Fractionation of Sediment-Bound Nitrogen: Bioavailability and Contamination Status of the Nutrient in Thrissur Kole Wetlands, Southwest India

Fractionation of sediment-bound nitrogen (N) in Thrissur Kole wetland was carried out to assess the mobility, bioavailability, and nutrient contamination status. Various N fractions were extracted using KCl equilibrium extraction method, and the concentration of each N fraction was analyzed by spectrophotometric method. Dominance of ammonia during all seasons was due to dissimilatory NO3− reduction to NH4+ and direct inputs from agriculture fields. Enrichment of urea during post-monsoon (181.78 ± 8.47 ppm) was attributed to fertilizer sources. NO3−-N deficit at central zone of the study area (monsoon and post-monsoon) was due to the anoxia. Comparatively higher values of NO2−-N during monsoon (0.07 ± 0.02 ppm) was due to the land run-off, leaching of fertilizers from agriculture fields, and higher rate of denitrification. Elevated content of total N [1819.94 ± 40.20 ppm (pre-monsoon); 3061.62 ± 13.21 ppm (monsoon); 2763.44 ± 9.65 ppm (post-monsoon)] reflected the inputs from in-situ production, agriculture, terrestrial run-off, waste disposal, and guano as well as carcass of birds. Average contribution of bioavailable N to total N was higher (36.16%) indicated the possibility for eutrophication risk during pre-monsoon. Agriculture dominated regions recorded severe contamination and eutrophication risk as indicated by organic pollution index, organic nitrogen index, and phosphorous pollution index.