Nickel (Ni) plays a central role in plant nitrogen (N) metabolism, yet the long-term physiological impact of nanostructured Ni fertilizers on soybean (Glycine max L.) remains insufficiently characterized. This study assessed the effects of Ni seed treatment and foliar application, using macro-, micro-, and nanosized Ni sources, on soybean cv. “IPRO Flecha” N metabolism and biological nitrogen fixation (BNF) throughout the crop life cycle. Ni was applied as seed coating (45 mg Ni kg–1 seed) and/or foliar spray (20 g Ni ha–1 at V4) using NiSO4·6H2O (macro), micrometric Ni(OH)2 (∼24 μm), or nanometric Ni(OH)2 (∼5 nm). Key physiological, enzymatic, and isotopic parameters were measured at V6, R2, and R7 stages, including nitrogenase, urease, and nitrate reductase activities, BNF contribution (δ15N), and N metabolites (N-NH3, N-NO3–, N-ureides). At R2, nano-Ni seed + foliar increased leaf dry weight by 28% compared with seed-only and 49% compared with the control, whereas at R5 the same treatment reduced leaf dry weight by 62% relative to nano-Ni seed-only, indicating a strong stage-dependent response. At maturity (R7), Ni application increased seed yield across sources, with mean yield rising from 12 g pot–1 in the control to 19–22 g pot–1, with the highest yield under nano-Ni (22 g pot–1). Ni supply increased urease and nitrate reductase activities relative to the control; however, nano-Ni reduced nitrate reductase at V6 (≈41–43% lower than macro- and micro-Ni), and nano-Ni seed-only reduced nitrogenase activity at R2 by ∼33% compared with the control. BNF in shoots ranged from 60.5–87.4% at R2 and was generally higher under Ni treatments at R7 (≈85–92%), except for nano-Ni seed + foliar (67.3%). Overall, Ni improved soybean N metabolism and yield, but nano-Ni effects were highly dependent on the growth stage and application method, highlighting the need for mechanistic studies on nanoparticle behavior over time to support consistent agronomic outcomes.
Oliveira et al. (Fri,) studied this question.