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Contributed Talk

Session: : Biogeochemistry: New Paradigms In Biogeochem Cycling 2

COS 29-3 - Global synthesis of symbiotic tree-based nitrogen-fixation

Monday, August 7, 2023
4:00 PM – 4:15 PM PDT
Location: B116
Abstract: Symbiotic nitrogen-fixing trees often represent the largest N input in ecosystems where they are abundant. In extreme cases, pure stands of N-fixing trees can sustain N inputs up to 200 kg N ha-1 yr-1, which greatly exceeds N deposition and other natural fixation sources. The efficiency of tree-based N fixation is thought to decline in colder, high latitude environments, though this pattern has not been evaluated using literature synthesis of field studies. As part of a global synthesis of terrestrial biological N fixation, we evaluated patterns in annual rates of tree-based symbiotic N fixation reported in the literature, with a particular focus on how rates vary by latitudinal zone (e.g., tropical, temperate, and boreal). We synthesized a total of 270 studies that either reported, or were easily converted to, annual area-based fixation rates (i.e., kg N ha-1 yr-1). These literature estimates were derived from studies that measured N-fixation in various ways, including acetylene reduction, nitrogen accretion, and stable isotope estimates of percent fixed nitrogen. In preliminary analyses, we found that most studies occurred in tropical latitudes (72%), followed by temperate (25%) and boreal (3%) zones. All boreal studies occurred in relatively undisturbed ecosystems, whereas a majority of temperate and tropical studies occurred in either plantations or in systems with varying legacies of human disturbance (87%). BNF rates (kg N ha-1 yr-1) across all studies were highest in boreal, then temperate, and lastly tropical regions, though the local abundance of N-fixing trees was generally greatest in high latitude studies. Consequently, when published BNF rates were standardized per unit basal area of N-fixing tree, estimates of BNF per unit basal area converged to similar values among boreal, temperate, and tropical latitudes. Globally, studies in young (< 10 yr) plantations and in strongly human-disturbed sites displayed the highest BNF rates per unit basal area, while rates in older plantations ( > 10 yrs) were similar to those in undisturbed systems. Finally, when evaluated by IGBP biome, nearly 85% of studies occurred in evergreen or deciduous broadleaf forests, evergreen needleleaf forests, or open savanna, highlighting a need for more studies in woody savanna and mixed forests. Overall, our findings reveal that BNF rates in N-fixing trees vary coherently by latitudinal zone and disturbance history, and that standardization of BNF rates per unit N-fixer abundance reveals striking convergence of global rates of tree-based BNF by latitude.