9527317 Johnson Although nitrogen is frequently the most limiting nutrient in ecosystems, continuous, elevated inputs of nitrogen may cause significant ecosystem stress. Deposition of anthropogenic nitrogen has been associated with recent losses of plant as well as fungal diversity throughout Europe. Little is known about the effects of atmospheric nitrogen deposition on the structure and function of North American grasslands and shrublands, although as much as 30 kg nitrogen per hectare per year is deposited in southern California, with lesser amounts elsewhere in the US. The three principal objectives of this research will be to (1) assess species composition changes of fungi during nitrogen eutrophication in mesic to semiarid grasslands using microscopy and immunoflourescence, (2) assess the kinds of mycorrhizal fungi under nitrogen eutrophication to indicate the mutualistic or parasitic effects of fungi, and (3) conduct bioassays to determine whether inferior mutualists predominate after nitrogen eutrophication. This study will examine the effects of nitrogen eutrophication on mycorrhizae at four experimental grasslands within the Long-term Ecological Research network and at a shrub-grasslands in southern California. These grasslands in Michigan, Minnesota, Kansas, New Mexico, and California form a precipitation and fertility gradient. The effects of nitrogen euthrophication will be studied on plants, mycorrhizal fungi and soils in experiments that have previously been established and in new plots at these sites. This research is designed to assess the ranges and variances of mycorrhizal responses to nitrogen at three scales: at individual plant-root interfaces, within grasslands communities, and across a climatic and soil gradient. This is a first step toward assessing the potential effects of nitrogen eutrophication on the structure and functioning of mycorrhizae-plant interactions in grasslands in the US. This research is important because grasslands are the basis for rang e resources and beef production in North America.
