Nitrogen (N) deposition is a serious environmental issue for soil fertility and human wellbeing. Studies on various terrestrial ecosystems yielded fragmented information on soil-N status (microbial biomassand mineral-N) and dynamics (N-mineralization and-leaching) whereas the holistic view on this issue is relatively unknown. A complete understanding of soil-N status and dynamics in response to N deposition is essential for sustainable management of ecosystem structure and function as needed for human wellbeing. Therefore, we conducted an experiment in the N-limited tropical grassland to explore the question whether N-deposition weakens the soil-N status and dynamics; if yes, then what could be the optimum amount of deposited N and the related controlling mechanism? We undertook a 3-year (2013-2016) experimental N fertilization (control, 30, 60, 90, 120, and 150 kg N ha −1 year −1) study (using urea as a source of N deposition). The data from a total of 72, 1 × 1 m plots (six treatments with 12 replicates) were collected and properly analysed with statistical software. N deposition caused significant differences in the parameters of soil-N status and dynamics. The responses of microbial biomass-N, N-mineralization, and mineral-N to the N deposition were quadratic (maximum values were in N 90) whereas N-leaching showed a linear response. Compared to control, N deposition (30-150 kg N) consistently enhanced (29-96%) leaching of N. As a mechanism, acidification induced aluminium toxicity, carbon to nitrogen ratio and litter decomposition governed the soil-N status and dynamics. N deposition over and above 90 kg ha −1 year −1 resulted in a negative feedback to soil N transformation and availability. Hence, N deposition below 90 kg ha −1 year −1 could be a limit for the sustainable functioning of the tropical or similar grasslands. Globally, N deposition has been identified as a major threat to the functioning of the sensitive ecosystems 1,2. Fossil fuel combustion, biomass burning, changes in land use pattern and use of N-fertilizer have been identified as major contributors of atmospheric-N depositions 1,2 .These N deposition sources doubled the global N cycle over the last century 3. Studies have suggested that in 1,860 the reactive-N deposition for terrestrial ecosystems was 15.88 Tg year −1 and at the beginning of the 1990s it was 63.5 Tg year −1 which is four times higher than that in 1860 4. According to Galloway et al. 5 and Zhou et al. 6 , recently, the global ecosystems are receiving a very high rate of N deposition, often > 100 kg N ha −1 year −1 and it may reach up to 125.2 Tg year −1 by 2050 4,7. The deposition is expected to increase by a factor of 2.5 over the next century 8. In the Asian region, the predicted reactive-N deposition by 2030 would be more than 1.5 times higher (from 67.7 to 105.3 Tg year −1) than that of 2000 9 and by 2020 it is likely to exceed the combined emissions of North America and Europe 10. On the other hand, the global N fertilizer uses in 1960 and 2000 were 3.5 and 87 million m...
