Nitrogen plays multiple key roles in climate change. Trees currently absorb about 1/3 of human-caused CO2 emissions, but their ability to continue to do so depends on how much nitrogen they can access. Thus, nitrogen helps us out with climate change. In a completely different way, though, nitrogen makes climate change (and other environmental issues) worse. Excess nitrogen in soils can be exported as nitrous oxide, which is a greenhouse gas that traps ~300x more heat per molecule than CO2, or as various water and air pollutants. Many of the unknowns in this area concern the process of symbiotic nitrogen fixation, wherein specialized bacteria inhabit roots of certain plant species. These bacteria can access the inexhaustible pool of dinitrogen gas in the atmosphere, and thus can bring new “fixed” nitrogen into ecosystems. A “Goldilocks” amount of nitrogen fixation—just enough newly fixed nitrogen to match tree nitrogen demand—could fuel sustained carbon storage. Too little nitrogen fixation could constrain trees’ ability to absorb CO2, but too much could result in excess nitrogen and the related environmental problems. Recent work in our lab has addressed many questions surrounding this issue, both in terms of current conditions and in terms of feedbacks with temperature. Where do N-fixing symbioses live? How might their distribution change as climate continues to change? Do they fix just the right amount of nitrogen, or too little or too much? Do they facilitate carbon storage? Do they exacerbate nitrous oxide emissions? How does symbiotic nitrogen fixation change with temperature? Finally, what do the answers to these questions mean for forest management? Should we plant nitrogen-fixing trees during reforestation efforts, as many have suggested?