Genotypic Variation in Root System Architecture Is Associated with Total Plant Nitrogen Accumulation in Peanut (Arachis hypogaea L.) Under Low-Nitrogen Conditions

Root system architecture can influence nitrogen (N) acquisition under low N, but genotypic variation in peanut remains poorly resolved. We evaluated 12 peanut (Arachis hypogaea L.) genotypes under a common low-N regime (0.15 mmol L−1 nitrate-N) using independent experimental modules spanning seedling, flowering–pegging, and late reproductive stages. Root morphology, TTC-reduction-based root metabolic activity, spatial biomass allocation, total plant N accumulation, nodulation, reproductive traits, and rhizosphere N-cycling enzyme activities were quantified. Yuhua 37 (YH37) showed concordant favorable performance across independent assays, ranking among the highest genotypes for N accumulation and combining long roots, high root biomass, a high root-to-shoot ratio, and high root metabolic activity. Across genotypes, seedling total N accumulation was positively correlated with total root length (Spearman’s ρ = 0.71, p = 0.0128) and below-9 cm root allocation (ρ = 0.68, p = 0.017); both remained significant after Benjamini–Hochberg adjustment (q = 0.034). Root dry weight and TTC-reduction-based root metabolic activity were positively but not significantly associated with total N accumulation. In the independent reproductive-stage assay, YH37 also showed high nodule, peg, and pod numbers and high rhizosphere urease and alkaline protease activities. Because mineral-derived and biologically fixed N were not partitioned, total plant N accumulation should not be interpreted as nitrate uptake alone. These results identify total root length and relative vertical root deployment as candidate phenotypic axes associated with total plant N accumulation under the tested low-N condition, while validation across N levels and field environments is still required.