Background
Citing some climate-alarmist thinking on the subject, the authors say that due to the climatic change that is predicted to occur during the twenty-first century, "individual drought events are projected to increase in duration (Carnicer et al., 2011) and spatial extent (Burke et al., 2006)," which in turn leads them to state that "how terrestrial ecosystems will respond to the increased frequency and duration of drought is unknown, creating uncertainties about the consequences for human well-being (e.g., ecosystem services) and changes to the global carbon balance."

What was done
"In experimental mesocosms," in the words of Wurzburger and Miniat, they "manipulated soil moisture to study the effects of drought on the physiology, growth and competitive interactions of four co-occurring North American tree species, one of which (Robinia pseudoacacia) is a symbiotic N2-fixer."

What was learned
The two U.S. researchers discovered that "under drought, growth declined across all species." However, they say that it "triggered an 80% increase in nodule biomass and N accrual for R. pseudoacacia, improving its growth relative to other species."

What it means
Wurzburger and Miniat say their results suggest that "drought intensified soil N deficiency and that R. pseudoacacia's ability to fix N2 facilitated competition with non-fixing species when both water and N were limiting." Under scenarios of moderate drought, therefore, they conclude that "N2 fixation may alleviate the N constraints resulting from low soil moisture and improve the competitive ability of N2-fixing species, and as a result, supply more new nitrogen to the [entire] ecosystem."

References
Burke, E.J., Brown, S.J. and Christidis, N. 2006. Modeling the recent evolution of global drought and projections for the twenty-first century with the Hadley Center climate model. Journal of Hydrometeorology 7: 1113-1125.

Carnicer, J., Coll, M., Ninyerola, M., Pons, X., Sanchez, G. and Penuelas, J. 2011. Widespread crown condition decline, food web disruption, and amplified tree mortality with increased climate change-type drought. Proceedings of the National Academy of Sciences USA 108: 1474-1478.