Preferential uptake of different nitrogen forms by tree species in Bornean mixed dipterocarp forest

Background/Question/Methods

Nitrogen in soil exists in three major forms available to plants, including ammonium (NH₄⁺), nitrate (NO₃^-), and organic nitrogen. Plant species often display preference for specific nitrogen forms, which may be a mechanism for niche partitioning if nitrogen form availability varies in different habitats. Evidence for this type of species sorting has been found in grasslands, alpine tundra, and temperate forests, but it is unknown whether this occurs in the tropics. Lambir Hills National Park in Malaysian Borneo is composed of a diverse assemblage of tree species, many of which specialize on different soil types along a soil gradient. Soils at Lambir have varying availability of nitrogen forms: clay has a higher concentration of NO₃^- than sandy loam. The purpose of this study was to determine if tree species specializing on sandy loam or clay soil at Lambir have differing preferences for NH₄⁺, NO₃^-, or organic nitrogen. If so, these preferences may influence the distribution of tree species across the soil gradient. An isotope tracer experiment using ¹³C and ¹⁵N was conducted on twelve tree species, including four congeneric pairs specializing on either clay or sandy loam soil, two additional sandy loam specialists, and two generalist species. Individuals were randomly treated with K¹⁵NO₃^-, ¹⁵NH₄⁺Cl, ¹⁵N¹³C glycine, or water (the control). After 48 hours one leaf per plant was harvested, dried, and ground, then later analyzed for stable isotopic contents. 

Results/Conclusions

Preliminary data suggest that of the twelve tree species, 50% showed greater uptake of ammonium relative to the control, compared with nitrate and glycine. These included one generalist, two clay specialists, and three sandy loam specialists. Greater uptake of nitrate relative to the control, compared with ammonium and glycine, was exhibited in 41.7% of species. These included one generalist, three sandy loam specialists, and one clay specialist. The ¹³C data showed that 25% of species displayed at least some evidence of uptake of organic nitrogen (glycine).  Our data thus far suggest a weak pattern of more rapid nitrogen uptake among sandy loam specialists compared to the control, especially for ammonium. Although there appears to be no dominant pattern between soil type specialization and nitrogen form preference, nitrogen form uptake does vary between tree species. Nitrogen form preference may be an unappreciated, yet important mechanism in the niche partitioning of tree species, which has implications for understanding the generation and maintenance of tropical forest diversity.