Vegetation and Soil Aggregates Shape Nematode Communities and Energy Flow on the Loess Plateau

Although soil nematodes are central to belowground energy flow, how vegetation and soil aggregate characteristics interactively regulate the nematode community structure and energy dynamics remains poorly understood. We investigated 80 soil samples from five vegetation types—Prunus armeniaca L. (AV), Pinus tabuliformis Carrière (PT), Caragana korshinskii (CK), Medicago sativa L. (MS), and native grass Stipa bungeana (SB)—and four aggregate sizes (LMA > 2 mm, MMA 0.25–2 mm, SMA 0.053–0.25 mm, and MA < 0.053 mm) on the Loess Plateau. Vegetation types showed clear functional differentiation, in which AV dominated bacterivore diversity and energy flux in LMA, CK enhanced fungivore and herbivore energy flow, SB supported omnivore–carnivore energy flux, and PT exhibited suppressed communities. Fauna analysis of the EI (enrichment index)–SI (structural index) plot revealed aggregate-dependent food web structuring, where all vegetation types clustered in quadrant C (structured, low enrichment) in small aggregates, while PT and MS shifted to quadrant D (structured, enriched) in larger aggregates. SEM showed that energy flux and energy uniformity are driven by nematode abundance (p < 0.01) and diversity (p < 0.01), respectively, with soil aggregates promoting uniformity (p < 0.05) but suppressing total flux (p < 0.05), thus revealing a trade-off between energy throughput and distribution equity. CK maximizes total energy flux, while AV maintains high energy uniformity; as such, they could be keystone restoration species in the study area. This study provides mechanistic insights into soil food web energetics and offers an empirical foundation for optimizing vegetation restoration strategies on the Loess Plateau.