Suppression of NH₃ and N₂O emissions by massive urea intercalation in montmorillonite

Purpose: A large amount of nitrogen (N) fertilizers has been broadcasted over soil surface for reliable crop production. Unfortunately, the broadcasted N vulnerable to volatilization and leaching can lead to serious environmental problems. As a new approach to mitigate N loss of broadcasted fertilizers, massive intercalation of urea into montmorillonite (MMT) was recently proposed to innovatively enhance the urea use efficiency. This study focuses on demonstrating the behaviors of the urea intercalated into MMT in soils. Materials and methods: A eutectic Ca_0.7/Mg_0.3(urea)₆Cl₂ complex was intercalated into MMT at a weight ratio of complex/MMT of 1.0 (the resulting product was referred to as Ca/Mg-UCM). Behaviors of broadcasted urea were examined on both bare and planted soils. Soil urea-N, ammonium, and nitrate were analyzed using a spectrophotometer after extraction of the soil by 2 M KCl solution. Ammonia emitted from the soil was collected by a closed-dynamic air flow method, and NH₃ concentration was determined by titration with a standard 0.005 M H₂SO₄. In particular, urea labeled with 5% ¹⁵N atom was used in evaluating N uptake. Emission of N₂O was evaluated by collecting all the gases from the soil using a PVC column. Results and discussion: A dramatic difference between the urea-broadcasted and Ca/Mg-UCM-broadcasted soils was found in ammonia emission. A peak emission rate decreased from 38.5 N mg m^-2 h^-1 of the urea-broadcasted soil to 28.3 N mg m^-2 h^-1 of the Ca/Mg-UCM-broadcasted soil. This remarkable suppression of the initial ammonia emission could be explained by the events such as delayed urea hydrolysis, dissolution of ammonia into soil waters, especially water adsorbed on MMT, adsorption of ammonia by MMT, and delivery of urea molecules into inner soils through deintercalation and diffusion. Uptake amounts of total N and ¹⁵N by whole perilla plants were substantially increased by Ca/Mg-UCM broadcasting, compared with urea broadcasting. The intercalation also suppressed N₂O emission noticeably. At the same dosage of urea, Ca/Mg-UCM broadcasting led to an increase in dry weight of kale shoot by 38%, compared with urea broadcasting. Conclusions: This study has successfully demonstrated that the urea intercalation led to considerable suppression of both ammonia and N₂O emissions compared to that with urea-broadcasted soils but without any noticeable effects on fast effectiveness of urea in the former. Thus, clay-intercalated urea as a fast effective fertilizer is expected to greatly contribute to mitigation of ammonia and N₂O emissions along with green and sustainable crop production.