The Ecology of Nitrogen Fixation

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Plant growth in temperate ecosystems is limited by nitrogen availability. Indeed, the success of modern agriculture is due primarily to the invention of nitrogen fertilizers. However, the use of nitrogen fertilizer requires intensive energy inputs and can result in eutrophication of the environment. Nitrogen can also be supplied to ecosystems by nitrogen fixing microbes. The symbiotic interaction between plants and nitrogen fixing bacteria frees plants from the limitations of soil nitrogen availability. The development of new nitrogen fixing symbioses could free us from our reliance on nitrogen fertilizer. However, this long sought goal of plant breeders has yet to be realized. Understanding the ecology of existing nitrogen fixing organisms may help us not only to understand their roles in natural systems, but also guide us in developing new nitrogen fixing systems and applications for them. The fundamental problem in the ecology of nitrogen fixing plants is explaining why they are not more common. In many nitrogen-limited ecosystems nitrogen fixing plants tend to be rare, whereas they often dominate systems where nitrogen does not limit growth. The proposed research will look at factors that can potentially limit the success of symbiotic nitrogen fixing plants. Theory predicts that nitrogen fixation is limited by its cost relative to the acquisition of other elements. Lab and field experiments will therefore determine if making carbon acquisition more difficult (by altering light levels), or soil nutrient uptake more efficient (using fertilizer and adjusting soil temperatures) reduces the effectiveness of the nitrogen fixation. Comparing nitrogen fixing plants with and without their symbionts will allow us to quantify the benefit of the symbiosis under varying environmental conditions. Plants typically harbor symbiont genotypes that vary in their effect on nitrogen fixation. I will manipulate interactions between bacterial genotypes of varying nitrogen fixing capability to understand how mutualistic relationships are maintained, rather than displaced by less mutualistic interactions. I will also determine how symbiotic nitrogen fixation contributes to the productivity of boreal forests across a regional transect. Comparing productivity and nitrogen fixation from the driest edge of the boreal forest to the center of its range will not only give us insight into conditions that favor nitrogen fixation, but will also help us understand how this may be modified with a changing climate. Finally, I will determine the input of fixed nitrogen by cyanolichens and mosses where they are super abundant, to determine if they can displace the role of symbiotic nitrogen fixation in a tropical cloud forest. Overall, this research will allow us to predict conditions that promote nitrogen fixation and discover new methods to manage nitrogen and ecosystem productivity.