Potential of Urea-Aluminosilicate Slow-Release Nanocomposites for Controlling Nitrous Oxide and Ammonia Emissions.

N inputs from fertilizers increase NH₄⁺ and NO₃^– soil concentrations and may increases the soil emissions of NH₃ and N₂O. Urea is the main N source for crop production in Brazil. The nitrogen loss by ammonia volatilization is one of the main factors for low efficiency of N-urea applied on the soil surface. Moreover, N₂O produced through nitrification and denitrification processes in the soil is one of main source of anthropogenic N₂O emissions. GHG emissions can be suppressed by addition of aluminosilicates minerals to N-fertilizer. However information on how urea−aluminosilicate slow-release nanocomposites might affect volatilization, nitrification and denitrification processes in the soil is limited. Zeolite minerals are crystalline hydrated aluminosilicates of alkali or alkaline-earth metals, structured in three-dimensional rigid crystalline network, formed by the tetrahedral AlO₄ and SiO₄, which come together to compose a system of canals, cavities and pores at nanoscale. These minerals are characterized by the retaining and releasing water and exchange cations without changes in structure. Other hydrated layered silicates clay minerals, like bentonite, are able to exchange cations, and intercalate neutral molecular species between the interlayer regions by interaction with structural water. In order to follow the gaseous N emissions and dynamics of the soil mineral N in which soil was amended with nanocomposites and standard N-sources three studies were conducted: a greenhouse-potted, a field and a laboratory experiment. Three different species of aluminosilicate were used: bentonite, clinoptilolite and stilbite. Nanocomposites were prepared at three ratios (w w⁻¹ basis) of aluminosilicate/urea: 1:1 (50% urea), 1:2 (66% urea), and 1:4 (80% urea). All treatments applied at the level of 100 kg ha^-1 of N on soil surface were compared with ammonium nitrate, urea and an unfertilized soil control. Ammonia volatilized was captured by an absorber with foam soaked with 0.5 N H₃PO₄ and covered by a politetrafluoroetilene tape. Brachiaria brizantha grass and corn were cultivated respectively in the greenhouse and field experiments. Both in the greenhouse-potted and in the field aluminosilicate and urea mixture reduced 50% the losses by volatilization observed with pure urea. The main action of aluminosilicates in partial reduction on NH₃ loss was due its clay high cation exchange capacity retains urea hydrolysis formed ammonium in the soil exchange complex, avoiding the volatilization. The reduction in ammonia losses by volatilization and the increased efficiency of N utilization when urea is used together with aluminosilicates was demonstrated in both greenhouse and field experiments. These results indicate that aluminosilicates minerals are able to improve the efficiency of nitrogen use, contribute to increasing N uptake through the control of retention of ammonium ion. Due the promising results in reducing losses by volatilization, the aluminosilicates were tested for formation of a new fertilizer formulation by extruding the mixture of urea powder with them. The preparation consisted of three steps: mixing, extrusion, and drying. The materials were extruded in a twin-screw extruder. Unlike zeolites, bentonite could effectively be processed by extrusion, due to its plasticity. So the urea-betonite-extruded materials were converted into pellets (3 mm X 5 mm), and dried at room temperature. Urea-zeolites materials remained in the form of powder mixture without aggregation. The aerobic laboratory incubation experiment consisted of 500 mL hermetic pots with 100 g of soil. Initially soil was kept at field capacity and treatments were applied. To assure the N₂O fluxes after seven days of incubation, soil moisture was increased until soil saturation. GHG fluxes were estimated by photoacoustic infrared multi-gas monitoring system. Mineral N content of the soil was extracted with 2 M KCl in a 1:10 (soil:extractant) ratio. Ammonium-N and NO₃–N contents of the extracts were determined by flow injection analysis. Nanocomposites helped to keep ammonium in soil and also minimized soil nitrate formation, which helps lower the risk of increased N₂O emissions. Results suggest that N release rates, as indicated by NH₄-N and NO₃-N, were related to GHG emissions. The challenge is meeting crop demand yet reducing GHG emissions by selection the adequate ratio of N and aluminosilicate nanocomposite.