The influence of nitrogen concentration and ammonium/nitrate ratio on N-uptake,mineral composition and yield of citrus

In short-term water culture experiments with different ¹⁵N labeled ammonium or nitrate concentrations, citrus seedlings absorbed NH₄⁺ at a higher rate than NO₃^–. Maximum NO₃^– uptake by the whole plant occurred at 120 mg L^–1 NO₃^–-N, whereas NH₄⁺ absorption was saturated at 240 mg L^–1 NH₄⁺-N. ¹⁵NH₄⁺ accumulated in roots and to a lesser degree in both leaves and stems. However, ¹⁵NO₃^– was mostly partitioned between leaves and roots.Adding increasing amounts of unlabeled NH₄⁺ (15–60 mg L^–1 N) to nutrient solutions containing 120 mg L^–1 N as ¹⁵N labeled nitrate reduced ¹⁵NO₃^– uptake. Maximum inhibition of ¹⁵NO₃^– uptake was about 55% at 2.14 mM NH₄⁺ (30 mg L^–1 NH₄⁺-N) and it did not increase any further at higher NH₄⁺ proportions.In a long-term experiment, the effects of concentration and source of added N (NO₃^– or NH₄⁺) on nutrient concentrations in leaves from plants grown in sand were evaluated. Leaf concentration of N, P, Mg, Fe and Cu were increased by NH₄⁺ versus NO₃^– nutrition, whereas the reverse was true for Ca, K, Zn and Mn.The effects of different NO₃^–-N:NH₄⁺-N ratios (100:0, 75:25, 50:50, 25:75 and 0:100) at 120 mg L^–1 total N on leaf nutrient concentrations, fruit yield and fruit characteristics were investigated in another long-term experiment with plants grown in sand cultures. Nitrogen concentrations in leaves were highest when plants were provided with either NO₃^– or NH₄⁺ as a sole source of N. Lowest N concentration in leaves was found with a 75:25 NO₃^–-N/NH₄⁺-N ratio. With increasing proportions of NH₄⁺ in the N supply, leaf nutrients such as P, Mg, Fe and Cu increased, whereas Ca, K, Mn and Zn decreased. Yield in number of fruits per tree was increased significantly by supplying all N as NH₄⁺, although fruit weight was reduced. The number of fruits per tree was lowest with the 75:25 NO₃^–-N:NH₄⁺-N ratio, but in this treatment fruits reached their highest weight. Rind thickness, juice acidity, and colour index of fruits decreased with increasing NH₄⁺ in the N supply, whereas the % pulp and maturity index increased. Percent of juice in fruits and total soluble solids were only slightly affected by NO₃^–:NH₄⁺ ratio.