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? Combining greenhouse with field experiments, we used a fine-scale 15N-labeled fertilizer gradient to quantify the degree of N limitation, along with rates of N fixation. We used these to assess the fixation strategies for several species of N-fixing trees, both actinorhizal and rhizobial, that grow across a latitudinal gradient.
Preliminary findings from several species in the greenhouse, as well as one species in the field, suggest that different species follow different regulation strategies. For example, Robinia pseudoacacia, grown in the field and in the greenhouse, down-regulates fixation with increasing rates of N fertilization based on both nodule biomass and amount of N derived from fixation. In the field, Robinia fixed about 80% of its N in the control (tracer-only) treatment. However, even at high rates of fertilization (150 kg N/ha/yr), about 20% of N in Robinia continued to come from fixation, suggesting that this species follows an “incomplete down-regulation” fixation strategy. Based on these data, woody legumes do not all follow a strictly facultative fixation strategy, as has been suggested by previous observational experiments.