Ecosystem nitrogen retention in response to 8 years of exposure to elevated CO2 and nitrogen pollution

Background/Question/Methods

Both nitrogen (N) pollution and rising CO₂ have direct physiological effects on plants and biogeochemical cycles that may alter the fate of N in tidal marshes. How these two global change factors independently and simultaneously affect N cycling in marshes remains largely unexplored. Our objective was to determine how elevated CO₂ and N pollution affect the three primary fates of N in a tidal marsh: 1) long-term retention in plants and soil, 2) denitrification to the atmosphere, and 3) tidal export. Our experimental marsh plots have been factorially exposed to two levels of atmospheric CO₂ (ambient or ambient + 340 ppm) and two levels of N addition (0 or 25 g N m^-2 y^-1) since 2006 (n=5). We tracked the fate of a ¹⁵N tracer that was added in 2006 to each plot to quantify the fraction of added N retained in vegetation and soil, as well as to estimate lateral migration of N. To assess the importance of gaseous N loss, N₂O flux was measured in situ using respiration collars and potential denitrification (N₂ + N₂O) was measured with the acetylene reduction technique using laboratory incubations of soil slurries. 

Results/Conclusions

Nitrogen-fertilized plots retained 47% as much ¹⁵N as control plots due to reduced above and belowground retention and increased N₂O flux and migration. Plots under elevated CO₂ only retained 3% more ¹⁵N than control plots despite a significant decrease in lateral migration, possibly due to denitrification activity not captured by our in situ measurements. Plots under the combined treatment retained 63% as much ¹⁵N as control plots primarily due to a reduction in belowground storage. Our results indicate that brackish marshes may not be capable of retaining more nitrogen as atmospheric CO₂ continues to increase, particularly in N polluted marshes.