A least-input nitrogen fertilizer reduction strategy sustains peanut yield with lower nitrogen inputs.

INTRODUCTION: Synthetic nitrogen (N) fertilizers underpin global food security but drive escalating environmental costs, including greenhouse gas (GHG) emissions and soil degradation. Conventional N-reduction strategies face a fundamental trade-off: lowering fertilizer inputs typically compromises crop productivity and farmer income. Nanotechnology offers a pathway to decouple yield from N dependency, yet scalable, low-cost solutions remain elusive. OBJECTIVES: This study aimed to develop a least-input foliar fertilization strategy using bio-derived nanocarbon (BNC) that enables a 30% reduction in synthetic N input in peanut (Arachis hypogaea L.) without sacrificing crop yield, nutritional quality, or economic returns, while elucidating the underlying physiological and microbiological mechanisms. METHODS: Field trials were conducted with peanut (cv. Huayu 22) over two consecutive growing seasons (2024-2025) in Laixi, Shandong, China, under four treatments: N (conventional N, 100% urea), N-30 (reduced N, 70% urea), N + BNC (100% urea + foliar BNC), and N-30 + BNC (70% urea + 18 g ha-1 foliar BNC). Photosynthetic parameters, oxidative stress markers, and 15N isotope tracing were assessed. Transcriptomic, metabolomic, and 16S rRNA microbiome analyses characterized systemic signaling and rhizosphere responses. GHG emissions were modeled using the DNDC framework, and economic benefits were calculated based on yield and input costs. RESULTS: BNC application under N-30% maintained yield parity with the N control by sustaining photosynthetic electron transport and reducing reactive oxygen species (ROS). This enhanced carbon status upregulated the transcription factor HY5, activating nitrate transporter AhNRT1.2 and ammonium transporter AhAMT1.1 and increasing 15N uptake. Changes in root-exudate composition were accompanied by shifts in the rhizosphere bacterial community, including higher relative abundance of taxa associated with nitrification and nutrient turnover (Nitrospira and Gemmatimonas), and coincided with 21% and 14% higher root NO3--N and NH4+-N contents, respectively. Nitrogen use efficiency (NUE) improved by 16.0%, total GHG emissions decreased by 34.8%, and net profit increased by 27.0%. CONCLUSION: A micro-dosage of upcycled BNC (∼$0.02 ha-1) orchestrates HY5-mediated shoot-to-root signaling and rhizosphere microbiome restructuring to sustain crop productivity under reduced N inputs. This commercially viable, low-input framework offers a scalable route for climate-smart agriculture that reconciles food security with environmental sustainability.