Effect of nitrogen on photosynthetic responses of Bromus tectorum

Background/Question/Methods

Bromus tectorum is the most widely distributed invasive species in the Great Basin and has the largest ecological impact in the region.  The success of B. tectorum is measured not only by how widely it spreads and the different environments it colonizes, but also by how the species modifies the environment to favor its long-term persistance.  In the Great Basin as in many other plant communities, nitrogen is a limiting element.  However nitrogen availability has increased in many ecosystems because of a rise in the deposition of nitrogen derived from anthropogenic sources. A positive relationship between B. tectorum and nitrogen availability has been previously documented.  However, the mechanisms responsible for such response remain unclear.  One possibility is that an increase in nitrogen availability leads to an increase in the photosynthetic rate of B. tectorum. In this study we aim to relate leaf nitrogen content to assimilation rates in B. tectorum growing under greenhouse and field conditions.  In the greenhouse, six modified Hoagland solutions were used to control available nitrogen.  In the field, available nitrogen will be directly estimated from soil samples.  Assimilation rates were measured using a gas exchange system; leaf nitrogen content will be obtained using a CH&N analyzer.

Results/Conclusions

Based on preliminary data, nitrogen treatments greatly increased biomass production.  A positive relationship was found between nitrogen treatment and number of tillers produced in plants grown in the greenhouse.  In terms of the effects of nitrogen treatment on assimilation rates of B. tectorum, a positive relationship was found between maximum assimilation rates and nitrogen treatments in B. tectorum individuals grown in the greenhouse.  When comparing the response of assimilation to light, individuals in the intermediate nitrogen treatments (half strength Hoagland) showed higher maximum assimilation rates.  More data needs to be added to the analyses in order to confirm the observed trends.  Changes in the assimilation rate of B. tectorum in response to changes in the available nitrogen may potentially lead to changes in the distribution of this invasive species in the Great Basin.