Assessment of soil organic carbon stock in pistachio orchards established under semi-arid climates of Siirt province, Türkiye

The management of agricultural lands plays a crucial role in the global carbon (C) cycle. Intensive soil tillage is one of the primary factors contributing to the mineralization of soil organic carbon (SOC), leading to its release into the atmosphere as CO₂. This study determined the soil organic carbon stock (SOCS) in pistachio-growing areas, which have expanded significantly in recent years in Kurtalan district, Siirt province. Spatial variation of SOCS was also mapped within the study area to ensure its long-term monitoring over time. For this purpose, soil samples were collected from a depth of 0–20 cm in 72 pistachio orchards. The soil organic C (SOC) content and bulk density (Bd) of the collected soil samples were determined, and SOCS values were calculated to generate spatial distribution maps. The SOC content ranged from 2.85 g kg⁻¹ to 26 g kg⁻¹, while SOCS varied between 8.97 t ha⁻¹ and 73.74 t ha⁻¹. Significant variability in SOC and SOCS was observed among soil samples belonging to different texture groups. Additionally, SOC and SOCS values significantly decreased as orchard age increased. The SOC and SOCS values decreased by 20.3% and 22.7%, respectively in pistachio orchards older than 20 years compared 5 years old or younger orchards. This decline in SOC content was also reflected in the SOC:Clay ratio, a key indicator of soil structure stability, which dropped from an average of 0.32 in younger orchards to 0.20 in older orchards. This suggests that agricultural practices in pistachio orchards disrupt soil aggregation, accelerating organic C mineralization and reducing the soil’s ability to retain organic matter over time. These results show the urgent need for sustainable soil management strategies to preserve soil integrity, enhance C retention, and support long-term agricultural sustainability in semiarid pistachio cultivation areas.