Elevated Nitrogen Deposition to Fire‐Prone Forests Adjacent to Urban and Agricultural Areas, Colorado Front Range, USA

Abstract

As humans increasingly dominate the nitrogen cycle, deposition of reactive nitrogen (Nr) will continue to have adverse consequences for ecosystems. In the Rocky Mountains, Nr deposition remains elevated and has become increasingly dominated by ammonium, despite efforts to reduce emissions. Currently, spatial models of Nr deposition do not fully account for urban and agricultural emissions, sources that contribute to the observed high rates of ammonium deposition in adjacent ecosystems. To address this gap in the Colorado Front Range, we measured Nr deposition along a transect from urban and agricultural plains to subalpine forests. We found elevated values of wet Nr deposition at the urban and foothill sites (4.7 and 4.4 kg N ha−1 yr−1, respectively), and lower values at the montane and subalpine sites (2.5–2.8 kg N ha−1 yr−1). Ammonium dominated wet and bulk Nr deposition, accounting for approximately 69% of bulk Nr deposition. Seasonally, bulk Nr deposition was highest in the spring months, when air masses from the plains are transported west into the mountains. Previous work has demonstrated that high elevations of the Colorado Front Range are especially sensitive to Nr deposition due to thin soil and minimal vegetation. Our results indicate that despite lower precipitation, the fire‐prone forested foothills receive even greater Nr deposition than higher elevations, due to proximity to urban and agricultural Nr sources. The interaction between elevated Nr deposition and wildfire in this region may pose a risk to water supplies and ecosystems, and is an important topic for future research.

A call for strategic water-quality monitoring to advance assessment and prediction of wildfire impacts on water supplies

“…Wildfire is often not the sole terrestrial disturbance affecting water-supply watersheds; pre-and post-wildfire logging, reforestation, mining, and other activities can confound interpretations of wildfire impacts (e.g., Silins et al, 2009;Harrison et al, 2014;Rhoades et al, 2018;Stevens-Rumann et al, 2018;Murphy et al, 2020;Burke et al, 2021), especially when such disturbances overlap in space and time. Compound stressors can extend to atmospheric processes such as elevated wet and dry deposition of constituents before wildfire (Riggan et al, 1994;Burke et al, 2010;Heindel et al, 2022) and smoke effects (Williamson et al, 2016;Evans et al, 2021;Boyer et al, 2022). Climate variability and change also confound detecting wildfire effects, and future shifts in precipitation regimes (Touma et al, 2022) may affect constituent mobilization.…”

Fire in the Earth System: Introduction to the Special Collection

“…Heindel et al. (2022) measured nitrogen deposition along a transect spanning a range of elevation and land use in the fire‐prone Colorado Front Range. They found elevated nitrogen deposition from urban and agricultural sources at much higher levels than previously modeled, indicating the need for more intensive nitrogen monitoring in the wildland‐urban interface because nitrogen and wildfire can combine to degrade water quality and fire‐prone ecosystems substantially.…”

“…Work by Corona-Núñez et al (2020) using GOES and MODIS satellite data determined that tropical forest fires produce 19% of Mexico's CO 2 emissions, equivalent to 4 to 11 times more than national emissions from deforestation. Heindel et al (2022) measured nitrogen deposition along a transect spanning a range of elevation and land use in the fire-prone Colorado Front Range. They found elevated nitrogen deposition from urban and agricultural sources at much higher levels than previously modeled, indicating the need for more intensive nitrogen monitoring in the wildland-urban interface because nitrogen and wildfire can combine to degrade water quality and fire-prone ecosystems substantially.…”