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

BACKGROUND: 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). RESULTS: 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. CONCLUSIONS: 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.