Estimating nitrogen fixation rates and controls in a tropical dry forest

Background/Question/Methods

More than a decade ago, 106 estimates of biological nitrogen (BNF) fixation from terrestrial ecosystems around the world were compiled. Since then, our knowledge about the controls over BNF has advanced considerably, especially in the form of conceptual frameworks, but less so in terms of empirical evidence that validates them. The major natural sources of newly fixed N to forest ecosystems have rarely if ever been measured in tropical dry forests (TDF), where N₂-fixing legume trees are ubiquitous. Our main goal was to determine the rates of and controls over BNF in a TDF in Costa Rica and compare this flux to other N inputs to these forests. We tested the hypothesis that legume trees employ a facultative strategy of N₂-fixation and that this process is regulated by resource availability including light, soil moisture and nutrients. We estimated the quantities of N₂ fixed annually by five common legume species, using the ¹⁵N natural abundance method, counting nodules and measuring nodule activity, and quantifying (in the field) or manipulating (in a shadehouse experiment) the variation in important environmental variables (soil nutrients, soil moisture, light). We compare the magnitude of this flux with other N inputs such as atmospheric N deposition.

Results/Conclusions

In the field, the ¹⁵N natural abundance approach successfully detected differences in N₂-fixation among species across the landscape. Levels of N₂-fixation ranged from 13 to 100 percent reliance on fixation and were negatively correlated to soil nitrate concentrations (p <.005). This relationship varied by species, where Lysiloma down-regulated fixation to a higher degree. Nodule mass was significantly different among different species (p<.01) and ranged from 52 to 182 g/m². We did not find any relationship between nodule mass and the percentage of plant N derived from the atmosphere (%Ndfa). Possible explanations for this include the reliance of legumes on retranslocated N that was fixed in a previous year, or ¹⁵N dilution from neighboring trees. In the shade house experiment, Ndfa varied from 4 to 100%, and seedling nodule mass ranged from 0 to 0.948 g. Brighter light conditions and phosphorus addition increased N₂-fixation. Different combinations of light and nutrients induce fixation in each species, which reveal a gradient in N₂-fixation strategies from fine-tuning (Lysiloma) to no tuning (Gliricidia). Defining N₂-fixation strategies has important consequences for understanding the role of different species in forest succession, how forest N cycles, and the evolutionary ecology of the legume lifestyle.