A spatially explicit, empirical estimate of tree-based biological nitrogen fixation in forests of the United States

Quantifying human impacts on the N cycle and investigating natural ecosystem N cycling depend on the magnitude of inputs from natural biological nitrogen fixation (BNF). Here, we present two bottom-up approaches to quantify tree-based symbiotic BNF based on the forest inventory data across the coterminous US plus SE Alaska. For all major N-fixing tree genera, we quantify BNF inputs using (1) ecosystem N accretion rates (kg N ha-1 yr-1) scaled with spatial data on tree abundance and (2) tree N demands and percent of N derived from the atmosphere (%Ndfa) scaled with tree growth rates. We estimate that trees fix 0.32 to 0.74 Tg N yr-1 across the study area. Tree-based N fixation displays distinct spatial variation that is dominated by two genera; Robinia (55% of tree-associated BNF) and Alnus (32%); the third most important genus, Prosopis, accounted for 6%. Compared to published estimates of other N fluxes, tree-associated BNF accounted for 0.53 Tg N yr-1, similar to asymbiotic (0.29 Tg N yr-1) and understory symbiotic BNF (0.46 Tg N yr-1), while N deposition contributed 1.10 Tg N yr-1 and rock weathering 0.0035 Tg N yr-1. Overall, our results reveal previously uncharacterized spatial patterns in tree BNF (the dominant N input in 62% of grid cells) that can inform large-scale N assessments and serve as a model for improving tree-based BNF estimates worldwide. This updated, lower BNF estimate indicates a greater ratio of anthropogenic to natural N inputs suggesting even greater human impact on the N cycle.

Impact/Purpose

In order to understand and manage nitrogen inputs to watersheds, scientists and managers accurate estimates of natural, background influences. Nitrogen fixation by trees is a very important component of the nitrogen cycle, in some places being the dominant N source, for example with red alder in western Oregon and Washington and black locust trees in the eastern US. Scientists at Columbia University and USGS invited EPA scientists to collaborate on a new approach to estimate N fixation across the coterminous US and southeast Alaska. This work determined that tree-based N fixation was similar to that of understory plants or asymbiotic fixers. This estimate was lower than previous estimates, and represents the current best estimate of N-fixation in the US. The tree-based rate was approximately half the rates of total N deposition across the nation, indicating that these rates are substantial and important to consider when examining watershed N budgets.

Citation

Staccone, A., W. Liao, S. Perakis, J. Compton, C. Clark, AND D. Menge. A spatially explicit, empirical estimate of tree-based biological nitrogen fixation in forests of the United States. American Geophysical Union, Washington, DC, 42(2):e2019GB006241, (2020). [DOI: 10.1029/2019GB006241]

DOI: A spatially explicit, empirical estimate of tree-based biological nitrogen fixation in forests of the United States