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RISING CO₂, RISING SEA LEVEL AND RISING (OR SINKING?) COASTAL WETLANDS

J. Patrick Megonigal, Adam Langley and Amy Wolf
Smithsonian Environmental Research Center
Bio

Coastal marshes have the potential to store soil C faster than other terrestrial ecosystems because rising sea level enhances detritus burial and reduces efficient aerobic microbial decomposition. However, marshes that cannot accrete (i.e. build) new soil at the rate of sea level rise will become submerged and disappear. Here we consider the effects of elevated CO₂ on two antagonistic processes with respect to soil carbon storage – plant growth and decomposition. Previous work in a temperate brackish marsh showed that elevated CO₂ stimulates root growth, which in turn should increase soil accretion and the rate of elevation rise. To access the effect of elevated CO₂ on soil carbon decomposition rates, we grew a plant depleted in ¹³C (δ¹³C = -27^o/oo to -39^o/oo), Scirpus olneyi, in a soil enriched in ¹³C (δ¹³C=-15^o/oo). Both plants and soils were collected from the site of the elevated CO₂ field experiment. The difference in C isotope ratio allowed us to partition the CO₂ arising from the soil into plant versus microbial sources. We found that elevated CO₂ increased microbial mineralization of soil organic carbon by 157%. This suggested that gains in soil surface elevation caused by increased root growth could be offset by faster decomposition of soil organic matter by microbes. The net effect of elevated CO₂ on plant growth versus decomposition should be apparent from changes in soil surface elevation. We recently initiated a new field experiment designed to measure this response variable in detail. Results to date show that the joint effects of elevated CO₂ crossed with elevated N – another important global change variable – will be to increase soil surface elevation. Thus, elevated CO₂ and N may allow some tidal marsh ecosystems to persist in a future of rapid sea level rise.