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

Intertidal wetlands capture and store carbon (C) for long periods of time, helping to reduce the concentration of CO2 in the atmosphere. Yet the processes, which govern the decomposition and subsequent long‐term storage of organic matter (OM) and C in these habitats, remain poorly understood. The Tea Bag Index (TBI) uses a standardized OM (green and Rooibos tea) and has the potential to shed light on OM decomposition across habitats, including saltmarshes. Here, we apply the TBI method at two saltmarshes within the same estuary with the aim of (i) reducing the influence of climatic variables and (ii) determining the role of the environment, including the soil characteristics, in the decomposition of OM. We extended the standard (3 months) incubation period over a full year in order to investigate the longer‐term decomposition processes at each site. The initial results partially support previous studies that the early stages of decomposition (leaching of the water‐soluble fraction) is governed by climatic conditions, but may be further enhanced by tidal flushing in saltmarshes. By extending the incubation period, we observed the initiation of midstage OM decomposition (cellulose degradation) upon which the soil characteristics appear to be the dominant control. These results highlight the importance of long‐term TBI incubations to understand early‐stage OM decomposition. The relationship between tea mass (OM) loss and C loss in these intertidal environments is not straightforward, and we would caution the use of the TBI as a direct universal proxy for soil C degradation in such intertidal wetlands. Plain Language Summary: Saltmarshes capture and lock organic matter (OM) and carbon (C) away in their underlying soils, preventing some of that C being released back to the atmosphere. Yet how this OM and C breaks down within these environments is poorly understood. The Tea Bag Index (TBI) uses standardized OM in the form of green and Rooibos tea to investigate decomposition. Burying the tea for different time periods allowed us to measure the mass and C loss to understand the breakdown of the material over time. Burying the tea bags in two different marshes within the same estuary allowed us to focus on the soil characteristics rather than climatic conditions. The yearlong incubations showed that the early breakdown of the OM is likely controlled by the climate and tidal flushing of the marshes, but that the second phase of decomposition is governed by the soil characteristics. The TBI is being used as a surrogate for the breakdown of soil C we tested, if mass loss of the tea relates to C degradation. The results suggest that the TBI is currently a poor proxy for C degradation in saltmarshes and that the TBI in these environments should be treated with caution. Key Points: Tea Bag Index (TBI) field incubation experiments should span a period of >1 year to ensure that all stages of early organic matter (OM) decomposition are observedSoil characteristics are a major control on long‐term OM decomposition in intertidal environmentsThe TBI should be used with caution in intertidal environments and is not a universal proxy for organic carbon (OC) degradation [ABSTRACT FROM AUTHOR]