Experimental Warming Affects Salt Marsh Litter Decomposition Through Changes in Plant Species Assemblage

Abstract

Questions: Salt marshes are highly productive coastal ecosystems that, like other coastal wetlands, play a key role in climate change mitigation due to their efficiency in sequestering and storing atmospheric carbon (C). This ability is partly explained by the low decomposition rates of organic matter that is buried in their sediments. However, given that decomposition dynamics are temperature-dependent, escalating temperatures potentially threaten salt marsh C sequestration capacity. The question addressed in this study is whether increasing temperature affects salt marsh detritus decomposition rates not only directly, but also by affecting plant community composition. Location: We performed a field manipulative experiment in a South Western Atlantic salt marsh in Mar Chiquita coastal lagoon, Argentina. Methods: We increased temperature using open-top chambers (OTCs). After 3 years of experimental run, we estimated plant species cover at each plot and estimated litter decomposition rates with a litterbag approach using standardized substrate (i.e. Spartina litter) and a community-based substrate.Results: Results show that the use of OTCs raised the temperature in warming treatments 1.05° C on average. After 3 years of experiment, the plant community structure under warming was characterized by increased dominance of Spartina densiflora, which produces a less labile detritus. Warming reduced the decomposition of community based litter by 50%, to levels similar to those of the standardized Spartina litterConclusions: Our results show that warming is likely to have significant consequences on salt marsh plant community structure, increasing the biomass of the dominant species and driving more recalcitrant litter production, which could slow decomposition dynamics. This mechanism should be considered when assessing the broader effects of global change.