The Bird Soil Biogeochemistry Lab studies carbon and nitrogen cycling below ground in both terrestrial and estuarine systems. Soils play a central role in governing how matter and energy move through the environment, and they are widely viewed as key to how the Earth responds to a changing climate. At the same time, soils release greenhouse gases to the atmosphere and serve as a substantial sink for the carbon and nitrogen that plants add. Our research examines how soil fauna and microbial communities, vegetation, climate and mineralogy together shape whether soil carbon and nitrogen are recycled, lost or held in place across boreal, temperate and tropical ecosystems. A large share of our work relies on stable isotope tracers (13C and 15N) to track carbon and nitrogen as they move among plants, soil microbes and mineral surfaces, helping us understand how soils sustain ecological productivity, the production of food and fiber, and environmental quality.