Nitrogen deposition across scales: Hotspots and gradients in a savanna landscape

Background/Question/Methods

Nitrogen (N) deposition is an element of global change that is receiving increasing attention, but is still relatively understudied in California.  Nitrogen deposition can affect biodiversity and ecosystem functions, but patterns of input vary spatially and temporally across scales.  This heterogeneity is critically important in savanna landscapes where it has long been recognized that savanna trees are hotspots of increased nutrient concentrations.  Scrubbing of the atmosphere by canopy foliage is a primary mechanism.  To investigate patterns of N deposition in a savanna landscape we asked whether a regional-scale gradient of deposition existed across north-central California oak savannas and whether the canopy received greater N deposition than the adjacent open grassland.  We hypothesized that the chemical form of N deposition would shift across the gradient such that sites closer to urban N emission sources received a greater proportion of N in oxidized form and sites closer to agricultural sources received a greater proportion of N in reduced form.  Using a throughfall/bulk deposition method, we quantified nitrate-N and ammonium-N inputs beneath solitary oak canopies and the adjacent open grassland in 6 oak savanna sites across north-central California during the 2008-2009 rainy season. 

Results/Conclusions Preliminary results indicate that a regional-scale deposition gradient exists with sites in closer proximity to urban and agricultural emission sources receiving significantly higher overall rates of input and sites in rural, remote areas receiving significantly less N overall.  Additionally, sites closer to urban sources of pollution have a greater proportion of N input as nitrate compared to other sites, consistent with an exposure to urban pollutant gases.  The oak canopy receives significantly greater N inputs compared to the adjacent open area at all sites, but the ratio of open to canopy deposition appears to be non-linear across sites.  Such a non-linearity suggests that the role of the canopy as a deposition hotspot becomes stronger in areas subjected to higher N deposition.  Average rates of N deposition beneath solitary oak canopies (up to 20 kg ^-1ha ^-1yr^-1) vastly outstrip estimates of N deposition for the California oak savanna as a whole.  The oak canopy functions as a deposition hotspot in the landscape and presents a unique opportunity to study the effects of rising rates of N deposition on oak savanna plant species interactions and community dynamics.