Data deficient saltmarsh species has climate relevant carbon stocks from the east coast of India

The saltmarsh ecosystems of India are least studied,  despite being considered as nature-based solutions (NbS) towards climate change mitigation. This study quantifies the seasonal variation in carbon stocks of mono-specific saltmarsh specie (Porteresia coarctata) and its potential to play an important role in India’s climate change mitigation plans. Seasonal (pre-and post-monsoon) sampling of P. coarctata surface water, biomass and 30 cm sediment core was collected across four estuarine locations on the east coast of India to quantify the sediment dry bulk density (DBD), organic matter (OM%), organic carbon (C_org%), Corg stocks of sediment and biomass, total carbon (C%) and nitrogen (N%) and stable isotope ratios of Carbon (δ¹³C) and Nitrogen (δ¹⁵N).  In general, the sediment DBD, OM and Corg of P. coarctata meadows was higher in post-monsoon and was influenced by salinity and pH changes. Carbon isotope ratio (δ¹³C) of sediment C sources suggested, particulate organic matter (POM) contribution was highest (0.04-0.79%) followed by P. coarctata (0.01–052%) and other macrophytes across our study locations.  The seasonal variation of δ¹³C showed increased contribution of marine derived particulate and dissolved organic matter into P. coarctata meadows in post-monsoon season.  Higher δ¹⁵N values observed in post-monsoon season suggest an anthropogenic input, that was utilized by P. coarctata to increase its above and below-ground biomass and shoot density. The combined ecosystem (30 cm sediment + biomass) C_org stocks of P. coarctata was 1.7-fold higher in post-monsoon (4021.20± 917 Mg C) compared to pre-monsoon (2297.36 ±647 Mg C) season among the four locations with a sediment Corg contribution >70%. Based on the International Panel for Climate Change Tier II assessment the P. coarctata meadows (443 ha) can help in avoiding the pre-and post-monsoon emissions of 8431.34 and 14757.84 Mg CO₂, respectively. The combined price of the total CO₂ equivalent stored in P. coarctata meadows in pre- and post-monsoon is calculated to be US$ 14.50 and US$ 25.38 million, respectively. Further studies quantifying the NbS potential of mono-specific and mixed P. coarctata meadows along the coast of India is important, along with the integration of saltmarsh ecosystems into India’s National Action Plan on Climate Change.

Keywords: Saltmarsh, Carbon stocks, Porterasia coarctata, Nature-based solution, Stable isotopes