Abstract
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 biomass- and 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 N90) 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.
Highlights
Nitrogen (N) deposition is a serious environmental issue for soil fertility and human wellbeing
If the hypothesis is supported, what could be the optimum amount of deposited N and what could be the controlling mechanism for soil-N status and dynamics for sustainability of tropical grasslands
Effect size analysis indicated a consistent decline of soil-pH due to increasing amount of N deposition (Fig. 2)
Summary
Nitrogen (N) deposition is a serious environmental issue for soil fertility and human wellbeing. To feed the hungry world (through massive food and agriculture production) has been identified as a major reason for tremendous hike in the global N fertilizer use and emission These studies reflected synchronization of atmospheric-N deposition and N fertilization[1,2,11,12,13] (N fertilization is referred to as N deposition). Studies from various terrestrial ecosystems showed disturbances in normal soil-N status ( NH4+-N, NO3−-N, total mineral-N, microbial biomass nitrogen; MBN) and dynamics The feedbacks of MBN to the atmospheric-N depositions from diverse ecosystems need urgent study for predicting its impact on the soil health and global N cycling[33]
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