The role of biogenic structures for greenhouse gas balance in vegetated intertidal wetlands

This chapter elucidates in detail the current knowledge on the role of biogenic structures for greenhouse gas biogeochemistry and dynamics in vegetated intertidal wetlands, i.e., mangrove forests and saltmarshes. The major types of biogenic structures formed by wetland plants and animals are portrayed and related to relevant biogeochemical processes affecting greenhouse gases. Subsequently, the impact of biogenic structures for greenhouse gas exchange is demonstrated by assessing and compiling the current knowledge on net primary production, carbon sequestration, and greenhouse gas emissions in mangrove forests and Spartina marshes. The data compilation clearly emphasizes the important role of biogenic structures (i.e., plant roots and infaunal burrows/tubes) for carbon cycling and greenhouse gas dynamics of intertidal wetlands. Emission of the greenhouse gases CO2, CH4, and N2O in mangrove forests with biogenic structures is increased 4.8, 29.9, and 3.8 times, respectively, compared with mangrove sediments devoid of these structures. Due to lack of reliable data on the role of burrow structures on greenhouse gas emission in Spartina marshes, only the impact of vegetation is available resulting in an increase of 2.7, 3.3, and 9.1 times, respectively. The strong enhancement of greenhouse gas emissions via biogenic structures largely counteracts the otherwise efficient capacity of carbon sequestration by these vegetated wetlands, leading to climate neutrality when embracing the global warming potential of the involved gases. This finding is surprising and contradicts previous estimates that indicated a distinct climate mitigation potential of mangrove and saltmarsh ecosystems.