Mitigating nutrient stress in faba bean in the Ethiopian Highlands: Improving biomass yield, carbon assimilation, and water‐use efficiency through Rhizobium and phosphorus management

Abstract Nutrient deficiency limits faba bean ( Vicia faba L.) biomass yield, carbon (C) assimilation, and intrinsic water‐use efficiency in highland agroecosystems. Carbon isotope composition (δ 1 3 C) is indicative of plant physiological responses. This study investigated the effects of phosphorus (P) fertilizer, Rhizobium inoculation, and variety on shoot biomass, carbon–nitrogen (C–N) dynamics, and δ 1 3 C of Vicia faba . A field experiment was conducted at two sites using three varieties (Dosha, Gebelcho, and Numan), four P application rates (0, 10, 20, and 30 kg ha −1 ), and Rhizobium inoculation (with or without the strain EAL‐110) in a split‐split plot design with three replications. Shoot biomass was significantly affected by variety and P application, ranging from 1.78 g plant −1 (control) to 3.86–3.89 g plant −1 at 20–30 kg P ha −1 , with a maximum of 4.6 g plant −1 under combined inoculation and 30 kg P ha −1 . Carbon concentration increased from 41.71% to >43.4% with P application. Nitrogen concentration increased from 3.75% (uninoculated) to 4.53% (inoculated). The Rhizobium × P interaction doubled C content from 0.75 to 1.78 g plant −1 , reaching 2.01 g plant −1 at high P with inoculation. The C:N ratio decreased from 11.55 to 9.76 with inoculation. Carbon isotope composition values were more negative at the Gorche site (ranging from −29.49‰ to −29.68‰) than at the Hagere Selam site (−28.59‰ to −28.98‰) due to better moisture availability. Phosphorus application increased δ 1 3 C by 0.6‰–1.0‰. Strong correlations existed between C and biomass ( r = 0.994), and between N and C:N ( r = −0.96). Phosphorus and Rhizobium critically controlled C assimilation, biomass yield, and δ 1 3 C, with inoculation promoting N fixation and balancing C:N. Integrated Rhizobium and P management effectively improves plant physiological attributes and yield.