The Indian Subcontinent occupies <3% of the total land area of the world; however, as much as 22% of the world’s human population lives here, in rough proportion to which the region accounts for ~19% (17 million tonnes of N annually) of the global synthetic N fertilizer consumption. Fossil fuel combustion is the other major source of new N introduced to the environment. The fate of the enormous N loading, which has increased by a factor of 50 over the past 4 decades, is largely unknown. Less than 5% of the anthropogenic N appears to reach the sea by river runoff; the rest presumably accumulates in the terrestrial aquatic systems where an unknown fraction may be removed as N2 or N2O through redox transformations, especially in anaerobic environments of the subsurface aquifers and hyperlimnions of stratified reservoirs and lakes. Given the serious health hazards of high nitrate levels in drinking water and the high greenhouse potential of N2O, both nitrate accumulation in natural waters and conversion of fixed N to N2O are of immense socio-economic significance. The proposed study will, for the first time, investigate N cycle processes in groundwaters and lakes/reservoirs in India. Three man-made reservoirs and one natural lake, and groundwater aquifers of three different types will be seasonally sampled for this purpose. The four major components of the project will be: (a) to study the spatial and temporal variations of various dissolved N species (organic N, nitrate, nitrite, ammonium, N2O and N2) in relation to organic matter loading and ambient dissolved oxygen levels; (b) to determine rates of redox transformations (denitrification and anaerobic ammonium oxidation) and assess their relative importance in N2 production; (c) to identify sources of N and understand mechanisms of its transformations through natural N and O isotope abundance measurements; and (d) to characterize through molecular analyses the microbial community involved in redox transformations.