Plants mediate soil organic matter decomposition in response to sea level rise

Abstract

Tidal marshes have a large capacity for producing and storing organic matter, making their role in the global carbon budget disproportionate to land area. Most of the organic matter stored in these systems is in soils where it contributes 2-5 times more to surface accretion than an equal mass of minerals. Soil organic matter (SOM) sequestration is the primary process by which tidal marshes become perched high in the tidal frame, decreasing their vulnerability to accelerated relative sea level rise (RSLR). Plant growth responses to RSLR are well understood and represented in century-scale forecast models of soil surface elevation change. We understand far less about the response of SOM decomposition to accelerated RSLR. Here we quantified the effects of flooding depth and duration on SOM decomposition by exposing planted and unplanted field-based mesocosms to experimentally manipulated relative sea level over two consecutive growing seasons. SOM decomposition was quantified as CO2 efflux, with plant- and SOM-derived CO2 separated via δ(13) CO2 . Despite the dominant paradigm that decomposition rates are inversely related to flooding, SOM decomposition in the absence of plants was not sensitive to flooding depth and duration. The presence of plants had a dramatic effect on SOM decomposition, increasing SOM-derived CO2 flux by up to 267% and 125% (in 2012 and 2013, respectively) compared to unplanted controls in the two growing seasons. Furthermore, plant stimulation of SOM decomposition was strongly and positively related to plant biomass and in particular aboveground biomass. We conclude that SOM decomposition rates are not directly driven by relative sea level and its effect on oxygen diffusion through soil, but indirectly by plant responses to relative sea level. If this result applies more generally to tidal wetlands, it has important implications for models of SOM accumulation and surface elevation change in response to accelerated RSLR.

Global change effects on decomposition processes in tidal wetlands: implications from a global survey using standardized litter

“…This finding is in accordance with an increasing number of studies demonstrating negligible direct effects of sea level on decomposition rate in tidal wetland soils Mueller et al, 2016;Janousek et al, 2017). A SLR-induced reduction in decomposition rate with positive feedback on tidal wetland stability seems therefore to be an unlikely scenario.…”

“…OM preservation were thus far overlooked, probably because stimulation of decomposition processes through increasing flooding is counterintuitive (Mueller et al, 2016). Here, we provide evidence that accelerated SLR is unlikely to slow down the decomposition rate of fresh OM inputs and additionally may strongly decrease OM stabilization and thus potentially the fraction of net primary production and other OM inputs to stable SOM.…”

“…We therefore conclude that other parameters exerted overriding influence on k, mainly masking temperature effects and have not been captured by our experimental design. For instance, we do not have data on plant-biomass parameters that are thought to exert strong control on decomposition in tidal wetlands through priming effects (Wolf et 325 al., 2007;Mueller et al, 2016;Bernal et al, 2017). Likewise, large differences in site elevation and hydrology could have induced high variability in k across sites and masked potential temperature effects.…”

“…Second, by conducting paired measurements in both high-and lowelevated zones of tidal wetlands worldwide, we were aiming to gain insight into potential effects of accelerated relative SLR on carbon turnover. Despite the dominant paradigm that decomposition is 130 inversely related to flooding, the existing literature on hydrology and SLR effects on OM decomposition in tidal wetlands yields equivocal results, which is often due to the overriding effect of OM quality on decomposition rate (Hemminga and Buth, 1991;Mueller et al, 2016). Additionally, by expanding our study to include fresh and brackish sites, we anticipated to capture the effects of salt-water intrusion into brackish and fresh systems, which is likely to affect 135 decomposition processes in tidal wetlands (Morrissey et al, 2014).…”

Sediment and carbon deposition vary among vegetation assemblages in a coastal salt marsh

“…within and among different species assemblages) to surface deposition and longer-term C accumulation remains unquantified and may vary with vegetation structure and geomorphic position. Regardless of OM source, the capacity of salt marshes to store carbon in the long-term remains dependent upon the balance between OM input and its decay (Mueller et al, 2016). While there is considerable debate as to which factors most influence the long-term retention of C in soils, litter quality has long been identified as a key driver of decay rates (Cleveland et al, 2014;Enríquez et al, 1993;Hemminga and Buth, 1991;Josselyn and Mathieson, 1980;Kristensen, 1994) and is of particular relevance to C stock accumulation in surface soils.…”