Changes of soil carbon along a topo-climatic gradient in rangelands of Iran: insights from 14C mean residence time and δ13C

Abstract

Context Soils can be the largest terrestrial carbon source and a potential sink of atmospheric CO2. Soil organic carbon (SOC) dynamics can be unravelled by 14C-derived mean residence times (MRT). Aims We aimed to understand SOC dynamics in surface and subsurface soils along a topo-climatic gradient in the rangelands of Khuzestan Province, Iran. Methods Study sites were selected under two contrasting regional climates in Izeh (MAT + 19.2°C, MAP 623 mm) and Ramhormoz (MAT + 27.5°C, MAP 200 mm). Soil physicochemical properties, SOC forms, and 14C MRT and δ13C signatures were determined in the control profiles. Key results The average MRT up to 1 m depth in Izeh and Ramhormoz were 2980 and 6582 years before present, respectively. On average, a loss of 300 Mg C ha−1 in SOC stocks and a rise of 430 years in SOC MRT up to 1 m can be expected per 1°C increase in MAT, 50 mm reduction in MAP, and 100 m decrease in elevation, highlighting the potential significance of MAT in SOC dynamics. Using optimistic and pessimistic carbon emission scenarios, carbon emissions in the upland areas were projected to be between 50 and 100 Mg C ha−1 over 80 years. Conclusions While the most influential element on SOC stock and its relative age was likely the temperature, other factors like erosion and deposition processes can cause enhanced SOC dislocation along the topo-climatic gradient. Implications Soil carbon pools stabilised for centuries to millennia are susceptible to alterations due to climate and land cover change.