Russian olive (Elaeagnus angustifolia L.) is an invasive non-native tree in western North America capable of nitrogen fixation through symbiotic actinorhizal associations. The high abundance of the tree may have important effects on ecosystem nutrient dynamics and consequent community responses. This study explored the influence of Russian olive on soil nitrogen along a section of the Rio Grande River riparian zone in Rio Grande Valley State Park, Albuquerque, New Mexico. Cottonwood trees (Populus deltoides, var. wislizenii) without subcanopy trees were paired with cottonwood having a Russian olive under the canopy, and soil nitrogen pools were assessed. Relative to open interplant areas, soils under subcanopy Russian olive showed a 55% increase in total nitrogen and nearly four times the amount of available nitrogen compared to soils under cottonwood alone. Organic matter accumulation followed a similar pattern relative to open areas with 73% more accumulation under subcanopy Russian olive compared with cottonwood trees alone. Acetylene reduction and δ15N revealed that nitrogen fixation occurred in Russian olive at the site during the time of sampling, and foliar chemistry averaged 2.58% nitrogen for Russian olive compared to 0.54% for cottonwood. Both soil texture and Russian olive presence proved to be important factors affecting the observed soil nitrogen patterns. Despite these nitrogen inputs, cottonwood trees appeared to utilize sources other than that derived from Russian olive. A soil respiration experiment assessed the influence of Russian olive on soil microbial productivity, which revealed that soils at this location are carbon limited and not nitrogen limited. Even after imposing a nitrogen limitation through the addition of carbon, Russian olive did not affect microbial productivity despite higher nitrogen levels in soils associated with it. The results of this study show that Russian olive enhances soil nitrogen resources in this semi-arid riparian environment, but the added nitrogen is not likely to facilitate cottonwood tree growth or further exotic weed invasion. Russian olive may instead compete with cottonwood for other resources.
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Vol. 28 • No. 4