Direct and indirect controls on organic matter decomposition in four coastal wetland communities along a landscape salinity gradient

Abstract

1. Coastal wetlands store more carbon than most ecosystems globally. As sea level rises, changes in flooding and salinity will potentially impact ecological functions, such as organic matter decomposition, that influence carbon storage. However, little is known about the mechanisms that control organic matter loss in coastal wetlands at the landscape scale. As sea level rises, how will the shift from fresh to salt-tolerant plant communities impact organic matter decomposition? Do longterm, plant-mediated, effects of sea-level rise differ from direct effects of elevated salinity and flooding? 2. We identified internal and external factors that regulated indirect and direct pathways of sea-level rise impacts, respectively, along a landscape-scale salinity gradient that incorporated changes in wetland type (fresh, oligohaline, mesohaline and polyhaline marshes). We found that indirect and direct impacts of sea-level rise had opposing effects on organic matter decomposition.3. Salinity had an indirect effect on litter decomposition that was mediated through litter quality. Despite significant variation in environmental conditions along the landscape gradient, the best predictors of above-and below-ground litter decomposition were internal drivers, initial litter nitrogen content and initial litter lignin content respectively. Litter decay constants were greatest in the oligohaline marsh and declined with increasing salinity, and the fraction of litter remaining (asymptote) was greatest in the mesohaline marsh. In contrast, direct effects of salinity and flooding were positive. External drivers, salinity and flooding, stimulated cellulytic activity, which was highest in the polyhaline marsh. Synthesis.Our results indicate that as sea level rises, initial direct effects of salinity will stimulate decay of labile carbon, but over time as plant communities shift from fresh to polyhaline marsh, litter decay will decline, yielding greater potential for long-term carbon storage. These findings highlight the importance of quantifying carbon loss at multiple temporal scales, not only in coastal wetlands but also in other ecosystems where plant-mediated responses to climate change will have significant impacts on carbon cycling. 656 | Journal of Ecology STAGG eT Al.

An Assessment of the Tea Bag Index Method as a Proxy for Organic Matter Decomposition in Intertidal Environments

“…Clearly, these intertidal environments are globally important sites for C sequestration and storage. However, there is a general lack of empirical data on saltmarsh C dynamics, notably regarding the environmental factors controlling in situ decomposition of organic matter (OM; Mueller et al, ; Janousek et al, ; Stagg et al, ). As C stored in saltmarshes can be autochthonous, understanding the rates and drivers of litter decomposition is essential to both manage these ecosystems and model how climate change may affect their C storage (Middleton & McKee, ).…”

Salinity Affects Topsoil Organic Carbon Concentrations Through Regulating Vegetation Structure and Productivity

“…For instance, Kleber et al () found that the old and stable soil organic matter was not necessarily chemically recalcitrant; and Caruso et al () demonstrated that environmental and ecological factors (e.g., spatially structured microbial activities), rather than molecular recalcitrance, were the predominant controls in determining carbon residence times. As a consequence, long‐term viability of salt marshes and their carbon sequestration depend greatly on the balance between organic matter production and decay responses to environmental changes (Kirwan & Mudd, ; Stagg et al, ).…”

“…Although the feedback of SOC concentrations to increased surface/pore water salinity and soil salinity has been studied for decades from different perspectives, there is no consensus regarding the effects of salinity on SOC concentrations. Some studies suggested that elevated salinities can decrease SOC concentrations (Hansen et al, ; Więski et al, ), some reported increases in SOC concentrations and inhibition of decomposition processes (Hu et al, ; Stagg et al, ), and others indicated no significant impact on SOC accumulation (Spalding & Hester, ). In this context, it is imperative to identify the relationship between soil salinities and SOC concentrations, especially under the accelerated sea‐level rises and aggravated saltwater intrusion scenarios.…”

Soil Respiration and Photosynthetic Carbon Gain on an Abundant Coastal Land After Plantation of Tamarix chinensis

“…The efficient use of such techniques on degraded land can be applied in conjunction with the cultivation of halophytes (Ashraf et al 2012). However, planting on saline land can substantially alter the biological and biochemical characteristics of soil, including soil biomass production, salt, and water regulation and microbial activity (Jin et al 2013;Cui et al 2016;Stagg et al 2017). In addition, the response of saline soil respiration to human activity also needs to be investigated, as this response has been among the key indicators in studies of ecological and environmental improvements (Olsson et al 2015;Adachi et al 2017).…”