Ammonium supply enhances growth and phosphorus uptake of mahogany (Swietenia macrophylla) seedlings compared to nitrate.

Nitrogen (N) is the mineral nutrient required in greater quantities by plants with deficiencies affecting crop production. Nitrate (NO3-) and ammonium (NH4+) are the inorganic N forms most readily available for plant uptake. We investigated the influence of ammonium and nitrate rates on growth, nutritional status, chlorophyll production, and nutritional efficiency in mahogany seedlings, grown in an Oxisol soil under greenhouse conditions. Treatments were five N rates (50, 100, 150, 200, and 400 mg dm−3 N) in ammonium and nitrate forms and control treatment (0 mg dm−3 N), in a factorial design with five replications. The plants were harvested for evaluation at 150 days after transplanting. N supply affected mahogany growth, mainly in the production of leaf tissue. The maximum economic efficiency of N rates for production of leaf tissue was around 190 mg dm−3 N. The ammonium supply enhanced leaf tissue production, with a 20% increase in leaf area and 15% in leaf dry matter compared to nitrate. Furthermore, the plants showed a preference for ammonium uptake in rates above 200 mg dm−3 N, which indicates that the preference for N form was dependent on the concentration of N in the soil. The ammonium supply enhanced nutritional status, mainly phosphorus, demonstrating high synergism on uptake of this nutrient. Conversely, a strong antagonism between nitrate and phosphorus uptake was observed. Thus, ammonium enhanced the growth and nutritional status of mahogany seedlings and must be prioritized in its fertilization. [ABSTRACT FROM AUTHOR]