture of soil-and fertilizeremitted NH 3 might have played a role in the variant exchange characteristics after fertilization Á particularly between those in April and May where an almost three times higher fertilizer amount applied in May (82.7 kg N ha (1 ) did not lead to significantly elevated emissions compared to the fertilization in April (28.5 kg N ha (1 ). According to studies of Nemitz et al (2000), Denmead et al (2008) and Bash et al (2010), closer plant canopies Á as was the case in May Á are likely to recapture more NH 3 than open plant canopies. However, for the June fertilization with a medium quantity of 44.6 kg N ha (1 applied, but followed by the substantial emission rates, maturity effects must have been predominated over canopy recapture effects.…”

A modified micrometeorological gradient method for estimating O<sub>3</sub> dry depositions over a forest canopy

“…Within these upper layers the vertical length scales of turbulence are probably smaller than the distance associated with changes in concentration and wind speed gradients (Baldocchi, 1988). Thus, the flux-gradient theory is likely applicable for estimating the vertical flux distribution of air pollutants within a plant canopy as has been used in previous studies (e.g., Baldocchi, 1988;Bash et al, 2010;Wolfe and Thornton, 2011). Applying the flux-gradient theory within the canopy, a height-dependent flux (F (z)) can then be calculated as…”