Differential nitrate leaching and mass balance of nitrogen-15-labeled nitrogen sources applied to turfgrass and citrus

Most crops including turfgrass and citrus require nitrogen (N) fertilization in order to maintain the desired level of quality and productivity. However, potential leaching losses of N present environmental and economic concerns. It has been suggested that nitrate (NO₃)-N can be retained by soil cation exchange in the form of a CaNO₃+ ion pair. Objectives of this research were (1) to determine the leaching potential and utilization of 5 soluble 15N labeled sources, ammonium nitrate-NH₄NO₃ = AN, potassium nitrate-KNO₃ = KN, urea-CO(NH₂)₂ = urea, ammonium sulfate-(NH ₄)₂SO₄ = AS, and calcium nitrate-5Ca(NO₃ )₂NH₄NO₃10H₂O = CN in turfgrass and citrus systems through mass balance studies; (2) to identify components responsible for differential leaching and utilization of applied 15 N; and (3) to determine the influence of Ca+2 on NO₃− retention, leaching, and utilization of applied 15N. Ammonium-N applied in nutrient sources AN, AS, and urea lowered the pH of the soil system and resulted in increased percent cover and color ratings during turfgrass field studies due to increased manganese (Mn) uptake. All N sources were equally effective in supplying N to plants except when NO₃− sources KN and CN were used with leachate pH > 6.5 in turfgrass field studies and in the glasshouse when AS exhibited lower coverage and higher leaching rates than the other sources. Total N recovery for all N sources was 72% based on 15N studies with 56% in plant, 4% in soil, and 12% in leachate. Unaccounted N was presumed to be lost to volatilization and denitrification. No difference in N leached or N retained in the soil was observed due to the presence of the proposed CaNO₃+ ion pair. These studies suggest that fertilizing with NO₃−-N sources increases the growth media and leachate pH which may result in negative influences on growth and quality of turfgrasses due to Mn availability that do not exist when NH₄ +-N sources are used. Additionally, in low exchange capacity sandy growing media, it does not appear that the formation of the proposed Ca(NO ₃)+ ion pair reduces the potential leaching loss of NO ₃−-N supplied through the application of calcium nitrate fertilizer.