PARENT SESSION
Thursday, August 10, 1:30-5:00 pm
COS 92 - Nitrogen cycling II
Sultana, Mezzanine Level, Cook Convention Center
Presiders: J Ely and S Baer

Facultative nitrogen fixation in a lowland tropical forest inferred by ¹⁵N and direct measures.

Barron, Alexander^*,1, Hedin, Lars¹, ¹ Princeton University, Princeton, NJ

ABSTRACT- Ostensible nitrogen (N) fixers are abundant in lowland Neotropical forests, and are thought to display high levels of fixation even though these forests tend to be N-rich. In contrast, physiologically based models predict that N fixation by trees should be down regulated if N is abundant. Methodological barriers have hindered the comparison of these two predictions. Traditional approaches (excavating root nodules and measuring enzyme activity) are effort-intensive and difficult to extrapolate. A more recent approach based on ¹⁵N natural abundance involves assumptions about how plants acquire and allocate N that have not been critically examined. Most importantly, these two approaches have never been directly compared in the field for tropical trees. We report on spatial and temporal variations in symbiotic N fixation across three types of lowland forest in Panama that differ in disturbance intensity. Nodule densities and N fixation differed significantly across forest types. Comparison of trees of the same size, light level, and genus within and across forests show that nodule formation and N fixation are highly facultative and can vary over three orders of magnitude, with rates very close to zero in mature forest. However, the natural abundance of foliar ¹⁵N did not capture this variation effectively and the ¹⁵N of non-fixing reference species was often very close to zero. Our results suggest that legumes can actively adjust N fixation rates to conditions in the landscape and that ¹⁵N patterns are not consistent with N fixation as the primary factor driving foliar ¹⁵N in legumes.

Key words: nitrogen fixation, tropical forest, ¹⁵N natural abundance