Epiphytic Moss as a Biomonitor for Nitrogen Deposition

Abstract:

Tracking nitrogen (N) deposition patterns is important for understanding how anthropogenic sources of nitrogen affect natural habitats, human health, and for evaluating computer models of future N deposition. It can also aid in tracking and modeling anthropogenic fossil fuel emissions. This pilot study investigated the use of Orthotrichum lyellii, a common urban epiphytic moss, as a possible bioindicator for N deposition through the analysis of total moss N content and N isotopic fractionation ( δ¹⁵N) for evaluating N sources. In the spring/summer of 2013 we collected 168 O. lyellii samples from the trunks of deciduous trees in 53 locations in the Portland metropolitan area. In the winter of 2013-14, we resampled the same locations to investigate the effect of seasonality. The averaged summer moss N content were plotted against a land use regression model (LUR) developed by taking NO_x samples from 144 sites in the Portland area within the Urban Growth Boundary. The correlation between moss N and modeled NO₂ was found to be significant at p < 0.001, r=0.625. Summer moss samples N content ranged between 0.71% and 3.36% (mean of 1.87%), the δ¹⁵N ranged -8.97‰ and 11.78‰ (mean of -0.91‰). Moss winter N content ranged between .77% and 3.12% (mean of 1.71%), and the δ¹⁵N ranged -10.40‰ and 10.27‰ (mean of -3.73‰). The average values for %N and δ¹⁵N fall within the range of previous studies in other moss samples, however the maximum values are higher than what other studies have typically found for both %N and δ¹⁵N. A significant correlation between δ¹⁵N and %N was found (r = 0.67). The moss samples showed a similar pattern of higher N content and δ¹⁵N near the urban center decreasing with distance from major roadways and other significant sources of fossil fuel derived NO_x. These results indicated the sensitivity of O.lyellii to N and the potential for its use as a biomonitor. With sufficient sampling density, using O. lyellii  as an inexpensive biomonitor to evaluate local bioavailable N deposition could be a useful tool in monitoring atmospheric N inputs at relatively high resolution and low cost.