Sugar beet residue outperforms foliar nano-MoO₃ and proline under deficit irrigation in a soybean–faba bean rotation on calcareous soil

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Abstract Aims Combined deficit-irrigation and amendment strategies represent an emerging avenue for stabilizing legume production on calcareous soils. We evaluated whether sugar beet residue, molybdenum oxide nanoparticles (n-MoO₃), and exogenous L-proline can simultaneously sustain yield, reinforce stress physiology, and improve profitability under reduced irrigation in a soybean–faba bean rotation on newly reclaimed Egyptian land. Methods Two irrigation regimes (75% and 100% of crop evapotranspiration, ETc) were combined with seven treatments: nontreated control, sugar beet residue at 12 and 24 t ha⁻ 1 (SB₁₂, SB₂₄), foliar n-MoO₃ at 200 and 400 mg L⁻ 1 (N-Mo₂₀₀, N-Mo₄₀₀), and foliar L-proline at 200 and 400 mg L⁻ 1 (P₂₀₀, P₄₀₀). Nutrient status, osmolyte and antioxidant accumulation, yield, water productivity, and economic return were assessed. Results SB₂₄ delivered the highest leaf and seed N, P, and K in both crops, while N-Mo₄₀₀ maximized leaf and seed Mo, raising soybean leaf Mo by ~ 89% above the deficit control. Leaf proline peaked under P₄₀₀ at 75% ETc; ascorbic acid declined under more effective treatments, indicating stress relief. The yield ranking SB₂₄ > SB₁₂ > N-Mo₄₀₀ > P₄₀₀ > N-Mo₂₀₀ > P₂₀₀ > Control was consistent across both crops. SB₂₄ at 100% ETc produced the highest seed yield and soybean oil content (21.45%), whereas SB₂₄ at 75% ETc achieved the highest water productivity (0.56 and 0.32 kg m⁻ 3 ). Conclusions SB₂₄ at 100% ETc was the most profitable option (BCR = 3.53 in faba bean; 2.32 in soybean), while SB₂₄ at 75% ETc is recommended as the most water-efficient strategy for newly reclaimed calcareous soils.

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