Oasis management and topography interactively shape soil inorganic carbon dynamics in hyper-arid soils.

Inorganic carbon (C) comprises a large fraction of the total C in arid and hyper-arid soils globally and therefore significantly contribute to terrestrial C sequestration. Soil inorganic carbon (SIC) derives from geological sources or from pedogenic carbonates formed by coupled biological–geochemical processes. Yet the extent to which oasis management influences and eventually reduces SIC through acidifying effects of N fertilisation and biological respiration remains poorly understood, despite the central role of oases for the agriculture and economy of arid and hyper-arid regions. We investigated SIC dynamics in southwestern Tunisia, sampling soils to 120 cm (0-5, 5-10, 10-30, 30-60, 60-90, and 90-120 cm) in three traditional and three modern oasis systems along topographic gradients (upper, midslope, and downslope positions) and beneath versus between date palms. Traditional oases are characterized by long-term organic inputs and high crop plant density and diversity, while modern oases have lower date palm density, plant diversity and greater reliance on synthetic fertilization. On average, SIC accounted for 76 % of soil total C to 120 cm depth, underscoring its role here as a dominant long-term soil C sink. The oasis types and relative tree positions did not differ in SIC contents and stocks (averaged 31.3 kg m⁻²), but uniquely distinct patterns in SIC content emerged from management–topography interactions, gypsum content, and biological activity. Modern oases showed higher SIC upslope due to limited inherent leaching, whereas traditional oases accumulated SIC together with gypsum in saline downslope positions. Carbon isotopes i.e. δ¹³C_SIC​ values (–8 to –5‰) indicated large biological contributions, comprising up to 40% of SIC in traditional oasis systems and 20% in modern oasis systems. Soil organic C (SOC) correlated negatively with δ¹³C_SIC​, pointing to microbial respiration and root-derived CO₂ as primary drivers of pedogenic carbonate formation. These results highlight the dual geochemical–biological origin of SIC and the potential of oasis management to stabilize management related losses of SIC in hyper-arid regions.