Citation:

Petach, A., D. Menge, W. Liao, S. Perakis, J. Compton, AND C. Clark. New, national bottom-up estimate for tree-based biological nitrogen fixation in the US. Ecological Society of America, Portland, OR, August 06 - 11, 2017.

Impact/Purpose:

Quantifying human impacts on the nitrogen cycle and investigating natural ecosystem nitrogen cycling both require an understanding of the magnitude of nitrogen inputs from biological nitrogen fixation (BNF). Scientists at Columbia, Princeton, USGS and US EPA are working together to develop a new bottom-up estimate of N fixation across the US. 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. Our estimates from this bottom-up technique are several orders of magnitude lower than previous estimates indicating that tree-based BNF may contribute less to the overall nitrogen cycle than previously understood.

Description:

Nitrogen is a limiting nutrient in many ecosystems, but is also a chief pollutant from human activity. Quantifying human impacts on the nitrogen 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 (kg N ha-1 yr-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 estimates from this bottom-up technique are several orders of magnitude lower than previous estimates indicating that tree-based BNF may contribute less to the overall nitrogen cycle than previously understood. According to our estimate, 69189 kg of nitrogen are fixed annually by trees across the continental United States (8986 – 129392 kgN/year range). This fixation is dominated by Robinia pseudoacacia which fixed 48.85% of all N in the FIA plots and Alnus rubra which fixed a further 45.15% of N from the FIA plots. Alnus rhombifolia and Prosopis glandulosa are prominent to a lesser extent but still contribute appreciably to the estimate. This indicates that the bulk of fixation is taking place in species that are both abundant and rapid fixers. Consequently, the regional fixation breakdown largely reflects the distribution of R. pseudoacacia dominates across Midwest and Northeast, and A. rubra across the Northwest. P. glandulosa dominates the South and Southeast-. Although these estimates do not include BNF from grasses, shrubs, or other small plants they still indicate a new range of nitrogen fixation estimates. This revised fixation estimate has implications for global climate models since a decreased BNF estimate would impact carbon storage potential and nitrous oxide losses.

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