Lignin is an important component of plant tissues that provides protection from herbivores and pathogens due to its complex chemical structure. When plant tissues die and enter the soil, lignin is often among the last components to decompose and may persist for decades or even longer. Despite the generally slow decay rates of lignin, recent work has suggested that many soils contain very little lignin in their organic matter. This apparent contradiction presents a key challenge for our understanding of soil organic matter formation, a topic which is critically linked to environmental services including soil fertility, water quality, and carbon storage. This research seeks to understand the environmental factors that control lignin decay rates and the ultimate contributions of lignin to soil organic matter in ecosystems across North America. The influence of differences in soil microorganisms and minerals on lignin decomposition across diverse ecosystems will be assessed. The broader implications of the findings for ecosystem functions and services will be studied by using a mathematical model. A postdoctoral researcher and graduate students will be trained, and findings will be communicated to the public and to land managers with targeted outreach activities.<br/><br/>This research seeks to resolve lingering inconsistencies in soil organic matter, where the context-specific importance of lignin may have been neglected due to spatial and methodological sampling biases. Interactions among lignin, soil geochemical characteristics, and microbial community composition may have significant impacts on lignin residence times and mineral sorption that emerge only when examined across continental-scale environmental gradients. Data collected from National Ecological Observatory Network (NEON) sites will be combined with new experiments to test two hypotheses: (1) Does lignin decomposition rates and its abundance in soil organic matter predictably vary with soil geochemical characteristics and microbial community composition at the continental scale (in contrast to both classical and modern paradigms of soil organic matter)?; and (2) Can the representations of lignin decomposition and carbon cycling in a state-of-the-art mechanistic ecosystem model be improved by inclusion of geochemical and microbial parameters (in addition to the traditional variables of climate, residue quality, and nutrient availability)? The NEON data will be combined with field and laboratory incubations of carbon isotope-labeled lignins and nuclear magnetic resonance spectroscopic measurements to test these hypotheses.<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.