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New insights into tropical dry forest nitrogen cycling from Costa Rica

By Jeff Atklins

Maga Gei, a postdoctoral researcher at the University of Minnesota, and her advisor, Jennifer Powers, have been working on tropical dry forest ecology in the Guanacaste Conservation Area in Costa Rica since 2009. At this year’s 100th meeting of the Ecological Society of America in Baltimore, Maryland, Gei presented recent work in her talk “Nitrogen fixation in dry tropical forests” in Monday’s ESA session “Disrupted Nitrogen Cycling in the Tropics: Tracking the Effects of Global Change Impacts on N Biogeochemistry from Soil to Stream.”

Gei’s talk highlighted the significance of nitrogen fixing tropical legumes in dry tropical forest nitrogen cycling and how the strong seasonality of precipitation acts as an environmental control.

TROPICAL DRY FORESTS

Tropical dry forests account for approximately 42% of all tropical forests on the globe. These forests occur throughout the tropics, including southern Mexico and Brazil. While rainfall ranges from 250 to 2000 mm per year in these forests, that precipitation is strongly seasonal, with nearly all annual rainfall occurring during 5–6 months of the year. This distinct seasonal component of precipitation has profound effects on biogeochemical cycling, and in particular, nitrogen cycling, within these forests.

“Nitrogen limitation is one of the main constraints on plant growth,” said Gei, “Though nitrogen is abundant as a gas in the atmosphere, plants can’t directly use it. Tropical legumes, through symbiosis, capture nitrogen and are an important source of new nitrogen for this ecosystem. Nitrogen-fixing legumes are also very abundant in tropical dry forests.”

NITROGEN-FIXATION

Nitrogen-fixation occurs in nodules that grow on the roots of legumes. Within these nodules, nitrogen-fixing bacteria convert the inert N2 gas in the atmosphere into plant-available nitrogen.

Bisections of healthy root nodules. (fromhttp://aces.nmsu.edu/pubs/_a/A129/)

To better understand environmental controls on nitrogen-fixing tropical legumes, Gei used a combination of manipulative greenhouse experiments. In one experiment, plants were grown with added phosphorus, while and equal amount of nitrogen was added to another set of plants. In a subsequent experiment, plants were amended with nitrogen equal to that in experiment one, while a second plant received double the amount of nitrogen. By looking at the change in nodule mass, Gei could calculate the effect of the various treatments.

UNEXPECTED, YET ILLUMINATING RESULTS

The hypothesis was that nitrogen addition would result in fewer nodules and suppressed nitrogen fixation. After all, why go to the trouble and ecological expense of fixing nitrogen when it becomes more freely available?

The expected outcome did not exactly match the actual. Adding excess nitrogen did not affect nitrogen fixation rates unilaterally, but instead resulted in a varied response by species.

“Tropical legumes are not a homogenous group as they are typically explained,” said Gei, “They have different nutrient acquisition strategies and while some fix a lot of nitrogen, others regulate the process as they are more responsive to the environment, with some being more obligate fixers while others are more facultative. ”

The overall change in nodule mass in the experimental treatments was not as pronounced as had been expected against controls. “The carbon costs of nitrogen fixation may not be as high as we expected,” said Gei, “And fixation rates varied more by species than by treatments in the experiments.”

But where did all the added nitrogen in the experiment go?

“Probably due to their ability to fix nitrogen, legumes are very efficient in many different levels, they have faster germination, faster growth rates, really high photosynthetic rates,” Gei offered, “And that could explain why they are so successful in tropical dry forests.”

FORECASTING FOR THE FUTURE

Gei went on to highlight the significance of this work in the light of climate change the associated projections of changing precipitation regimes, “Under the severe drought conditions this area is expected to experience, nitrogen inputs to these forests could be severely compromised.”

Tropical legumes represent the most substantial input of nitrogen into dry tropical forests. Fundamental research, such as Gei’s, helps us to better understand the complex dynamics of this ecosystem and aids in understanding potential impacts from climate change.

A note on style…Maga Gei’s presentation style is progressive, forgoing the tired over-text-laden style of some scientists, with bold, colorful pictures. She allows the slides to serve as a backdrop to her words — a style that should be more widely adopted. Nothing can make good science seem more dull than reading a slide to the audience.

— Jeff Atkins for PLOS Ecology Field Reports

JEFF ATKINS is a Ph.D. candidate at the University of Virginia and Field Scientist for the Shenandoah Watershed Study. His research focuses on the interaction of vegetation and landscape position to influence biogeochemical cycles within complex terrain and the effects of inter-annual climate variability on ecosystems. You can reach him via Twitter (@atkinsjeff).

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