Objective Nematodes are a key functional group in soil ecosystems, and their community structure and metabolic activities can indicate soil ecological stability and nutrient cycling status. They are important biological indicators for assessing the ecological effects of agricultural management practices. This study aims to elucidate the effects of biochar application under fertilization conditions on the community composition, ecological functions, and metabolic characteristics of soil nematodes in greenhouse vegetable fields in the North China Fluvisol region. It also explores the potential ecological pathways affecting crop yield.

Methods In the core greenhouse vegetable production area of the Beijing-Tianjin metropolitan circle, six treatments were established: conventional fertilization (N2), conventional fertilization with biochar (N2B), reduced fertilization (N1), reduced fertilization with biochar (N1B), biochar-only application (B), and a blank control (CK). Biochar was applied at a rate of 3.75 t/hm² and incorporated into the soil by rotary tillage along with organic fertilizer. After two years of application, soil nematodes were separated and identified. The Shannon diversity index (H'), structure index (SI), and metabolic footprint (BFMF) of nematodes were analyzed. These indicators were combined with soil physicochemical properties and structural equation modeling (SEM) to explore the coupling relationship between ecological functions and crop yield.

Results 1) Compared with the control, fertilization and biochar application treatments (N1, N2, N1B, N2B) significantly improved the soil structure of greenhouse vegetable fields, increased soil organic matter content and nitrogen retention capacity. Although they significantly reduced the total abundance of nematodes, they increased the taxonomic richness (number of genera). 2) The N1B treatment significantly increased the structure index of nematode communities and reduced community dominance, indicating a more even distribution of species within the community. 3) Metabolic analysis showed that the N2B treatment significantly enhanced the metabolic footprint (BFMF) of bacterivorous nematodes, indicating an increase in the efficiency of carbon flow dominated by the bacterial trophic pathway. Structure-enrichment quadrant analysis further revealed that biochar application reduced ecosystem disturbance, enhanced community energy enrichment, and improved structural stability, highlighting its pivotal role in regulating the microbial-nematode interaction network. 4) SEM analysis revealed that soil organic carbon (SOC) had a significant positive effect on yield, while pH showed a negative effect. The N1B treatment simultaneously enhanced ecological functions and achieved dual optimization of ecosystem functioning and crop productivity.

Conclusions Conventional fertilization combined with biochar enhanced bacterivore-driven carbon energy flow and increased community dominance, which was unfavorable for ecosystem stability. In contrast, reduced fertilization combined with biochar application improved soil structure and nutrient enrichment, decreased dominance, increased H' and the proportion of high c-p groups, and consequently enhanced the maturity index (MI) and structural stability (SI) of the nematode community. Overall, integrating fertilization reduction with biochar amendment is an effective approach to strengthen soil ecological stability and support sustainable greenhouse vegetable production in the Fluvisol region of North China.