Here in Arizona's Sonoran Desert around Phoenix, we receive a fair amount of nitrogen from air pollution. That means that soil inside the city of Phoenix has more nitrogen than soil outside the city. A lot of our research tries to understand what happens to that nitrogen that's deposited by air pollution. Nitrogen can fertilize plants (which is why you might put it on your yard or garden), but too much of it can cause a problem, both for human health and for the ecosystem's health!