Soil properties shape nematode communities more than vegetation in Robinia pseudoacacia plantations on the Loess Plateau

Soil nematodes are often used as indicators of soil ecosystem health due to their large numbers, diverse species, ease of acquisition and identification, and sensitivity to disturbances. Black locust ( Robinia pseudoacacia ) has been widely planted in the Loess Plateau due to its rapid growth and high tolerance to environmental stress. Compared to the above-ground plant communities and soil microorganisms, little research has been conducted on how black locust plantation ecosystems affect soil nematodes. To fill this knowledge gap, we investigated understory vegetation and soil nematode communities in mature black locust plantations across three vegetation zones (steppe, forest-steppe, and forest) in the Yanhe River catchment along environmental gradient. The primary objective was to understand how soil nematode communities respond to the introduction of black locust along environmental gradient, thereby providing insights into the complex interactions within black locust plantation ecosystems. Our results found that: (1) Black locust growth increased understory vegetation richness and enhanced leaf photosynthetic traits (LPT) but indirectly reduced understory aboveground biomass, with limited effects on root functional traits; (2) The steppe zone exhibited superior soil nematode community structure and functionality compared to the forest-steppe and forest zones, with distinct differences in trophic group composition among the three zones; (3) Soil total nitrogen, total phosphorus, and organic carbon were identified as primary determinants of soil nematode community structure and function, whereas the growth of black locust and the structure and functional traits of understory vegetation had no significant influence on nematode communities. Our findings revealed the regulatory mechanism of environmental factors on understory soil nematode communities in black locust plantations, with the development of black locust plantations acting as a mediating variable, regulating the effects of meteorological factors on understory vegetation structure and functional traits, while soil nematode communities were predominantly governed by the direct regulation of climatic factors and soil properties. Furthermore, genus-level taxonomic for nematodes resolution revealed distinct ‘genus fingerprints’ for each vegetation zone and potential biotic constraints within the plantations, thereby highlighting the critical value of detailed taxonomy in ecological assessment.