Topsoil Carbon Response to Aspect under Neltuma juliflora Afforestation: Implications for Carbon Sequestration in Semi-Arid Ecosystems

This study investigated the influence of slope aspect on soil and aboveground biomass carbon storage in Neltuma juliflora (family Fabaceae) plantations located in the Chah-Mari region near Behbahan County, southwestern Iran, an area characterized by semi-arid climate and rolling hills. Slope aspects along the four cardinal directions—north, south, east, and west—were selected, with four plots (30 × 30 m) established in each aspect. Soil and tree measurements were conducted within these plots to quantify variations in carbon accumulation. Results revealed that slope aspect had a significant effect on both soil and biomass carbon stocks. The western slope exhibited the highest soil carbon storage (4.5 Mg ha–1), approximately 56% greater than the southern slope, while the north-facing slope supported the highest biomass carbon (297.9 kg ha–1). Enhanced soil carbon on the northern and western slopes was associated with higher soil nitrogen, and phosphorus contents, as well as greater clay proportion, which contributed to improved carbon stabilization. These findings suggest that microclimatic conditions, particularly reduced solar radiation and higher soil moisture on northern and western slopes, promote carbon accumulation by fostering vegetation growth and organic matter inputs. Overall, slope aspect acts as a critical environmental factor governing carbon sequestration efficiency in arid and semi-arid regions. Prioritizing north- and west-facing slopes for afforestation and restoration can thus enhance ecosystem carbon storage potential and support climate change mitigation efforts in dryland environments.