Mineral N and Rhizobium Management in Shisham (Dalbergia sissoo)–Jack Bean (Canavalia ensiformis) Agroforestry Under Semi-Arid Conditions: Effects on Tree–Crop Performance, N Uptake and Soil N Status

Nitrogen management is an important component of legume-based agroforestry, particularly where mineral nitrogen inputs and biological nitrogen acquisition interact. The present study evaluated contrasting mineral nitrogen and Rhizobium management treatments in a shisham (Dalbergia sissoo Roxb.)–jack bean (Canavalia ensiformis (L.) DC.) agroforestry system under the semi-arid conditions of Bundelkhand, India. The experiment was conducted during 2022–2023 and 2023–2024 at the Forestry Research Farm, Rani Lakshmi Bai Central Agricultural University (RLBCAU), Jhansi, India, using six treatments in a randomized complete block design with four replications. The present study evaluated the effects of contrasting mineral N levels and Rhizobium inoculation on jack bean growth, yield, nodulation and N uptake, together with shisham growth and soil N status, under a semi-arid agroforestry system. The treatments comprised sole jack bean with 0 and 100% recommended nitrogen, shisham–jack bean with 0, 50 and 100% recommended nitrogen, and Rhizobium-inoculated shisham–jack bean without mineral nitrogen. Nitrogen management had a considerable impact on the growth, production, nodulation, and nitrogen concentration of jack beans. The sole jack bean receiving 100% of the recommended N (T2) exhibited the significantly highest plant height (108.75 cm), number of pods per plant (11.63), seeds per pod (7.70), and seed yield (899 kg ha−1), whereas the shisham–jack bean system with 100% recommended nitrogen (T5) yielded a similar seed output of 876 kg ha−1. Rhizobium-inoculated shisham–jack bean without mineral nitrogen (T6) showed the most significant nodulation (37.35 nodules per plant), nodule dry weight (303.25 mg plant−1), seed nitrogen concentration (2.98%), soil NH4+-N (10.15 mg kg−1) and NO3−-N (6.68 mg kg−1). Total nitrogen concentration by plants was significantly highest in T5 (53.63 kg N ha−1) and was statistically at par with T2 and T6. The growth and biomass accumulation of shisham were similarly maximized in T5, which achieved a tree height of 5.31 m, a diameter at breast height of 8.99 cm, and a total biomass of 9.60 Mg ha−1. The findings indicate that suitable mineral nitrogen supplementation augmented productivity in the shisham–jack bean system, but Rhizobium inoculation enhanced nodulation and was associated with greater plant N concentration and soil mineral N under zero mineral-N conditions.