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

Abstract

The quantification of atmospheric additions of nitrogen (N) to an ecosystem is often desirable, but difficult for many locations including many ecological manipulation experiments. Ideal methodologies for the complete and chemically speciated quantification of dry N deposition (e.g. tunable diode laser absorption spectrometer, thermaldissociation laser-induced fluorescence) are expensive, technologically challenging to maintain, and rarely colocated with important global change manipulation experiments. Here we present an alternative method for obtaining an approximation of total N deposition using short-term eddy flux and concentration measurements, annual regional concentration estimates, and modeling for the Duke Experimental Forest. The motivation for generating estimates for this location was to inform the long-term elevated CO 2 experiment conducted at Duke Forest. We estimated the total annual atmospheric N deposition to the forest to be 13.7 kg N ha À1 . Of this total, $58% was in dry-deposited forms. Surprisingly, and contrary to some previous predictions, nitric acid (HNO 3 ) was not the dominant portion of the total dry-deposited N, implying strongly that other forms of gaseous N like organic peroxy and alkyl nitrate compounds are a significant portion of the total flux. Furthermore, this study sheds light on the completeness of the estimates derived from Clean Air Status and Trends Network (CASTNet) and other dry deposition networks. CASTNet does not measure organic forms of dry deposition. In fact, CASTNet only quantifies HNO 3 in the gas phase and may significantly underestimate total N deposition in many environments.

Observations of reactive nitrogen oxide fluxes by eddy covariance above two midlatitude North American mixed hardwood forests

“…9) calculates a net daily deposition of 0.14 mg (N) m −2 day −1 . These are of the same order of magnitude as the handful of previously reported summertime NO y fluxes from other forest sites across North America (Sparks et al, 2008;Munger et al, 1998;Hogg, 2007). Table 1 compares mean results from previous NO y eddy covariance flux studies in mg (N) m −2 day −1 , with a focus on summertime observations.…”

“…Munger et al (1998) describe the budget for Harvard Forest using NO y eddy covariance flux data collected from 1990 to 1996, and show that, on an annual basis, dry deposition contributes 34 % of total NO y inputs; the same paper summarizes data collected during the summer of 1990 over a black spruce woodland in northern Quebec which show that contributions of dry deposition are roughly the same. Sparks et al (2008) report a much higher dry NO y deposition contribution to total nitrate on an annual basis of around 60 % above a North Carolina pine plantation. Other studies using eddy covariance have applied novel instruments to directly determine the contribution of individual species over short-term (several months or less) intensive field campaigns at a number of sites across North America (Horii et al, 2004;Turnipseed et al, 2006;Farmer and Cohen, 2008;Wolfe et al, 2009;Min et al, 2012;Zhang et al, 2012).…”

“…Data from the National Geophysical Data Center show the timing of this event coincided with large smoke plumes that were observed over north-central Ontario and northern Quebec for several days (http://maps.ngdc.noaa.gov/viewers/ firedetects). In light of the high PM 2.5 that was recorded at an air quality monitoring site 20 km west of the HFWR tower Sparks et al (2008). (hourly average concentrations greater than 20 µg m −3 , when they were typically less than 6 µg m −3 in the preceding and following weeks), we take this as evidence that the source of the high concentrations could have been forest fire.…”

“…At several locations across Canada, a deposition model that was applied to short-term measurements of individual NO y species estimated that dry deposition of nonnitrate species contributes equally or significantly more at every site (Zhang et al, 2009). At a site in North Carolina, nitrate was estimated not to be the dominant deposited species (Sparks et al, 2008). Similarly, at Harvard Forest, MA, nitric acid was estimated to contribute between 38 and 73 % of the total gaseous NO y flux, with NO x only partly compensating for the rest (Horii et al, 2006).…”

Evaluating the calculated dry deposition velocities of reactive nitrogen oxides and ozone from two community models over a temperate deciduous forest

“…However, a recent field study over a coniferous forest (Turnipseed et al, 2006;Sparks et al, 2008) estimated that HNO 3 accounted for only w24% of the NO y flux and PAN exhibited a close portion of w20%. Zhang et al (2009) investigated the total nitrogen flux budget over eight rural sites across eastern Canada, and estimated that HNO 3 constituted less than half of the NO y flux (46%), and the flux from other measured gaseous nitrogen species (NO 2 þ PAN þ PPN) were also a significant portion (35%).…”