Contribution of N₂O emissions to the atmosphere from Indian monsoonal estuaries

Abstract

A B S T R A C T Estuaries are known to contribute a significant amount of nitrous oxide (N 2 O) to the atmosphere; however, the contribution from the Indian estuaries is unknown. We made an attempt to estimate emissions of N 2 O from the Indian estuaries by collecting samples from 28 major and minor estuaries along the Indian coast during the wet and dry periods. The N 2 O was mostly saturated in all measured Indian estuaries during the study period (72Á631%), with exceptionally high saturation in the Ponniyaar estuary (5902%) during the wet period. The N 2 O 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 N 2 O in the Indian estuaries. The negative relation between salinity and N 2 O saturation suggests inner estuaries are a strong source compared to outer estuaries. The annual mean N 2 O saturation (2049137%) and fluxes (1.3 mmol N 2 O m (2 d (1 ) in the Indian estuaries were significantly less than European estuaries (271% andThe estimation of flux of N 2 O 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 N 2 O 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.

Nitrous oxide and methane in two tropical estuaries in a peat-dominated region of northwestern Borneo

“…Dissolved N 2 O concentrations were mostly close to atmospheric equilibrium concentrations (i.e., 100 % saturation), which is in line with other comparable studies in the tropics and subtropics (Richey et al, 1988;Zhang et al, 2010;Rao and Sarma 2013;Borges et al, 2015). High N 2 O concentrations were reported for eutrophic and hypoxic coastal waters in the western Indian continental shelf (up to 533 nmol L −1 ; Naqvi et al, 2000) and for the Peruvian upwelling (up to 986 nmol L −1 ; Arevalo-Martinez et al, 2015).…”

“…flux densities between 0.7 and 49 mmol m −2 yr −1 in the estuaries of the Sundarban mangrove ecosystem. However, we caution that the comparison suffers from the different approaches to determining the gas exchange velocity k. Koné et al (2010), Shalini et al (2006), Biswas et al (2007), and Rao and Sarma (2013) used empirical equations relating k to wind speed, while we used estimates based on floating chamber measurements. We refer to our discussion in Müller et al (2016), where we showed that our estimates derived by floating chamber measurements yielded higher values than if we had used empirical relationships with wind speed.…”

Upwelling intensity modulates N2O concentrations over the western Indian shelf

“…Biogeochemical cycling over the inner shelves is to a large extent influenced by anthropogenic discharges from adjacent estuaries, the waters of which are expected to be highly supersaturated with N 2 O [ Barnes and Owens , ; Bange , ; Barnes and Upstill‐Goddard , ]. However, the Indian estuaries show low N 2 O concentrations due to their capacity for rapid assimilation of nitrogen [ Rao and Sarma , ]. The Kochi estuary is no exception to this and due to its very fast turnover times for anthropogenic nitrogen [ Bhavya et al ., ] its N 2 O concentrations are very low during both the dry (12 ± 5.6 nM) and wet (28 ± 15 nM) seasons (unpublished data), which is in agreement with earlier studies [ Rao and Sarma , ].…”

“…However, the Indian estuaries show low N 2 O concentrations due to their capacity for rapid assimilation of nitrogen [ Rao and Sarma , ]. The Kochi estuary is no exception to this and due to its very fast turnover times for anthropogenic nitrogen [ Bhavya et al ., ] its N 2 O concentrations are very low during both the dry (12 ± 5.6 nM) and wet (28 ± 15 nM) seasons (unpublished data), which is in agreement with earlier studies [ Rao and Sarma , ]. Moreover, most of our nearshore observations were made during the flood tide (see Table of Gupta et al .…”

Influence of river discharge on abundance and dissemination of heterotrophic, indicator and pathogenic bacteria along the east coast of India

“…3f). The significant north-south variations in physical associated with biogeochemical properties along the east coast of India were mainly driven from discharge from various rivers with different characteristics (Sarma et al, 2012a,b;Rao and Sarma, 2013).…”