Impact of nitrogen deposition and critical load exceedances on India’s ecologically sensitive regions

Human-induced nitrogen (N) deposition poses a serious threat to biodiversity in ecologically sensitive protected areas. India, a major hotspot of reactive nitrogen (Nr) emissions, has so far lacked high-resolution assessments of nitrogen deposition impacts over its national parks (NPs). This study presents the first high-resolution modelling of total nitrogen deposition over India using an updated ammonia (NH3) emission inventory, which remains subject to substantial uncertainty. Ecological risks were evaluated under a baseline scenario (BS-2015) and a future emission scenario (FES-2030) by comparing simulated deposition with critical load (CL) thresholds prescribed for ecosystems in the United States (USA), China, and Europe. Under BS-2015, 6, 12 and 18 NPs already exceeded China, Europe and USA CL values, respectively, whereas under FES-2030, the number of NPs CL exceedances doubled, with 25 NPs exceeding USA CLs and 12 exceeding European CLs. The exceedances of Chinese CLs were less frequent, but a relative increase in the total N deposition ranging from 5 to 124% were projected across the various ecosystems. The findings indicate increasing N stress through 2030 and highlight the need for long-term field studies to establish region-specific critical load thresholds for India.