Factors controlling spatiotemporal variability of soil carbon accumulation and stock estimates in a tidal salt marsh

Abstract. Tidal salt marshes are important contributors to soil carbon (C) stocks despite their relatively small land surface area. Although it is well understood that salt marshes have soil C burial rates orders of magnitude greater than those of terrestrial ecosystems, there is a wide range in storage rates among spatially distributed marshes. In addition, wide ranges in C storage rates also exist within a single marsh ecosystem. Tidal marshes often contain multiple species of cordgrass due to variations in hydrology and soil biogeochemistry caused by microtopography and distance from tidal creeks, creating distinct subsites. Our overarching objective was to observe how soil C concentration changes across four plant phenophases and across three subsites categorized by unique vegetation, hydrology, and biogeochemistry, while also investigating dominant biogeochemical controls on soil C concentration. We hypothesized that subsite biogeochemistry drives spatial heterogeneity in soil C concentration, and this causes variability in soil C concentration at the marsh scale. In addition, we hypothesized that soil C concentration and porewater biogeochemistry vary temporally across the four plant phenophases (i.e., senescence, dormancy, green-up, maturity), causing further variation in marsh soil C that could lead to uncertainty in soil C estimates. To test these hypotheses, we quantified soil C concentrations in 12 cm sections of soil cores (0–48 cm depth) across time (i.e., phenophase) and space (i.e., subsite), alongside several porewater biogeochemical variables including dissolved organic carbon (DOC), EEMs/ UV-VIS, redox potential, pH, salinity, reduced iron (Fe²⁺), reduced sulfur (S^2-), and total porewater element (Fe, Ca) concentrations in three distinct subsites. Soil C concentration varied significantly (p<0.05) among the three subsites and was significantly greater during plant dormancy. Soil S, porewater sulfide, redox potential, and depth predicted 44 % of the variability in soil C concentration. Our results show that soil C varied spatially across a marsh ecosystem up to 63 % and across plant phenophase by 26 %, causing variability in soil C storage rates and stocks depending on where and when samples are taken. This shows that hydrology, biogeochemistry, and ecological function are major controls on saltmarsh C content. It is, therefore, critical to consider spatial and temporal heterogeneity in soil C concentration when conducting blue C assessments to account for soil carbon variability and uncertainty in C stock estimates.