Contribution of N₂O emissions to the atmosphere from Indian monsoonal estuaries

Abstract

Estuaries are known to contribute a significant amount of nitrous oxide (N₂O) to the atmosphere; however, the contribution from the Indian estuaries is unknown. We made an attempt to estimate emissions of N₂O from the Indian estuaries by collecting samples from 28 major and minor estuaries along the Indian coast during the wet and dry periods. The N₂O was mostly saturated in all measured Indian estuaries during the study period (72–631 %), with exceptionally high saturation in the Ponniyaar estuary (5902%) during the wet period. The N₂O saturation displayed a strong relation with dissolved inorganic nitrogen (DIN; nitrate+nitrite and ammonium), ammonium and dissolved oxygen saturation, suggesting that nitrification is the major source of N₂O in the Indian estuaries. The negative relation between salinity and N₂O saturation suggests inner estuaries are a strong source compared to outer estuaries. The annual mean N₂O saturation (204 ± 137%) and fluxes (1.3 μmol N₂O m⁻² d⁻¹) in the Indian estuaries were significantly less than European estuaries (271% and ∼2.7 μmol N₂O m⁻² d⁻¹, respectively). The estimation of flux of N₂O from the European estuaries was also biased due to the inclusion of an exceptionally high supersaturation value from a small UK estuary, Colne (2645%). However, low N₂O saturation and fluxes in the Indian estuaries were related to mean low concentration of DIN that led to low nitrification rates compared to world estuaries. Despite India ranking second in artificial fertilizers use, high flushing rates during the wet period reduce residence time leading to less modification within the estuary.