Litter decomposition and prokaryotic decomposer communities along estuarine gradients

Climate change-driven sea-level rise and saltwater intrusion will impact carbon cycling in estuarine marshes by altering litter decomposition. We examined how litter quality and environmental conditions affect litter decomposition and prokaryotic communities along salinity and flooding gradients of the Elbe Estuary. Native and standardized litter (Tea Bag Index) were incubated in soils of various marsh types (freshwater, brackish, salt) and flooding frequencies (daily, monthly, yearly). Prokaryotic communities colonizing litter and soil were identified using 16S rRNA gene amplicon sequencing. Our results showed that litter quality is a key factor for litter decomposition in the estuary. Native litter decomposition increased with increasing salinity and decreasing flooding frequency. This was driven by litter chemistry, with higher lignin content and lignin:N ratio in the freshwater compared to the salt marsh vegetation and in pioneer zone compared to higher marsh vegetation. In contrast, tea litter decomposition declined with rising salinity, indicating adverse environmental conditions for decomposition with increasing salinity. Flooding effects varied with litter quality: mass loss of recalcitrant litter (rooibos tea) decreased with higher flooding frequencies, while mass loss of labile litter (green tea) increased. Prokaryotic communities in native and tea litter displayed distinct assemblages and lower diversity than the local soil community, indicating selective colonization of litter, which was particularly pronounced for tea litter. Tea mass loss benefited from a diverse soil prokaryotic community, while native litter mass loss was driven by an adapted local soil prokaryotic community. Our findings highlight biotic (litter quality, prokaryotic community) and abiotic (salinity, flooding) controls on litter decomposition in estuarine environments, suggesting that projected changes in salinity and hydrodynamics due to climate change could alter decomposition dynamics in these environments. • Salinity decreases tea decomposition; flooding decreases rates and stabilization. • Litter quality controls litter decomposition along the estuarine gradients. • Soil prokaryotic community is adapted to local vegetation. • Increased diversity of the prokaryotic soil community stimulates tea mass loss.