Fertile island effects across soil depths in drylands of the Tibetan Plateau

Background and aims: In dryland regions, patches of vegetation play a crucial role in sequestering carbon and vital nutrients, creating what have been described as ‘fertile islands’ within the surface soil beneath their canopies. Nevertheless, a persistent question remains regarding the variability of fertile island effect in relation to soil depth. Methods: To investigate this, we sampled soils at two different depths (0–30 cm and 30–50 cm) within perennial vegetation patches and open areas across 54 drylands scattered across the Tibetan Plateau. Our study focused on six key soil variables related to carbon, nitrogen, and phosphorus stocks, allowing us to assess the differences in fertile island effects at varying depths. Results: Among the soil fertility parameters evaluated, organic carbon exhibited the most pronounced fertile island effect at both soil depths. The fertile island effect responded differently to environmental factors at the two depths assessed. In the surface soil layer, the impact of vegetation on soil carbon content intensified with increasing precipitation seasonality and precipitation in the wettest quarter but declined with increasing temperature during the driest quarter. Conversely, in the deeper soil layer, the fertile island effect for nitrogen and phosphorus intensified with greater soil sand content and vegetation greenness. Conclusions: The fertile island effect can extend beyond the surface soil, while different environmental factors regulate the size of this effect with depth. These findings offer fresh and valuable insights into the relationships between biotic and abiotic factors and fertile island effects, a key feature of dryland ecosystems worldwide.