Nitrogen Content and Isotopic Composition of Spanish Moss (Tillandsia usneoides L.): Reactive Nitrogen Variations and Source Implications Across an Urban Coastal Air Shed

Abstract

In order to assess the use of Spanish moss (Tillandsia usneoides L.) as a biomonitor for atmospheric reactive nitrogen (Nr), T. usneoides samples were collected in urban Wilmington, NC, USA and at a remote barrier island 30 km away (Bald Head Island, NC, USA), and were analyzed for N content and δ15N. The mean urban N content (n = 64) was 0.8 ± 0.5 % whereas the island samples (n = 4) mean was 0.1 ± 0.04 %. δ15N values for Wilmington samples ranged from −13.9 to +23.6‰ (mean = −2.2 ± 9.3‰) while δ15N for island samples (n = 4) ranged from −12.4 to −9.8‰ (mean = −11.2 ± 1.1‰). Both the N content and δ15N values in the urban air shed were significantly higher than those at the barrier island due to urban anthropogenic Nr emissions including inputs from vehicles. δ15N of T. usneoides was correlated with road density, and δ15N values at a road transect decreased with distance from the road, indicating the importance of vehicle emissions as a source of Nr to urban ecosystems. A 3.5 kg ha−1y−1 bulk N deposition rate was estimated for the air shed using average T. usneoides N content (0.8 %) and a previously developed model relating N content in European moss from 16 countries to bulk N deposition. This deposition is remarkably similar to the N deposition rate (3.19 kg ha−1y−1) measured by the U.S. EPA Clean Air Status and Trends Network at the nearest coastal collection site. This suggests the relationship between European moss N content and bulk N deposition is also true for T. usneoides in the southeastern USA and underscores the efficacy of this method. Results presented here are significant because they indicate that T. usneoides can be used as an effective and inexpensive biomonitor for atmospheric N deposition.