Decomposing the Tea Bag Index and finding slower organic matter loss rates at higher elevations and deeper soil horizons in a minerogenic salt marsh

Abstract. Environmental gradients can affect organic matter decay within and across wetlands and contribute to spatial heterogeneity in soil carbon stocks. We tested the sensitivity of decay rates to tidal flooding and soil depth in a minerogenic salt marsh using the tea bag index (TBI). Tea bags were buried at 10- and 50- cm along transects sited at lower, middle, and higher elevations that paralleled a headward eroding tidal creek. Plant and animal communities and soil properties were characterized once while replicate tea bags and porewaters were collected several times over one year. TBI decay rates were faster than prior litterbag studies in the same marsh, largely due to rapid green tea loss. Rooibos decay rates were comparable to natural marsh litter, potentially suggesting that is more useful as a standardized organic matter proxy than green tea. Decay was slowest at higher marsh elevations and not consistently related to other biotic (e.g., plants, crab burrows) and abiotic factors (e.g., porewater chemistry), indicating that local hydrology strongly affects organic matter loss rates. Tea BI rates were 32–118 % faster in the 10 cm horizon compared to 50 cm. Rates were fastest in the first three months and slowed 54–60 % at both depths between 3- and 6- months. Rates slowed further between 6- and 12- months but this was less dramatic at 10 cm (17 %) compared to 50 cm (50 %). Slower rates at depth and with time were unlikely due to the TBI stabilization factor, which was similar across depths and decreased from 6 to 12 months. Slower decay at 50 cm demonstrates that rates were constrained by the environmental conditions of this deeper horizon rather than the molecular composition of litter. Overall, these patterns suggest that hydrologic setting, which affects oxidant introduction and reactant removal and is often overlooked in marsh decomposition studies, may be a particularly important control on organic matter decay in the short term (3–12 months). transects sited at lower, middle, and higher elevations that paralleled a headward eroding tidal creek.