Description

In the Chesapeake Bay, sea levels are rising more quickly than anywhere else in the eastern U.S. Relative sea-level rise in the Chesapeake measures 3.4 millimeters per year--twice the global average (1.7 mm/yr). Wetlands are often able to combat this by raising themselves up, through soil accretion or the formation of peat. But it's unclear how the other global change factors at work could jeopardize wetlands' ability to build elevation. In 2006, GCREW launched its first study to measure elevation change as a response to both carbon dioxide and nitrogen.

Though there is a great deal of interest in how rising CO₂ and nitrogen impact ecosystems together, very few long-term field studies have manipulated both. These plots were established in 2005 to experimentally manipulate atmospheric CO₂ and soil nitrogen. The greater size of plots allowed for sensitive measurement of soil surface elevation along with a keener focus on belowground processes.

Visit the Global Change Research Wetland homepage

The Questions

This second experiment addresses questions raised by the original Drake experiment. A great deal of work in other systems suggested that plants could continue absorbing higher CO₂ only if they had enough nitrogen. Can we enhance the ecosystem response to elevated CO₂ by adding nitrogen?

Tidal marshes must gain surface elevation to keep up with sea level rise. The original Drake study found that plants under elevated CO₂ were taking up a lot of carbon that couldn’t be accounted for in plant biomass. Could the missing carbon contribute to soil elevation change?  And if so, is it possible that by adding CO₂ and nitrogen, we can help marshes keep up with sea level rise?

Methodology

The lab of Dr. Pat Megonigal built 20 open-top chambers that each surrounded 3.4m² marsh plots. Inside half the chambers, they are raising carbon dioxide concentrations from 340 to 700 parts per million. But half of the plots also receive 25 g of nitrogen per year, simulating a more polluted marsh. In total, the experiment contains four types of experimental chambered plots:

1. Elevated CO₂ (Elev)
2. Ambient CO₂ with increased nitrogen (Amb + N)
3. Elevated CO₂ with increased nitrogen (Elev + N)
4. Control with no change (Amb)

Each plot also contains deep-rod soil elevation tables, allowing the researchers to make sensitive measurements of elevation change in each plot.

At the end of the season, they counted the individual plants to determine the abundance of the two dominant species (C3 Schoenplectus americanus and C4 Spartina patens). They also harvested some of the plants to compare the biomass of their roots belowground versus their shoots aboveground, and tracked the overall elevation change in the chambers.

Discoveries

Species composition:

A wealth of ecological theory suggests that adding nitrogen should increase CO₂ fertilization effects by adding a critical limiting nutrient. But the GCREW team found just the opposite. After four years of nitrogen addition, C4 grasses increased tenfold in biomass. C4 grasses are insensitive to elevated carbon dioxide, so the total ecosystem level CO₂ response actually declined where N was added. This result showed that shifts in plant community composition can reverse responses expected based strictly on biogeochemistry.

However, elevated CO₂ did cause a strong boost in soil elevation gain by the second full year of the study, likely by stimulating root productivity. This result suggests that rising carbon dioxide, while causing climatic warming and accelerating sea level rise, may also afford marshes an enhanced ability to keep up in the short term.

Download data from the marsh elevation experiment

View all downloadable data from the Global Change Research Wetland

The Future

Adding nitrogen greatly stimulated C4 grasses for several years. But since then, sea level has risen sharply, which favors the C3 sedge and nearly negates the nitrogen effects. A decade of observations is showing how one dominant factor (sea level) interacts with resource availability to control plant community composition and productivity. Carbon dioxide's tendency to boost wetland elevation has fluctuated greatly over the course of the study. The continuing record of elevation responses will allow for insights into how resources affect the marsh as it becomes more stressed by sea level rise.