Using 15N stable isotope to trace the movement of deposited N in a roadside ecosystem

Background/Question/Methods

Human activities have changed nitrogen (N) biogeochemical cycling from local to global scales. Traffic related N pollutants (NOx and NHy) are often deposited heavily in roadside ecosystems and could negatively affect soil and water quality. Other pollutants, including heavy metals and winter deicing salt, could interact with deposited N and affect ecosystem structure and function and the sustainability of our environment. In 2010, we set two parallel transects, one next to the highway (0-m transect), another 50 meters away (50-m transect), in a roadside grassy/shrubby ecosystem next to the Interstate Highway 81 near Binghamton, NY. In each transect, replicate sets of plots, containing control, salt, and nitrate treatments, were established to evaluate the long-term roadside impacts (0-m vs. 50-m) and test short-term experimental effects. Previous results showed that salt treatment increased soil conductivity 10 fold and reduced soil CO₂ emission 10-20% in the 0-m transect and 25-60% in the 50-m transect, but had no effect on net N mineralization and nitrification rates. In 2012, we conducted a ¹⁵N tracer experiment by spraying ¹⁵N-enriched NaNO3 solution (0.02 g N/m²) on all of our experimental plots, and followed the movement of N by sampling plants and soils.  

Results/Conclusions

¹⁵N applied on June 20, 2012 was sampled after 1-day, 7-days, 4-weeks and 4-months by taking subsamples of above- and belowground plant materials and soils at 0-5 and 5-15 cm depths. On Day-1, most aboveground biomass were dead thatch but by Day-7, variable amount of vegetation cover was recorded, generally higher in the 50-m than 0-m transect, and <5% in salt plots. Plant cover increased substantially by Week-4 except in salt plots. In the 50-m transect, aboveground N concentrations increased from 11.2 on Day-1 to 13.5 mg/g on Week-4 in the control and nitrate plots, but was low in salt plots. Plants in the 0-m transect had lower aboveground N concentrations, but higher root N concentrations than the 50-m transect. ¹⁵N tracer significantly raised aboveground del¹⁵N (300-465‰) and root del¹⁵N (35-99‰). Over time, aboveground ¹⁵N declined steadily, especially in the salt plots, but root ¹⁵N remained unchanged or even increased by Week-4. Both above- and belowground ¹⁵N declined sharply when plants senesced. Our results suggest that road salt and other pollutants could affect plant tissue N concentrations and the movement of deposited N. We will continue our analyses on soils and construct a ¹⁵N budget for this roadside ecosystem.