Responses of marsh plant biomass allocation to elevated CO2: A case study at Chesapeake Bay

Background/Question/Methods

Marsh plants regulate two feedback loops: aboveground and belowground processes which may have different implications for soil carbon sequestration in coastal wetlands. Plant belowground part regulates soil nutrient and water uptake, and influences plant aboveground part growth and biomass allocation. To investigate the responses of marsh plant biomass allocation to elevated CO₂ (eCO₂), open top chambers (OTC, double the CO₂ concentration in the elevated chambers compared to the ambient chambers) were set up in pure C3 sedge Scirpus olneyi, C4 grass Spartina patens, and C3&C4 mixed community at Kirkpatrick Marsh, Rhode River Estuary, Chesapeake Bay. C3 and C4 plant shoot, root and rhizome biomass, carbon (C) and nitrogen (N) concentration were measured annually.

Results/Conclusions

After 28 years study, we found that eCO₂ significantly increased C3 sedge total biomass in C3 community by 21.9%. However, eCO₂ significantly decreased C4 grass total biomass in C4 community by 9.8%. Biomass allocation results showed that eCO₂ significantly increased plant Below:Above ratio by 13.7%, 25.3% and 41.1% in the Mixed, C3 and C4 community respectively. Root:Shoot ratio also increased by 11.4%, 24% and 47.1% in the corresponding communities under eCO₂. However, Rhizome:Root ratio showed no significant change in the Mixed, C3 and C4 community. In addition, the Shoot C:N ratio all increased in C3 in Mixed (13.1%),  C4 in Mixed (4.4%), C3 (17.7%) and C4 (6.3%) community to eCO₂, indicating that N dilution occurred under eCO₂ condition. The results reveal that under eCO₂ plant trends to allocate more biomass to the belowground part which may favor the rhizome and root extending to the deeper layer for nutrient uptake. The increase in Below:Above biomass ratio suggests that eCO2 may help marsh plant build up the sediment and keep up with the future sea level rise.