Nitrogen Deposition Effects on Plant Diversity and Ecosystem Carbon Balance in Montane Grasslands of Obudu Plateau, Cross River State

Nitrogen deposition from agricultural emissions and biomass burning is increasing across sub-Saharan Africa, with potential consequences for plant diversity and carbon cycling in montane grassland ecosystems. The Obudu Plateau montane grasslands of Cross River State represent a biodiversity hotspot with endemic plant communities that have never been assessed for nitrogen deposition sensitivity. This study combines field gradient analysis along a measured nitrogen deposition gradient, manipulative nitrogen addition experiments, and biogeochemical process measurements to elucidate nitrogen deposition effects on plant diversity and ecosystem carbon balance. Thirty study plots were established across the nitrogen deposition gradient defined by atmospheric passive sampler measurements. Experimental plots received six nitrogen addition levels from 0 to 80 kilograms of nitrogen per hectare per year over three years. Plant species richness, functional diversity, and above-ground biomass were monitored annually. Soil carbon dioxide and methane fluxes, net nitrogen mineralisation, and soil carbon fractions were measured seasonally. Results reveal a significant negative relationship between nitrogen deposition and plant species richness along the field gradient, with a 35 percent reduction in species richness across the deposition range. Experimental additions suppressed forb diversity most strongly, consistent with competitive exclusion by nitrophilous grasses. Carbon dioxide flux increased with nitrogen addition, driven by enhanced microbial decomposition of labile soil carbon, while methane uptake declined, shifting the net greenhouse gas balance of these grasslands adversely. The study documents, for the first time, nitrogen deposition as an emerging threat to Obudu Plateau montane grassland biodiversity and recommends emissions management strategies to limit further deposition increases. Keywords: nitrogen deposition, montane grassland, plant diversity, carbon balance, Obudu Plateau.