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