Drivers of Grassland Biodiversity: Disentangling the Soil-Biome Interaction Across Physiographic Regions in Nepal’s Protected Areas

Grassland ecosystems within protected areas serve as crucial reservoirs of biodiversity, yet the interplay between their vegetation structure and soil properties across environmental gradients remains inadequately explored. This study examined plant diversity, biomass, and soil characteristics along a physiographic gradient in Nepal, spanning protected areas (PAs) in the Himalayas, mid-hills, and low-altitude Terai regions. The Terai is a lowland belt of flat alluvial plains and marshy grasslands along the Nepal-India border. A total of 288 soil samples were collected using stratified random sampling from April 2024 to February 2025. Pronounced divergence in floristic composition emerged: the Himalayas displayed the highest species richness and highest diversity. Multivariate analyses identified distinct plant communities, with the Terai dominated by lowland grasses and the Himalayas and mid-hills by high-altitude species. Mantel tests demonstrated a strong soil-plant diversity relationship in the Himalayas, a moderate relationship in the mid-hills, and a weak, nonsignificant relationship in the Terai, suggesting a decoupling of biogeochemical and diversity dynamics in the latter. Soil organic carbon stocks peaked in the mid-hills and generalized additive models identified soil bulk density as the primary predictor of carbon variation. These results reveal that ecological drivers of grassland structure and functions are not uniform but shift substantially with elevation, underscoring the need for region-specific conservation strategies tailored to localized drivers within PAs.