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

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.