WHEN IS 2+2 ! 4? INTERACTIVE PRIMING OF PYROGENIC ORGANIC MATTER, SOIL ORGANIC CARBON, AND PLANT ROOTS IN NATURAL AND MANAGED ECOSYSTEMS Thea Leslie Whitman, Ph. D. Cornell University Soils hold a globally important stock of carbon (C) and can act as both a C source and sink, depending on management and environmental conditions. Pyrogenic organic matter (PyOM) is produced naturally during fires, and contains relatively stable forms of C. Its intentional production has also been proposed as a mechanism for C management (in such cases PyOM is often referred to as "biochar"). However, the impact of natural or anthropogenic PyOM production on soils is complex and depends on many factors. In particular, the effects of PyOM on existing soil organic C (SOC) dynamics is poorly characterized or understood. Understanding the mechanisms behind these interactions, often referred to as "priming", is essential to predict the impact of PyOM additions to soils. In a greenhouse study, PyOM additions counteracted positive priming of SOC by corn plants, almost completely eliminating net C losses either by decreasing SOC decomposition or increasing corn C additions to soil. This highlights the importance of including plants in studies of PyOM-SOC interactions. In an incubation trial, the relative mineralizability of PyOM as compared to SOC predicted priming interactions, where the soil with lower SOC mineralization was more susceptible to short-term increases in SOC mineralization with PyOM additions, which were proportional to the C mineralization in the added PyOM. Soils also experienced net long-term decreases in SOC mineralization with PyOM additions, possibly due to stabilization of SOC on PyOM surfaces, an example of which we imaged using nanoscale secondary ion mass spectrometry. In a field trial, PyOM additions temporarily increased total soil CO2 fluxes, dramatically less than the addition of fresh corn stover, which likely increased SOC mineralization. Three-part stable isotopic partitioning revealed significantly greater root-derived CO2 fluxes with PyOM additions than without, and significantly lower PyOM-C derived CO2 fluxes when plants were present. PyOM additions only resulted in significant changes to the soil microbial community on day 79, while stover additions induced shifts by day 11. This work informs our understanding of PyOM-soil-plant-microbe interactions and contributes to progress toward a comprehensive predictive framework of PyOM effects on C cycling in soils.