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.