Collaborative Research: Does a transition in nitrogen fixation strategy explain the latitudinal distribution of nitrogen-fixing trees?

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Nitrogen (N) is a necessary nutrient for plant growth. The atmosphere consists mainly of nitrogen gas (N2), and this form of N is extremely stable. Most plants can't directly access N2 and their rate of growth is instead limited by the presence of available N in the soil. Therefore, some species of trees have evolved a symbiosis with a special class of bacteria that fix atmospheric N2 and convert it into a plant-available form. These nitrogen-fixing trees have abundant access to nitrogen, but there is a cost to this symbiotic association: these trees provide carbon to bacteria and therefore sacrifice some of their own growth to maintain the symbiosis. As a result, nitrogen-fixing trees comprise only a small proportion of tree species on earth. In theory, nitrogen fixation is only advantageous in ecosystems that have very limited amounts of N in the soil. N-limited forests tend to be more common at high latitudes than at lower latitudes (the tropics and subtropics), but curiously, N-fixing trees are actually most common at lower latitudes. In fact, N-fixing trees are ten times less abundant at higher latitudes than at lower latitudes. Why is this the case? This proposal will test a potential explanation. The "differential regulation hypothesis" states that variation in the degree to which N-fixing trees regulate N fixation can explain the pattern. N-fixing trees that rapidly adjust N-fixation to meet N demand should be more abundant, as is the case in lower-latitude forests. In contrast, species that cannot regulate N-fixation should be less abundant, as observed in higher-latitude forests. This project will measure the extent to which the regulation of N-fixation actually varies across latitudes, as well as the metabolic costs of regulating N-fixation and the relationship of these costs to temperature. The results will provide a long sought after explanation for the distribution of N-fixing trees on earth, which is an essential component of the growth of forests worldwide.The project will involve a field experiment in forests at different latitudes as well as two greenhouse experiments. The field experiment will fertilize forests with labeled N to vary the degree of N limitation and measure the regulation of N fixation in the field. The experimental locations are Oregon (with Alnus rubra), New York (with Robinia pseudoacacia), and Hawaii (with Morella faya, Casuarina equisetifolia, Acacia koa, and Sophora chrysophylla) and span the latitudinal threshold for N-fixing tree abundance (35° N). There are two major types of N fixing symbioses (actinorhizal and rhizobial) and the field sites were chosen to capture both. The greenhouse experiments will test a larger number of species and involve more highly controlled conditions. The greenhouse experiments will also evaluate how quickly plants can regulate N fixation. The experiments and laboratory analyses will involve participation of Title 1 high school students as well as undergraduate and graduate students in New York, California and Oregon.