Nitrogen (N) inputs from symbiotic N fixation are fundamental to the functioning of the biosphere, but which environmental factors regulate these inputs remains unresolved. Symbiotic N-fixing plants have the greatest potential for natural N inputs into the biosphere. Because it is energetically favorable for these “N fixers” to take up N from the soil when it’s available rather than fix it themselves, the availability of soil N has been long thought to be a primary regulator of fixation rates. However, light availability that fuels the carbon energy for these N-acquisition processes may be as or more important for regulating symbiotic N fixation than soil N availability. To test whether symbiotic N fixation is more strongly regulated by the lack of soil nitrogen or by the availability of light used to fuel fixation, we measured nitrogen fixation activity in 170 Pentaclethra macroloba seedlings exposed to varying levels of soil nitrogen and light in a controlled shadehouse setting at La Selva Biological Station, Costa Rica. Light treatments were 8%, 20%, and 40% of direct sunlight, and soil N treatments were 0, 20, and 40 g m-2 of ammonium nitrate added to a soil/sand mixture that had lower soil N than natural conditions.
Results/Conclusions
Nitrogen fixation varied over 500-fold more strongly across light than across soil N treatments. Nitrogen fixation rates increased with increasing light availability and decreasing soil N availability, as expected. Mean nodule biomass was 505.34, 5.91, and 0.25 mg for plants in high-, medium-, and low-light, respectively, while it was 295.77, 182.62, and 65.45 mg for plants in low-, medium-, and high-N treatments, respectively. Low and medium light levels (which were even more well-lit than forest understories) effectively completely shut down N fixation, whereas soil N levels that far exceeded natural soil N availability did not fully down regulate fixation. These results strongly suggest that light is a more important regulator of symbiotic N fixation that soil N availability, and that even very N-rich environmental conditions may not fully shut off N inputs from symbiotic fixation. Light regulation of N fixation is a departure from decades of focus on soil nutrient availability, and has important implications for our understanding of nutrient limitation and coupling the global carbon and nitrogen cycles.