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    Supporting Information for Fire presence affects patterns and controls on asymbiotic nitrogen fixation in seasonally flooded forests of southern Amazonia
    (University of Oregon, 2019) Bomfim, Barbara; Silva, Lucas C. R.; Marimon-Júnior, Ben H.; Marimon, Beatriz S.; Doane, Timothy A.; Horwath, William R.
    In this study, we focus on the biogeochemical consequences of fire on seasonally flooded (SF) forests of southern Brazilian Amazonia, where recent widespread rising tree mortality has been linked to changes in fire regimes. Recent studies have hypothesized that a quasi-permanent state-shift transition (from typical Amazon forests to open savannas) can occur when fire results in further depletion of already impoverished nutrient pools. Nitrogen (N) inputs via asymbiotic dinitrogen-N2 fixation (ANF) could be important for predicting the likelihood of post-fire forest recovery, but fire effects on ANF have yet to be quantified in this region. Here, we quantified ANF through combined field sampling and laboratory measurements using 15N-labeled dinitrogen (15N2) incubations, and quantified 14 biogeochemical parameters in surface (0−10 cm) and subsurface (10−30 cm) soils from burned and unburned areas within five SF forest stands selected to represent a gradient of fire disturbance, from low (once in 13 years) to high (five times in 13 years) frequency. We find that ANF rates are ~24% lower in burned relative to unburned surface soils, with over 50% of the variance in ANF explained by soil carbon (C) to N ratio and phosphorus (P) availability. Our data indicate that interactions between soil C:N:P stoichiometry and fire presence affect ANF in a predictable way. Taken together, these findings show that changes in soil carbon and nutrients explain a decrease in ANF with potential for long-term decline in C sequestration from altered successional trajectories in forest ecosystems of southern Amazonia.