PS 4-52 - New, national bottom-up estimate for tree-based biological nitrogen fixation in the US

Monday, August 7, 2017
Exhibit Hall, Oregon Convention Center
Anika R. Petach1, Duncan N. L. Menge1, Wenying Liao2, Steven Perakis3, Jana E. Compton4 and Christopher M. Clark5, (1)Ecology, Evolution, and Environmental Biology, Columbia University, New York, NY, (2)Ecology and Evolutionary Biology, Princeton University, Princeton, NJ, (3)US Geological Survey, Corvallis, OR, (4)US EPA, NHEERL, Western Ecology Division, Corvallis, OR, (5)National Center for Environmental Assessment, US Environmental Protection Agency, Washington, DC

Nitrogen is a limiting nutrient in many ecosystems, but is also a chief pollutant from human activity. Quantifying human impacts on the N-cycle and investigating natural ecosystem nitrogen cycling both require an understanding of the magnitude of nitrogen inputs from biological nitrogen fixation (BNF). A bottom-up approach to estimating BNF—scaling rates up from measurements to broader scales—is attractive because it is rooted in actual BNF measurements. However, bottom-up approaches have been hindered by scaling difficulties, and a recent top-down approach suggested that the previous bottom-up estimate was much too large. Here, we used a bottom-up approach for tree-based BNF, overcoming scaling difficulties with the systematic, immense (>70,000 N-fixing trees) Forest Inventory and Analysis (FIA) database. We employed two approaches to estimate species-specific BNF rates: published ecosystem-scale rates (kgN ha-1yr-1) and published estimates of the percent of N derived from the atmosphere (%Ndfa) combined with FIA-derived growth rates. Species-specific rates can vary for a variety of reasons, so for each approach we examined how different assumptions influenced our results. Specifically, we allowed BNF rates to vary with stand age, N-fixer density, and canopy position (since N-fixation is known to require substantial light).


Our preliminary bottom-up estimates of symbiotic tree-associated N-fixation across the US are several orders of magnitude lower than previous estimates. This suggests that symbiotic BNF may contribute less to the overall N-cycle than previously understood. According to our estimate, 69 Mg of nitrogen are fixed annually by trees across the continental US (9-129 Mg yr-1 range based on sensitivity analyses). Tree-based N fixation in the continental USA is dominated by only two species; Robinia pseudoacacia in the northwestern accounting for 49% and Alnus rubra in the western US which fixed 45% of BNF tree-associated in FIA plots. Less than 5% of tree BNF was associated with Alnus rhombifolia and Prosopis glandulosa, the next two most important tree species. Consequently, the regional fixation breakdown largely reflects the distribution of N-fixing trees with 45% of BNF in the Northeast and 37% of BNF in the South. This indicates that the bulk of tree-associated BNF is occurring in species that are both abundant and rapid fixers.

The lower estimate of tree-associated BNF that we calculated indicates that anthropogenic N sources and other BNF inputs may be more important than previously recognized in shaping regional patterns of watershed N export, greenhouse gas fluxes, and ecosystem carbon storage.