Inland and coastal wetland ecosystems, including a range of freshwater and saline ecosystems with different inundation regimes, (1) have a high capacity for soil carbon storage due to anaerobic waterlogged soils and the accumulation of internal and external carbon sources. (2,3) Within these ecosystems, the vast majority of carbon is stored belowground (e.g., 62–99% in coastal wetlands and 50–93% in freshwater wetlands). (4−6) The development of belowground carbon pools is largely dependent on inputs of carbon into the system and the microbial processes that moderate the decomposition of carbon-rich organic matter (OM). In addition to being fundamentally important for soil fertility and supporting biodiversity, (7) the decomposition process in wetland soils is vital for soil carbon (trans)formation and sequestration in wetlands, (8) and thus the global carbon cycle itself. (9,10)

Identifying the drivers of wetland litter decomposition processes is essential for predicting feedbacks to environmental and climate change. Elevated temperature can enhance OM breakdown through increased microbial metabolism, which in turn might increase the decay rate for a range of wetland types. (10−12) Changes in soil water content (e.g., through inundation patterns, wetland drainage, tidal cycles, and interannual variability in precipitation) can control much of the decay process in wetland soils, influencing initial mass loss through leaching and providing access to nutrients that support microbial metabolism, while also influencing oxygen availability, potentially limiting microbial mineralization under anaerobic conditions. (2,9,13−15) Anaerobic conditions caused by waterlogging can dampen the enhanced decomposition caused by elevated temperature during the later stages of decay when decay processes are dominated by enzymatic breakdown of plant structural compounds, (2,16) thereby complicating the impacts of warming temperatures in the long term. Further, variability in litter chemical composition (e.g., nitrogen concentration, tissue type, phylogenetic history) can be a major influence on the decomposition dynamics in wetlands and can result in litter-specific responses to external factors like salinity, inundation, and warming. (10,17−22)

Although there is greater potential for belowground OM deposits (e.g., roots, rhizomes) to contribute to wetland soil carbon, decomposition studies in wetlands often focus on aboveground litter decomposition, (10,23,24) despite high litter export rates and potential for herbivory. (25−27) Additionally, existing global models of long-term decomposition are primarily based on terrestrial ecosystems and do not represent belowground wetland decay well due to different decomposition dynamics (e.g., wet conditions). (28) Long-term field datasets on belowground OM decomposition may help develop paradigms on the controls of wetland decay, including sea-level rise, (29,30) soil water content, climate, and decomposers. (31,32) Global wetland decay datasets may also help parametrize global carbon cycling through earth system models (33,34) and by means of satellite-based models. (35)

Since litter chemical composition strongly affects the decomposition process, it is difficult to draw broader conclusions when a range of litter types are used over regional and global scales. (36,37) A promising approach to advance our knowledge of belowground decomposition across ecosystems and climates is to use standardized substrates in lieu of local plant litter. (38,39) Standardized green tea and rooibos tea “litters” have water-soluble-dominant (labile, rapid leaching) and lignin-dominant (recalcitrant, stable) compositions, respectively. (39,40) Tea litter has been valuable at revealing short-term, 3 month drivers of belowground litter decay at regional and global scales, although a limited collection of studies for wetland and aquatic ecosystems exists. (41,42) There are limitations and challenges in extrapolating the drivers of short-term incubations to inform longer-term predictions and processes of decomposition, i.e., linked to carbon storage and sequestration. Longer standardized tea litter incubations (e.g., ∼1 year) show how differences in inundation and oxygen conditions and temperature may be driving wetland OM decay, particularly for the more recalcitrant rooibos tea litter, (43,44) suggesting different ecosystem responses to anthropogenic changes in the future...