COS 78-7
Symbiotic N2 fixation facilitates ecosystem resilience to hydroclimate variability

Wednesday, August 12, 2015: 3:40 PM
303, Baltimore Convention Center
Jeffrey M. Minucci, Odum School of Ecology, University of Georgia, Athens, GA
Chelcy Ford Miniat, USDA Forest Service Southern Research Station, Coweeta Hydrologic Laboratory, Otto, NC
Nina Wurzburger, Odum School of Ecology, University of Georgia, Athens, GA

Global climate change is predicted to shift both the mean and variance of hydroclimates. While the effect of changing mean conditions has been studied extensively, relatively little is known about how increased hydroclimate variance will affect ecosystem processes such as symbiotic N2 fixation. Increased temporal variability in precipitation, for example, may result in unexpected responses of symbiotic N2-fixers and their capacity to deliver new N to the ecosystem. The objective of this study was to determine how temporal variability in soil moisture affects the growth, ecophysiology and symbiotic N2 fixation rate of Robinia pseudoacacia, a widely distributed leguminous tree native to the Eastern US. R. pseudoacacia seedlings were grown in the greenhouse and exposed to five different 14-week moisture regime treatments: constant high moisture, constant moderate moisture, and three variable soil moisture treatments with low, moderate or high variability of wet/dry cycles.  Every two weeks, we harvested five individuals from each treatment group and measured symbiotic N2 fixation via acetylene reduction assay, and ecophysiological traits including growth, biomass partitioning, pre-dawn leaf water potential and leaf gas exchange.


Multivariate analysis of physiological responses demonstrated that increased variability in soil moisture resulted in a more severe plant response to drought, including reduced photosynthesis and biomass growth rate relative to constant moisture controls. However, these reductions were compensated for by accelerated photosynthesis and growth during wet periods. As a result, final plant biomass did not differ between moisture variability treatments and constant moisture controls. Nodule biomass increased significantly during periods of compensatory growth in the variable soil moisture treatments. These results suggest that R. pseudoacacia is highly resilient to increased soil moisture variability, and that symbiotic N2 fixation facilitates compensatory growth, and hence, this resilience. Symbiotic N2 fixation may play a critical role in alleviating both individual- and ecosystem-level impacts of climatic variance induced by global change.