Early Trophic Reorganization of the Rhizosphere Food Web Differs Between Two Cultivars of Rehmannia glutinosa

Continuous cropping can cause soil degradation and replant problems, yet it remains unclear whether soil food-web differentiation begins during the first cropping season. In this study, we investigated temporal changes in nematode community composition and food-web functioning during first-cropping Rehmannia glutinosa. Genus-level nematode composition, trophic structure, ecological indices, and metabolic footprints were assessed across different growth stages, cultivars, and soil compartments. Soil physicochemical properties, metabolite profiles, and bacterial and fungal communities were further integrated to examine their associations with nematode community variation and functional allocation. Nematode assemblages changed markedly over the growing season, with sampling time representing the dominant source of community variation. Bacterivores formed the major trophic background, whereas plant-parasitic and fungivorous nematodes showed cultivar- and stage-dependent increases. Soils associated with the Dunwang (DW) cultivar, particularly the rhizosphere, exhibited a persistent shift toward the plant-feeding metabolic channel. In several treatments, plant-parasitic nematodes contributed disproportionately to the total metabolic footprint despite representing a much smaller proportion of total abundance, indicating clear abundance–footprint decoupling. Integrated analyses showed that nematode community composition was broadly associated with soil physicochemical conditions and bacterial, fungal, and metabolite profiles, whereas metabolic-footprint allocation was more closely related to microbial community composition, especially fungi. These findings demonstrate that nematode community and food-web differentiation can emerge within the first cropping season, before continuous-cropping obstacles become fully established. Metabolic footprints provide functional information that is not captured by abundance alone and may serve as sensitive indicators of early belowground ecological risk in R. glutinosa cultivation.