Some types of biological nitrogen fixation are well studied, but in many ecosystems a majority of nitrogen is likely fixed by cryptic sources that are difficult to study, including microorganisms in decomposing roots and wood, soil crusts, lichens, and mosses. An international group of researchers conducted a literature review to explore the rates, patterns, and controls of cryptic nitrogen fixation in both natural and agricultural ecosystems. They found high variability in reported rates of biological nitrogen fixation, which underscores the need for better and more geographically widespread measurements. Understanding patterns and controls of all sources of nitrogen fixation helps improve our ability to predict food production, ecosystem response to climate change, and human alteration of the global nitrogen cycle. This review was included in the 25th anniversary issue of the journal Ecosystems and was partially supported by the USGS John Wesley Powell Center. 

Cleveland, C.C., Reis, C.R., Perakis, S.S., Dynarski, K.A., Batterman, S.A., Crew, T., Gei, M., Gundale, M.J., Menge, D.N., Peoples, M., Reed, S.C., Salmon, V.G., Soper, F.M., Taylor, B.N., Turner, M.G., and Wurzburger, N., 2022, Exploring the Role of Cryptic Nitrogen Fixers in Terrestrial Ecosystems- A Frontier in Nitrogen Cycling Research: Ecosystems, Online. https://doi.org/10.1007/s10021-022-00804-2