Current research
A new method for measuring nitrogen fixation in plant-bacterial symbioses
Our understanding of how environmental variables (temperature, light, soil nutrients, etc.) affect nitrogen fixation rates has been constrained by available methods for studying nitrogen fixation. We have developed a novel method for making real-time, continuous and repeatable measurements of nitrogen fixation, photosynthesis and respiration at the whole-plant scale in plant-bacterial symbioses. This method allows us to quantify interactions between carbon exchange and nitrogen fixation and how they change over rapid (seconds-minutes) to long (weeks-months) timescales.
Representative publications: Bytnerowicz, TA, E Min, KL Griffin, & DNL Menge. 2019. Repeatable, continuous and real-time estimates of coupled nitrogenase activity and carbon exchange at the whole-plant scale. Methods in Ecology & Evolution. 10: 960-970. Bytnerowicz, TA, PR Akana, KL Griffin, & DNL Menge. 2022. Temperature sensitivity of woody nitrogen fixation across species and growing temperatures. Nature Plants. 8: 209–216. Mifsud, IEJ, PR Akana, TA Bytnerowicz, SR Davis, & DNL Menge. 2023. Nitrogen fixation in the stag beetle Ceruchus piceus (Coleoptera: Lucanidae): Could insects contribute more to ecosystem nitrogen budgets than previously thought? Environmental Entomology. nvad053. |
Temperature sensitivity of nitrogen fixation
Temperature is the best predictor of global nitrogen fixing tree abundance. Understanding how nitrogen fixation rates change with temperature is also critical for making accurate predictions of current and future global nitrogen and carbon dynamics in terrestrial biosphere models.
Using the method that we developed (Bytnerowicz et al. 2019 Methods in Ecology & Evolution), we have made precise measurements of the temperature sensitivity of nitrogen fixation while testing the roles of taxonomy (actinorhizal or rhizobial symbiosis), acclimation to growing temperature, adaptation to climate of origin (temperate or tropical), and how it compares to the temperature response of photosynthesis and other plant carbon fluxes. We have shown that optimal temperatures for tree nitrogen fixation acclimate to growing temperature, range from 29-37 °C, and are 5 °C higher than optimal temperatures of net leaf photosynthesis. Our temperature response functions are now beginning to be utilized in terrestrial biosphere models such as NOAA's GFLD Land Model. We are continuing work on the temperature sensitivity of nitrogen fixation through a combination of empirical investigations in partnership with Duncan Menge and Kevin Griffin at Columbia University and modeling investigations across several terrestrial biosphere models. Representative publications: Bytnerowicz, TA, PR Akana, KL Griffin, & DNL Menge. 2022. Temperature sensitivity of woody nitrogen fixation across species and growing temperatures. Nature Plants. 8: 209–216. Kou-Giesbrecht, S, S Malyshev, I Martínez Cano, SW Pacala, E Shevliakova, TA Bytnerowicz, & DNL Menge. 2021. A novel representation of biological nitrogen fixation and competitive dynamics between nitrogen-fixers and non-fixers in a land surface model (GFDL LM4.1-BNF). Biogeosciences. 18: 4143-4183. |
Regulation of nitrogen fixation
The impact of nitrogen fixation depends on the degree to which nitrogen fixation changes in response to resource limitation. Theory says obligate (fixation independent of plant nitrogen demand) or incompletely facultative fixation (versus perfectly facultative fixation, where plants only fix the nitrogen they need) reduces the competitive ability of nitrogen fixers and increases ecosystem nitrogen losses. However, regulation strategies are largely unresolved. We are investigating nitrogen fixation strategies across several species and biomes in field and greenhouse experiments.
There is some evidence for certain species regulating nitrogen fixation in a facultative manner. Theory says that time lags in how quickly nitrogen fixation rates can be up- and down-regulated in response to changes in soil nitrogen availability can create a mismatch with the environment, leading to consequences for nitrogen fixer competitive ability and ecosystem nitrogen cycling. By applying the method that we developed (Bytnerowicz et al. 2019 Methods in Ecology & Evolution), we have been quantifying time lags in the regulation of nitrogen fixation. Representative publications: Menge, DNL, AA Wolf, JL Funk, SS Perakis, PR Akana, R Arkebauer, TA Bytnerowicz, KA Carreras Pereira, AM Huddell, S Kou-Giesbrecht, & SK Ortiz. 2023. Tree symbioses sustain nitrogen fixation despite excess nitrogen supply. Ecological Monographs. e1562. |
Successional trajectories of nitrogen fixers
On average nitrogen fixers are more common in than in older forests, however there is remarkable variation around this average pattern. By analyzing US Forest Inventory Analysis data we showed that bimodality in nitrogen fixer abundance is the norm within most of the United States, meaning that when nitrogen fixers are present, they usually are dominant within a forest plot. This is the case for both young and old forests. To probe the mechanisms that could generate this pattern, we analyzed a simple theoretical ecosystem model that allows trees to be limited by nitrogen or a secondary resource across forest succession. We analyzed the model with a combination of analytical and simulation techniques to show that the successional patterns we observed can be generated by interactions between nitrogen supply, long-term transients, priority effects, and the fixation strategy.
Representative publications: Bytnerowicz, TA & DNL Menge. 2021. Divergent pathways of nitrogen-fixing trees through succession depend on starting nitrogen supply and priority effects. American Naturalist. 198(6): E198-E214. |
Leaf nitrogen, photosynthesis, and water use efficiency in nitrogen fixing trees
Critical to the success of nitrogen fixers across the various environments that they occupy is the effect that nitrogen fixation has on leaf nitrogen, and the effect that leaf nitrogen has on carbon assimilation and water use efficiency. We have been investigating these relationships across a set of field fertilization experiments spanning tropical and temperate biomes. Specifically, we have been testing for the role of nitrogen limitation status and fixation status (nitrogen fixer vs. non-fixer) on these relationships.
Representative publications: Bytnerowicz, TA, JL Funk, DNL Menge, SS Perakis, & AA Wolf. 2023. Leaf nitrogen affects photosynthesis and water use efficiency similarly in nitrogen-fixing and non-fixing trees. Journal of Ecology. DOI: 10.1111/1365-2745.14194 |
Previous research
Growth and reproduction of a native plant (Zannichellia palustris) in river restoration
As part of an effort to restore a spring-fed river in Northern California that had been invaded by Eurasian watermilfoil, I investigated the controls over the growth and development of a native plant (Zannichellia palustris) that was dominant in the river prior to invasion. In addition to characterizing the key role of temperature in controlling the growth and germination of Zannichellia palustris, we showed that a nonlinear biophysical model outperformed traditionally used linear germination models.
Representative publications: Bytnerowicz, TA & RI Carruthers. 2014. Germination characteristics of Zannichellia palustris from a Northern California spring-fed river. Aquatic Botany. 119: 44-50. Bytnerowicz, TA & RI Carruthers. 2014. Temperature-dependent models of Zannichellia palustris seed germination for application in aquatic systems. Environmental and Experimental Botany. 104: 44-53. |