Biological fixation, transfer and balance of nitrogen in passion fruit (Passiflora edulis Sims) orchard intercropped with different green manure crops

Abstract

Green manures can replace or supplement mineral fertilization and add organic matter to the soils, ensuring greater sustainability to fruit growing in semiarid regions. Biological fixation, transfer and balance of nitrogen were determined on an irrigated yellow passion fruit orchard (Passiflora edulis Sims) intercropped separately with three cover crops: sunn hemp, Crotalaria juncea (L.); pigeon pea, Cajanus cajan (L.) Mill; and jack bean, Canavalia ensiformis (L.) DC. In a fourth treatment, legumes were not planted, but spontaneous vegetation was left to grow freely. The legumes were croped for 90 days in three lines (0.5 m apart) inside the passion fruit plant lines (2.5 m apart). Fixation and transfers were determined by the 15N natural abundance technique, using sunflower as a reference plant. The three planted legumes nodulated abundantly and fixed nitrogen in high proportions (between 50 and 90% of their N), forming symbiosis with bacteria naturally established in the soil. Jack bean produced more biomass than sunn hemp and pigeon pea, and as much as the spontaneous plants, of which 23% were legumes. The amounts of fixed N (150, 43, 30 and 29 kg ha-1) were determined mainly by the biomass of legumes. More than 40% of the N of passion fruit plants came from the biological nitrogen fixation of the intercropped jack bean, which provided an amount of N higher than that exported in the fruits, generating a positive balance of more than 100 kg ha-1. Therefore, it is recommended to intercrop jack bean in irrigated passion fruit orchards.