Study the N Turnover of Legume Seed Meals for Designing a Slow-Release Nitrogen Fertilizer

To mitigate environmental problems and synchronize releasing nitrogen (N) with crop demand, slow-release N fertilizers can be a solution. In this research, the mechanism of the N immobilization in stable sources (not unstable sources such as microbial biomass and extractable organic N) for finding an appropriate compound in designing a slow-release N fertilizer was investigated. The experiments were carried out in a randomized complete block design using an incubation chamber to study the N mineralization in coarse and fine fractions of yellow lupin, blue lupin, and faba bean. The results showed that the major N immobilization occurred at 10 to 17 days after incubation. At this phase, only the polyphenols had a significant correlation coefficient with the N immobilization (r = 0.80). At 17 to 31 and 31 to 61 days after incubation, the N immobilization had significant relationship with cellulose (r = 0.96) and hemicellulose (r = 0.89), respectively. It seems that with advancing incubation time, cellulose and hemicellulose were released from cell walls, and similarly to polyphenol were bound to nitrate N (NO₃^?-N), ammonium N (NH₄⁺-N), or extractable organic N through different interactions. Although the main mechanisms of N immobilization in soil after adding plant materials with a high carbon (C)/N ratio are described in the literature, the available data do not yet present an appropriate composition of targeted, innovative, and slow-release N fertilizers. According to the obtained results, tests are suggested to find the optimum nitrification inhibitor using the powder of plant residues with different ratios of these compounds incorporated with inorganic fertilizers.