Organic carbon stocks in aboveground biomass and soils in hyper-arid AlUla County, Saudi Arabia

Dryland ecosystems, which cover nearly half of the Earth's terrestrial surface, play a considerable role in global carbon dynamics yet remain underrepresented in carbon stock assessments. This study evaluates organic carbon stocks in six protected areas within the hyper-arid AlUla County, Saudi Arabia, focusing on aboveground biomass (AGB) of herbaceous plants, trees and shrubs, as well as soil organic carbon (SOC). Across six protected areas, 172 plots were sampled using species- and growth form-specific allometric equations and soil cores (to 30 cm depth) to estimate organic carbon stocks for eight distinct habitat types. Mean total organic carbon (TOC) stocks ranged from 2.054 ± 0.379 t.ha−1 in basaltic rock or ‘harrats’ habitat, to 12.831 ± 1.921 t.ha−1 in abandoned agricultural lands. SOC accounted for more than 95% of average TOC stocks across all habitat types, except in arid thorn woodlands where SOC contributed 53.71% to the TOC stocks. Arid thorn woodlands also had the highest AGB carbon stocks (1.755 ± 0.564 t.ha⁻1), with trees comprising 54.61% of the AGB carbon pool. Organic carbon stocks in hyper-arid AlUla are predominantly soil-based, while AGB contributes little to the TOC stocks except in habitats with persistent woody vegetation. These patterns align with the lower end of reported ranges for other hyper-arid systems and establish an empirical foundation for future research on carbon storage in hyper-arid ecosystems of the Arabian Peninsula.