Plant response to elevated CO2 and nitrogen drives soil accretion in a high salt marsh

Tidal salt marsh plants may regulate soil accretion to maintain an optimal soil elevation relative to mean sea level. The ability of these important coastal ecosystems to match increasing rates of sea level rise, therefore, may be modified by other anthropogenic perturbations which strongly influence plant performance. The first year of experimentation in a marsh dominated by Scirpus olneyi and Spartina patens revealed strong plant productivity responses to elevated CO₂ and nitrogen addition, particularly when the treatments were applied in combination. We have outfitted these plots with soil elevation tables (SETs) to make high-resolution measurements of soil surface elevation. Initial SET measurements indicate that soil elevation follows trends in plant growth, with the most productive treatments yielding the largest gains in rooting-zone soil accretion. In this first growing season, patterns of elevation change likely arose from stimulation of subsurface plant biovolume. Long-term response of elevation in this highly organic marsh will depend on the balance of plant productivity and organic matter decomposition. Elevated CO₂ and nitrogen additions will likely continue to stimulate plant productivity, but the response of decomposition remains uncertain.