Blue carbon stocks and cycling in tropical tidal marshes facing grazing pressure

As the urgency to mitigate climate change becomes more important, so too does the need for effective policies and management that deliver effective carbon storage and sequestration world-wide. Tidal marshes are one of several 'blue carbon' ecosystems where enhanced management offers natural climate mitigation solutions. However, their capacity to store carbon can be diminished by anthropogenic land use. We measured carbon stocks and rates of decomposition in tropical tidal marshes that have experienced disturbance from ungulates (cattle and feral pigs). We found that tidal marsh carbon stocks were higher near the surface of the sediment relative to estimates from deeper in the soil profile. Generally, both carbon stocks and decomposition rates were higher in areas with greater tidal inundation, apart from a unique location where tidal flow has been severely restricted by a road. Remarkably, the fenced wetland had the lowest surface soil carbon ratio in the surface sediments (30 cm), a response presumably due to the wetland being used by cattle (seasonal grazing) that are more concentrated because they are trapped in the fenced enclosure (not outside the fenced protection area), compared to open wetland areas that also still have grazing impacts, but apparently causing much less impact because of a distributed use regime. Our study outlines that fencing wetlands to hold out ungulates but to then use the restored area for seasonal grazing seems counter-intuitive and a misuse of government funds.