PARENT SESSION
Organized Oral Session 48: Nitrogen cycling in terrestrial ecosystems: New insights from isotopes and experiments
Organizer(s): MC Mack, P Templer, and K Nadelhoffer
Thursday, August 11, 1:30 PM - 5:00 PM, Meeting Room 510b, Level 5, Palais des congrès de Montréal

Patterns of nitrogen fixation along a long-term soil chronosequence in New Zealand rainforests.

Menge, Duncan^*,1, Hedin, Lars¹, ¹ Princeton University, Princeton, NJ, USA

ABSTRACT- Soil nutrient availability is thought to be a key factor regulating the process of nitrogen (N) fixation, which in turn is thought to be an important control on ecosystem processes such as N losses, N limitation, productivity, and carbon storage. However, because organisms fix N within different biogeochemical niches (e.g. symbiotic plants vs. non-symbiotic bacteria), they likely have access to different nutrient pools in spatially heterogeneous systems, so it may be difficult to predict ecosystem-level N fixation from one pool of available nutrients. To test this idea we measured the abundance and nitrogenase activity (using acetylene reduction) of four classes of N-fixers (Frankia in nodules of the symbiotic tree Coriaria arborea, lichens, cyanobacteria living with bryophytes, and heterotrophic bacteria living on leaf litter) along a 120,000-year rainforest developmental sequence in New Zealand. Similar to most long-term chronosequences, soil N increases and soil phosphorus declines with age in these sites. We report that while C. arborea was excluded after sixty years, the abundance of N-fixing lichens and bryophytes peaked with soil fertility in the intermediate-aged sites. Nitrogenase activity in C. arborea nodules was similar in the young sites it occupied. However, nitrogenase activity did not vary with soil age in lichens, bryophytes, or leaf litter, suggesting that soil nutrients have little to no effect on the physiological regulation of N fixation in these classes. The contrasting patterns of abundance and nitrogenase activity within N-fixer classes reported here suggest that nutrient regulation of N fixation in this system acts at the community level, not the physiological level. Our data suggest that different classes of N-fixers have different controls, and by extension that spatial heterogeneity in nutrient availability may play a key role in determining ecosystem-level properties.

Key words: N fixation, ecosystem development, soil chronosequence, Coriaria arborea