Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest: implications for tropospheric chemistry

Abstract

Tropical forest soils are known to emit large amounts of reactive nitrogen oxide compounds, often referred to collectively as NO y (NO y = NO + NO 2 + HNO 3 + organic nitrates). Plants are known to assimilate and emit NO y and it is therefore likely that plant canopies affect the atmospheric concentration of reactive nitrogen compounds by assimilating or emitting some fraction of the soil-emitted NO y . It is crucial to understand the magnitude of the canopy effects and the primary environmental and physiological controls over NO y exchange in order to accurately quantify regional NO y inventories and parameterize models of tropospheric photochemistry. In this study we focused on nitrogen dioxide (NO 2 ), which is the component of NO y that most directly catalyzes the chemistry of O 3 dynamics, one of the most abundant oxidative species in the troposphere, and which has been reported as the NO y species that is most readily exchanged between plants and the atmosphere. Leaf chamber measurements of NO 2 flux were measured in 25 tree species growing in a wet tropical forest in the Republic of Panama. NO 2 was emitted to the atmosphere at ambient NO 2 concentrations below 0.53-1.60 ppbv (the NO 2 compensation point) depending on species, with the highest rate of emission being 50 pmol m -2 s -1 at <0.1 ppbv. NO 2 was assimilated by leaves at ambient NO 2 concentrations above the compensation point, with the maximum observed uptake rate being 1,550 pmol m -2 s -1 at 5 ppbv. No seasonal variation in leaf NO 2 flux was observed in this study and leaf emission and uptake appeared to be primarily controlled by leaf nitrogen and stomatal conductance, respectively. When scaled to the entire canopy, soil NO emission rates to the atmosphere were estimated to be maximally altered ±19% by the overlying canopy.

Application of thermal-dissociation laser induced fluorescence (TD-LIF) to measurement of HNO&lt;sub&gt;3&lt;/sub&gt;, Σalkyl nitrates, Σperoxy nitrates, and NO&lt;sub&gt;2&lt;/sub&gt; fluxes using eddy covariance

“…Upward NO 2 fluxes may be due to a combination of several processes. As the NO 2 mixing ratios are low, a compensation point may cause plants within the ecosystem to release NO 2 , as observed on the leaf scale for tropical plants (Sparks et al, 2001). As the ground was covered by snow during the winter, a direct release of NO 2 as a result of snow photochemistry could also cause emission (Domine and Shepson, 2002).…”

“…A compensation point is the atmospheric mixing ratio above which a compound is deposited and below which the compound is emitted. Compensation points for NO 2 of 0.53-1.60 ppb were reported for different plant species by Sparks et al (2001).…”

Dry nitrogen deposition estimates over a forest experiencing free air CO2 enrichment

“…NO 2 is also produced by soils, but is primarily produced in urban and suburban areas by the burning of fossil fuels. NO 2 can be either emitted or taken up by vegetation depending upon the ambient concentration (Sparks et al, 2001). The remaining components of NO y consist of more oxidized forms of N such as organic nitrates, peroxynitrates, and HNO 3 .…”

“…Estimates of dry deposition of reactive N to individual plants or entire ecosystems have been made using dynamic and open-topped chambers (Gessler et al, 2000;Sparks et al, 2001) and by measuring precipitation throughfall after it passes through the plant canopy (Lovett et al, 2000). However, these methods preclude deposition to nonplant surfaces, chemical transformations within or above the canopy, and attempts to scale chamber measurements to whole-system fluxes are associated with a high degree of uncertainty (Hanson et al, 1989).…”

Field investigations of nitrogen dioxide (NO&lt;sub&gt;2&lt;/sub&gt;) exchange between plants and the atmosphere

“…In addition to the root uptake of NO − 3 , atmospheric NO 2 uptake may occur directly via plant stomata (Hanson et al, 1989;Hanson and Lindberg, 1991;Rondón et al, 1993;Neubert et al, 1993;Hereid and Monson, 2001;Sparks et al, 2001;Chaparro-Suarez et al, 2011). Plant metabolic consumption of NO 2 has been investigated by using the 15 N…”