Carla R. G. Reis Steven S. Perakis Katherine A Dynarski Sarah Batterman Timothy Crews Maga Gei Michael J. Gundale Duncan Menge Mark Peoples Sasha C. Reed Verity Salmon Fiona M. Soper Benton Taylor Monica Turner Nina Wurzburger Cory Cleveland 2022 <p><span>Biological nitrogen fixation represents the largest natural flux of new nitrogen (N) into terrestrial ecosystems, providing a critical N source to support net primary productivity of both natural and agricultural systems. When they are common, symbiotic associations between plants and bacteria can add more than 100&nbsp;kg&nbsp;N&nbsp;ha</span>⁻¹<span>&nbsp;y</span>⁻¹<span>&nbsp;to ecosystems. Yet, these associations are uncommon in many terrestrial ecosystems. In most cases, N inputs derive from more cryptic sources, including mutualistic and/or free-living microorganisms in soil, plant litter, decomposing roots and wood, lichens, insects, and mosses, among others. Unfortunately, large gaps remain in the understanding of cryptic N fixation. We conducted a literature review to explore rates, patterns, and controls of cryptic N fixation in both unmanaged and agricultural ecosystems. Our analysis indicates that, as is common with N fixation, rates are highly variable across most cryptic niches, with N inputs in any particular cryptic niche ranging from near zero to more than 20&nbsp;kg&nbsp;ha</span>⁻¹<span>&nbsp;y</span>⁻¹<span>. Such large variation underscores the need for more comprehensive measurements of N fixation by organisms not in symbiotic relationships with vascular plants in terrestrial ecosystems, as well as identifying the factors that govern cryptic N fixation rates. We highlight several challenges, opportunities, and priorities in this important research area, and we propose a conceptual model that posits an interacting hierarchy of biophysical and biogeochemical controls over N fixation that should generate valuable new hypotheses and research.</span></p> application/pdf 10.1007/s10021-022-00804-2 en Springer Exploring the role of cryptic nitrogen fixers in terrestrial ecosystems: A frontier in nitrogen cycling research article