Interactive effects of invasion and hydrology influence C storage along a nitrogen gradient in a simulated clonal wetland ecosystem

Background/Question/Methods

Wetlands are highly dynamic ecosystems in terms of hydrology and N cycling, and are one of the most invaded habitats worldwide.  Therefore, it is important to disentangle the complex set of mechanisms and feedbacks influencing potential alterations to C cycling due to invasive wetland plants, hydrology, N loading, and their interaction.  MONDRIAN is a newly created individual based model, simulating growth and competition for nutrients among individual ramets, as well as clonal connections and horizontal expansion.  Simulated ecosystem N cycling both drives, and is driven by, plant growth, litter production, and biogeochemistry of litter and sediment organic matter.  Using MONDRIAN, we set up a factorial experiment testing the effects of (1) N loading, (2) hydrology, and (3) invasion on the ability of coastal wetlands to store C in living biomass, litter, and SOM, along with the sum of all pools (ecosystem C).   The gradient of N loading ranged from oligotrophic to eutrophic and invasion was by hypothetical species that differed from the native community only by size (equal to, 10x, or 20x the size of the largest native).  Wetland sediments were either always anaerobic (flooded), always aerobic, or intermediate with the water level just beneath the soil surface.    

Results/Conclusions

Overall, the highest ecosystem C pools (EcoC) occurred in the invasion scenario with the largest invader at the highest N loading level when hydrology was intermediate.  The lowest EcoC occurred in the native community at the lowest N loading level in flooded conditions.  Invasion by the smallest invader did not significantly affect EcoC compared to the native community, but invasion by the 10x and 20x species did significantly increase EcoC.  Increasing N loading increased all C pools in all scenarios, with EcoC usually doubling from the low to high end of the gradient.  Hydrology played an important role in C storage; intermediate hydrology had the highest levels of EcoC, followed by aerobic conditions, and then flooded conditions.  Litter carbon pools in flooded conditions were about 2-5x greater than the other hydrologic regimes.  EcoC was strongly related to productivity (R² = 0.92), which is why EcoC was usually lowest in flooded conditions where N mineralization was reduced causing decreased N uptake and reduced growth.  Intermediate hydrologic conditions had the highest EcoC because productivity remained high, while sediment mineralization was low.   These simulations show the importance of considering wetland hydrology, invasion, and N loading when determining a wetland’s ability to sequester carbon.