COS 120-2
Tracking the fate of nitrogen deposition in a spatially heterogeneous savanna landscape: A short-term 15N labeling experiment

Thursday, August 13, 2015: 1:50 PM
321, Baltimore Convention Center
Elise M. Tulloss, Department of Plant Sciences, University of California, Davis, Davis, CA, USA
Mary L. Cadenasso, Department of Plant Sciences, University of California, Davis, Davis, CA, USA
Background/Question/Methods

Nitrogen deposition changes ecosystem function, but the N-cycling mechanisms by which these changes occur are less well-understood.  In California, urbanization and intensive agriculture expose adjacent oak savannas to enhanced N deposition of nitrate- and ammonium-N, respectively.  Within the oak savanna, tree canopies are hotspots of N deposition.  We analyzed ecosystem N budgets to assess the effect of a regional N deposition gradient and local deposition hotspots on a California oak savanna.  A field experiment using fertilizer to simulate deposition was conducted in 2010-2012.  Fertilizer was applied to experimental plots in understory and open areas at four treatment levels simulating a gradient in deposition rates received across the region.  At the onset of the 2011-12 growing season, short-term ecosystem N budgets were manipulated by injecting a 15N tracer into soils within the experimental field plots.  Separate 15nitrate and 15ammonium cores were located in each experimental plot. Samples were collected 3 days after isotope application and analyzed for 15N tracer signatures in the inorganic soil, microbial biomass, aboveground plant biomass and litter N pools.  A mass-balance approach was used to analyze the results and compare ecosystem N budgets of the treatments. 

Results/Conclusions

There was no evidence of N loss as the ecosystem retained nearly 100% of the added 15N with the majority (40-80%) of it in the microbial biomass N pool.  Soils were characterized by rapid nitrification rates based on the heavier label present in 15nitrate compared to 15ammonium pools.  Plant N uptake was greater in the open compared to the understory.  In addition, there was generally less N uptake by plants at higher fertilizer levels in both open and understory, but there was a larger decrease in uptake with fertilizer in the open.  This suggests that plant communities in the open will become N saturated more readily than plants in the understory.  Litter 15N was significantly greater in understory versus open and at higher fertilizer levels.  In the short-term, N deposition was rapidly incorporated into microbial biomass regardless of chemical form of the deposition.  Greater N retention by microbes occurred at higher fertilizer levels, but there was little difference between understory and open.  Future analyses will compare these data to samples collected 6 months after isotope labelling to examine longer-term trends.