Preferences or plasticity in nitrogen acquisition by understorey palms in a tropical montane forest

Soil nitrogen (N) occurs in a range of chemical forms from simple inorganic compounds, such as nitrate (NO3 ) and ammonium (NH4 +), to organic compounds, such as amino acids. Plants differ in their capacity to use these various forms, which might influence the distribution of species across environmental nutrient gradients. We tested the hypothesis that the distribution of understorey palm species along a soil N gradient in a tropical montane forest in Panama is related to preferences for different chemical forms of N. We conducted a field experiment using 15N-labelled ammonium, nitrate and glycine to examine whether tropical plants show preferences for, or are flexible in, their use of chemical forms of soil N. All species used N from inorganic and organic sources and showed no preference for chemical forms of N. However, across all species, the overall N acquisition pattern was glycinenitrateammonium. Species from low-nutrient sites dominated by ammonium and organic N forms had inherently slow N uptake rates. Synthesis. Patterns in the distribution of understorey palms were related to nitrogen (N) uptake rates rather than preferences for N chemical forms. Down-regulation of N uptake rates may be an important adaptation for plant species associated with low-N soils, with plasticity in N acquisition patterns from various N sources important in alleviating competition for soil N.