Scattered wild pistachio trees profoundly modify soil quality in semi-arid woodlands

Abstract

Scattered wild pistachio trees (Pistacia atlantica Dest.) support various ecological functions and services, including soil conservation, in semi-arid woodlands of the Irano-Turanian zone, but over-exploitation puts the populations at risk. The current paucity of information precludes a thorough assessment of the effect of wild pistachio trees on soil quality. Therefore, we quantified their role in soil quality of semi-arid woodlands dominated by a Pistachio-Amygdalus community. At three sites in the southern Irano-Turanian zone, we contrasted moisture, pH, electrical conductivity (EC), organic carbon (SOC), total nitrogen (TN) and nutrient concentrations of bare soil with below-crown soil. In addition, we examined soil microbial respiration (Rmic), substrate-induced respiration (SIR), microbial biomass carbon (Cmic) and nitrification potential to evaluate the influence of pistachio trees on soil microbial activity. Soil pH and C/N ratio were similar between bare and below-crown soils but significantly greater soil moisture (+41 %), EC (+13 %), SOC (+82 %) and TN (+75 %) were observed below pistachio crowns. Most soil nutrients were more abundant beneath tree crowns, especially zinc and manganese. Soil microbial activity was greater below crowns compared to bare soil as evidenced by significantly elevated Rmic (+11 %), SIR (+86 %), Cmic (+61 %) and nitrification potential (+49 %). SOC-normalization of microbial variables revealed that a SOC-driven increase in Cmic was largely responsible for the enhanced soil microbial activity below crowns. Microbial communities in bare soil showed a higher metabolic quotient (Rmic/Cmic) than in below-crown soil, suggesting low microbial efficiency and higher metabolic costs. Our findings indicate that the soils below wild pistachio crowns represent fertility islands, where soil quality and microbial activity are supported by an increased supply of organic matter and more favourable soil moisture conditions. The few woodlands that have remained relatively intact in this area urgently require conservation action while restoration planting of wild pistachio trees in degraded areas may help restore soil quality.