Soil Nematode Community Composition and Energy Structure in the Root Zones of Woody Plants in the Ili River Valley: A Comparison Between Near-Pure-Species Trees and Mixed Shrub Communities

As a typical mountain ecosystem in the western Tianshan Mountains, the Ili River Valley possesses abundant vegetation resources. Soil nematodes are effective biological indicators for evaluating soil micro-food webs. Nevertheless, the response mechanisms of nematode community structure to distinct vegetation types, especially native trees and forest-edge shrubs, remain poorly understood in this region. In this study, two dominant tree species (Picea schrenkiana and Malus sieversii) and two forest-edge shrub species (Berberis heteropoda and Berberis sibirica) were investigated. We analyzed the composition, diversity, and energy structure of rhizosphere soil nematodes and further compared their differences among plant species. The results indicated that tree rhizospheres had significantly higher amounts of nitrate nitrogen (NO3−-N and microbial biomass carbon (MBC), along with a lower amount of extractable organic carbon/extractable total nitrogen (EOC:ETN) than shrub rhizospheres (p < 0.05). Picea schrenkiana (PS) exhibited greater root carbon storage, higher root biomass, and a higher root carbon-to-nitrogen ratio (RC:RN) than Berberis heteropoda (BH) and Berberis sibirica (BS) (p < 0.05). The genus Chiloplacus dominated the nematode community across all four woody plants. The relative abundance of omnivore-predatory nematodes was markedly higher in shrubs (BH and BS) than in trees (PS and MS). The soil food webs of PS and MS were degraded, whereas shrub food webs were in a transitional state between structured and degraded habitats. Shrubs presented a higher maturity index, structural metabolic footprint, and energy flux of omnivore-predatory nematodes, but a lower energy flux of bacterivorous nematodes. Additionally, PS had the highest nematode carbon use efficiency (NCUE) and the lowest energy flux uniformity (U). NO3−-N extractable total nitrogen (ETN), soil organic carbon (SOC), and root traits were the primary factors driving variations in nematode communities and carbon indicators. Therefore, nematode carbon indicators closely associated with soil carbon and nitrogen cycling have the potential to serve as sensitive auxiliary biological metrics for evaluating material cycling and energy flow in pure forests and shrub ecosystems. This study provides empirical support for the assessment of regional ecosystem stability.