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Fate of anthropogenic nitrogen in aquatic systems of India

Final Report Summary - AQUATIC N IN INDIA (Fate of anthropogenic nitrogen in aquatic systems of India)

Project context and objectives

Report of the project ‘Aquatic N in India’.

The overarching goal of the project was to understand the fate of the enormous amount of reactive nitrogen (N) released to terrestrial aquatic systems in India due to human activities. The four specific objectives were: (1) to study the spatial and temporal variations of dissolved N species; (2) to determine the rates of reduction-oxidation (redox) transformations (denitrification and anaerobic ammonium oxidation (anammox)), and assess their relative importance in reactive N loss; (3) to identify the sources of N and understand mechanisms of N transformations through natural isotope abundance measurements in dissolved N species; and (4) to characterise the composition of microbes involved in various redox transformations. The project was implemented as four well-defined investigations, each of which dealt with at least two of these objectives.

Project results

1. Nitrogen cycling in freshwater reservoirs

Eight dams located within a wide latitudinal band (9.8°N-31.4°N) were selected for the study. The reservoirs of these dams vary greatly in size and they also experience widely different climatic conditions. Thus, the data should be representative of reservoirs of South Asia as a whole. All reservoirs were sampled in the summer and additional observations were made in winter/early spring/late autumn in three reservoirs (Idukki, Koyna and Markandeya). The Tillari reservoir, located close to Goa, was selected for detailed observations: 24 visits to this site on a monthly basis provided time series data extending over two years. In addition to various physico-chemical measurements, on 15 field trips incubation experiments were carried out with labelled tracers to measure rates of various redox transformations in all but one reservoir with repeated observations in Tillari (6), Markandeya (3) and Idukki (2).

Contrary to expectations, the reservoirs sampled were not highly eutrophied - in a majority of cases, chlorophyll a and nitrate concentrations were below 3 mg m-3 and 3 µM, respectively. During summer stratification, anaerobic conditions developed in most reservoirs with apparent loss of oxidised N. However, incubations with 15N-labelled substrates yielded very low, often undetectable, rates of denitrification as well as anammox. Such was also the case for dissimilatory nitrate reduction to ammonium (DNRA) that was also measured in selected reservoirs. Incubations of samples spiked with 15N-labelled nitrite in the presence of methane greatly enhanced production of labelled N2 in Tillari and Markandeya reservoirs where methane has been found to accumulate within the hypolimnia during the summer. Samples collected for the molecular analysis are still being analysed but preliminary results suggest abundance of methanotrophs. Our results are of high environmental significance because the build-up of methane, characteristic of anaerobic open freshwater systems, would not only control eutrophication but it would also provide a negative feedback to the production of methane itself by limiting organic production in the surface layer. Significantly, unusually large accumulation of nitrous oxide was not observed in the reservoirs.

2. Nitrogen cycling in groundwaters

Sampling for groundwaters was carried out on six field trips - four to northern/northwestern India and two to central India. Samples during these trips were collected from the existing hand pumps and tube wells. Unlike central India, groundwaters in the northern region were invariably anaerobic. Although these expeditions provided valuable information on the distribution of dissolved N species including nitrous oxide, incubation experiments yielded low denitrification rates, except when the water was turbid. Since the denitrifiers were obviously particle-associated, experimental bore wells were dug at three sites for sampling of water along with the soil. One of these sites was located in a thickly-populated locality of Amroha, a typical medium-sized town situated in the alluvial plain of northern India; the other two sites were outside the town. At all the three sites, high denitrification activity was measured down to the maximum depth of sampling (~50 m). However, while at the two rural sites denitrification led to pronounced depletion of nitrate (generally < 10 µM), high concentrations (> 300 µM) persisted below 30 m at the urban location. These data indicate that while denitrification can efficiently remove a large part of N added to the groundwater through agricultural activities in rural northern India, such is not the case in some urban areas, presumably due to very high municipal loading.

3. Nitrogen cycling in Lonar Lake

The Lonar Lake is a brackish, alkaline lake (depth < 7 m) located in central India. Under this project, we studied biogeochemistry of this lake for the first time. On six visits to the lake during different seasons from 2008 to 2011, the water was always found to be anoxic (sulphidic) below 1-2 m. Despite the high inputs of nitrate through groundwater and sewage discharge from Lonar town, the nitrate+nitrite concentrations were generally close to the detection limit even in the well-oxygenated surface layer. Incubation experiments carried out on all occasions yielded very high potential denitrification rates in anoxic waters when 15NO3 was used as a substrate, but surprisingly not with 15NO2. However, high DNRA activity was measured in both cases, which is supported by the results of molecular analysis. Due to the high pH of water (~10), a large fraction of ammonia produced from sulphate reduction and DNRA should be in the unprotonated NH3 form, the emission of which should be an important pathway of fixed N loss from the lake.

4. Nitrogen cycling in coastal waters

The world’s largest naturally formed coastal low-oxygen zone develops over the western Indian shelf every summer as a combined consequence of weak upwelling and freshwater run-off from land. It is believed that the oxygen deficiency has intensified in recent years due to enhanced anthropogenic nutrient loading from land. This is manifested by the prevalence of sulphidic conditions in bottom waters over the inner shelf during late summer/early autumn. However, redox N transformation rates have not been previously measured in these waters. Under this project we generated data on denitrification and anammox rates during the period of oxygen depletion for four years (2008-2011). The observations were made covering a wide latitudinal range from off Mumbai to off Calicut, although repeat measurements were made only off Goa (all years), off Karwar (2009-2011) and off Mangalore (2008-2011). Incubation results revealed denitrification rates that are the highest ever reported from any marine system, but these were largely confined to a narrow band close to the coast. Anammox rates were much lower and DNRA, measured on selected samples, was not detectable. The data suggest an active coupling between the N and sulphur cycles. This is being followed up by ongoing molecular analysis.

Potential impact

Conclusions and potential socio-economic/societal relevance of the project

The most important result of the project is that the presence of methane stimulates reactive N loss from Indian freshwater reservoirs controlling eutrophication as well as further methane build-up. In groundwaters of the Indo-Gangetic Plain, high denitrification rates greatly reduce nitrate levels in rural areas, but in some thickly populated urban areas, N loss through denitrification does not seem to keep pace with high municipal loading. These results have high societal relevance because the processes studied determine the quality of drinking water. Unusual N cycling was observed in the Lonar Lake, and an active coupling between redox N and S cycles is revealed by data from the Indian shelf that houses the largest naturally-formed shallow low oxygen zone. As the latter has turned sulphidic in recent years, the results are of considerable biogeochemical, climatic and socio-economic significance.