Changes in Ecosystem Nitrogen and Carbon Allocation with Black Mangrove (Avicennia germinans) Encroachment into Spartina alterniflora Salt Marsh

Increases in temperature are expected to facilitate encroachment of tropical mangrove forests into temperate salt marshes, yet the effects on ecosystem services are understudied. Our work was conducted along a mangrove expansion front in Louisiana (USA), an area where coastal wetlands are in rapid decline due to compounding factors, including reduced sediment supply, rising sea level, and subsidence. Marsh and mangrove ecosystems are each known for their ability to adjust to sea-level rise and support numerous ecosystem services, but there are some differences in the societal benefits they provide. Here, we compare carbon and nitrogen stocks and relate these findings to the expected effects of mangrove encroachment on nitrogen filtration and carbon sequestration in coastal wetlands. We specifically evaluate the implications of black mangrove (Avicennia germinans) encroachment intoSpartina alterniflora-dominated salt marsh. Our results indicate that black mangrove encroachment will lead to increased aboveground carbon and nitrogen stocks. However, we found no differences in belowground (that is, root and sediment) nitrogen or carbon stocks between marshes and mangroves. Thus, the shift from marsh to mangrove may provide decadal-scale increases in aboveground nitrogen and carbon sequestration, but belowground nitrogen and carbon sequestration (that is, carbon burial) may not be affected. We measured lower pore water nitrogen content beneath growing mangroves, which we postulate may be due to greater nitrogen uptake and storage in mangrove aboveground compartments compared to marshes. However, further studies are needed to better characterize the implications of mangrove encroachment on nitrogen cycling, storage, and export to the coastal ocean.

Automated summary

Increases in temperature are expected to facilitate the encroachment of tropical mangrove forests into temperate salt marshes. The study conducted in Louisiana, USA, compared carbon and nitrogen stocks in marshes and mangroves and found that black mangrove encroachment leads to increased aboveground carbon and nitrogen stocks. However, belowground nitrogen and carbon sequestration may not be affected. Further studies are needed to better understand the implications of mangrove encroachment on nitrogen cycling, storage, and export to the coastal ocean.