Plant nitrogen acquisition strategies and community assembly in a tropical montane forest

Background/Question/Methods

Soil nitrogen (N) occurs in a range of chemical forms, including nitrate, ammonium, and amino acids. Plant species can differ in their capacity to use these various forms of soil N. Both the absolute and relative availability of soil N compounds can vary spatially and temporally. Thus, in N-limited ecosystems, partitioning of soil N can influence both local species coexistence and species distribution patterns along environmental nutrient gradients. We conducted a series of field experiments to examine the use of chemical forms of N in a tropical montane forest using ¹⁵N-labeled compounds. Our aim was to determine how soil-based habitat filtering and preferences for soil N compounds influence local scale (α) and mesoscale (ß) diversity. First, using eight species within the Arecoideae subfamily with contrasting distributions along a soil N gradient, we examined whether taxonomically related species with contrasting soil associations differ in their preferences for chemical forms of N, thereby promoting ß-diversity. Second, using eleven species with either arbuscular, ecto-, or orchid mycorrhizae, N₂-fixing bacteria, or cluster roots, we examined whether sympatric species with differing root specializations show contrasting preferences for soil N compounds, thereby promoting species coexistence.

Results/Conclusions

Across both studies, all species used N from both inorganic and organic N sources. In study 1, palm species with contrasting soil associations showed no preference in chemical forms of N taken up. However, palm species from low nutrient sites, in which the soluble N pool was dominated by ammonium and organic N, had inherently slow N uptake rates. Thus, habitat filtering of soil N gradients was related to N uptake rates, rather than preferences for soil N compounds. In contrast, species in study 2 with contrasting root associations took up N in following the pattern: nitrate > ammonium > glycine (F_{2, 14} = 40.48, P < 0.0001). However, the relative uptake of the N compounds differed among root association types (F_{8, 84} = 4.87, P < 0.001). Thus, differences in the capacity to use soil N compounds may facilitate coexistence among species with differing root association types. We suggest that N uptake strategies of species with contrasting habitat and root associations may be important in promoting species diversity in tropical montane forests.