Urea fertilization reduced biological N2 fixation but did not impact the development of legume trees in an agroforestry system

Background and aims

Understanding how inorganic N fertilizers applied to agroforestry systems impact N₂-fixation and development of legume trees is essential for optimizing management practices. This study hypothesized that inorganic N fertilization with urea (CH₄N₂O) could reduce the quantity of N derived from the atmosphere in the leaves, and decrease the overall development of the legume tree Mimosa caesalpiniifolia Benth. under an agroforestry system.

Methods

The trial was settled in a randomized complete block design with three repetitions, and evaluated four different rates of N fertilization (0, 100, 200, and 400 kg N ha⁻¹ year⁻¹), over a three-year trial. The agroforestry was composed of M. caesalpiniifolia double rows, grass pasture (Megathyrsus maximus Jacq. cv. Massai), and dispersed babassu palm trees (Attalea speciosa Mart. ex Spreng), located in the Amazon region of Brazil.

Results

This study proved that increasing rates of inorganic N fertilization using urea, up to 400 kg ha⁻¹ year⁻¹, did not affect the regular development of M. caesalpiniifolia trees under an agroforestry system; however, the level of N applied, especially the highest dosage (400 kg ha⁻¹ year⁻¹), was capable of reducing by approximately 45% the quantity of N derived from the atmosphere.

Conclusion

These findings indicate a negative impact of the inorganic N fertilization with urea on the symbiotic N₂-fixation of M. caesalpiniifolia trees. This study also evidenced that as the trees of M. caesalpiniifolia aged they tended to display a significant reduction of the content of N derived from the atmosphere in their leaves.