N-fixing trees are ten times more abundant in tropical areas than in temperate latitudes. This seems counterintuitive given that many lower-latitude forests have high N availability, which should select against N-fixing trees. There is no consensus explanation for this latitudinal abundance paradox. In this project, we addressed the question: does a transition in nitrogen fixation strategy explain the latitudinal distribution of nitrogen-fixing trees? Alternatively, does phylogeny predict fixation strategy better than latitude? We used a fine-scale 15N-labeled fertilizer gradient to quantify rates of N fixation of 8 N-fixing tree species in a greenhouse experiment. We used these to compare the fixation strategies of N-fixing trees, both actinorhizal and rhizobial, across a latitudinal gradient, and to examine the effects that increasing N deposition may have on overall N budgets in these different systems.
Results/Conclusions
Our greenhouse data suggest that different species follow different regulation strategies, though rhizobial (legume) trees generally turn down rates of N fixation when more N is available from the soil. This is true for both tropical (e.g., Leucaena leucocephala) and temperate (e.g., Robinia pseudoacacia) species. Nodule biomass as a percentage of total plant biomass was highest (3-7%) at very low N fertilization levels (<2 kg N/ha/yr), tapering to less than 0.5% at high N fertilization levels (>150 kg N/ha/yr), demonstrating that fixation contributes a relatively higher percentage of plant N at low N levels. However, total nodule biomass demonstrated a hump-shaped relationship with N fertilization level, peaking at moderate N fertilization levels. In L. leucocephala, for example, total nodule biomass averaged about 3mg/plant at very low N fertilization, 17mg/plant at mid N fertilization, and 0g/plant at very high N fertilization. These data suggest that while legumes rely relatively less on N fixation for their N needs at higher soil N availability, total amount of fixed N entering the system continues to climb from low to mid N fertilization levels. As a consequence, increasing N deposition may increase total rates of N fixation.