Sediment bound nitrogen speciation: Contamination status and risk assessment in Ashtamudi Ramsar wetland, Southwest...

Sediments in estuarine wetlands serve as major repositories of nitrogen, which initiates transformation processes and subsequently influence nutrient cycling. This comprehensive study aims to evaluate sediment bound N speciation, bioavailability and ecological risk assessment in the Ashtamudi Ramsar Wetland, a tropical estuarine system in the southwest coast of India. Surface sediment samples collected from 28 stations of the wetland during non-monsoon and monsoon, were analysed for grain-size distribution, geochemical characteristics and influence of various nitrogen fractions- nitrite-N (NO₂^-N), nitrate-N(NO₃^--N), ammonia-N (NH₄⁺-N), urea-N (NH₂ -CO-NH₂-N), residual-N, Kjeldahl-N and Total-N. The geochemical partitioning of the sediment-bound N revealed the dominance of residual-N, accounting for the contribution of 89.61% to 96.69% of the TN, suggesting long term storage of N within sediments. NH₄⁺-N, being the major bioavailable form exhibited elevated concentration (non-monsoon average = 309.36 mg/kg; monsoon average = 55.34 mg/kg), in the southern arm of the wetland, reflecting influence of enhanced OM mineralisation as a result of direct disposal of waste, effluent discharges from domestic fish processing units and tourism sector. Multivariate statistical analyses further endorsed that close association of OM and sediment textural characteristics are the major factors governing N transformation and potential loading in the system. The assessment of risk indices revealed severe nitrogen enrichment, with average OPI values, 1.71 (non-monsoon) and 1.18 (monsoon), and ONI values, 0.28 (non-monsoon) and 0.23 (monsoon) further substantiates that wetland is under stress of severe contamination. The findings emphasize the need of sustainable nutrient management strategies to ensure the ecological integrity of the wetland.