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