Nitrogen fixing plants can shift the competitive balance away from stress-tolerating long lived perennials in favor of invasive plants with the ability to thrive in N enriched soils (Huenneke et al. 1990; Carson and Barrett 1988). Nitrogen enrichment may also increase the dominance of a few species, resulting in a loss of overall diversity (Wilson and shay 1990; Tilman 1987). The spread of sweetclovers (Melilotus spp.) following intentional introductions may have similar effects throughout the grasslands of the Northern Great Plains.
Yellow (M officinalis) and white (M alba) sweetclovers are biennial, nitrogen fixing legumes native to Eurasia. In western North America, they are planted for forage and as cover crops. Yellow sweetclover is drought resistant, provides cover for ducks and upland game birds, and is highly palatable to most wild and domestic ungulates, so it is also seeded into pastures and planted with grasses in rangeland revegetation. It is used or recommended by federal agencies, including the U.S. Fish and Wildlife Service, Natural Resources Conservation Service, and the U.S. Forest Service. Yellow sweetclover can be highly invasive on the Northern Great Plains and many range managers believe it is becoming more common in native rangeland throughout eastern Montana.
Sweerclover, when inoculated with Rhizobium, vigorously fixes atmospheric nitrogen and transfers a portion to the soil and neighboring plants every year in Alaska. In pure stands, yellow sweetclover can add up to 84 kg N [ha.sup.-1] on the Great Plains (Plants of South Dakota Grasslands 1970), and fields planted to a grain/sweetclover rotation in Saskatchewan had 56% greater N supplying ability after 34 years (Campbell et al. 1993). Of course these data refer to agronomic systems where plants were plowed into the soil to optimize the gain of organic nitrogen; relatively little N was lost to volatilization as would be the case in native pasture systems.
Nitrogen accumulations in short and mixed grass prairie ecosystems of the Northern Great Plains may be accelerated by the presence of yellow sweetclover. In the summer of 2000, we excavated multiple yellow sweetclover plants from naturalized stands in the native prairie of each of six eastern Montana counties and found all plants to be nodulated to some degree. Therefore, a conservative estimate for nitrogen fixation by yellow sweetclover is 40 kg N ha [yr.sup.-1], assuming a stand half as dense as in a crop system. It is estimated that 50% volatilization of N associated with the shoots and 40% mineralization of the remaining root and shoot organic N occurs during the following year (Thomas 1992), with additional mineralization in subsequent years (Harris and Hesterman 1990; Stevenson and Cole 1999). Thus, it is possible that sweet clover has the potential to release inorganic N to the mineral soil at a rate of approximately 12 kg ha [yr.sub.-1].
This conservative estimate of potential N accumulation by sweetclover is 2-3 orders of magnitude greater than estimates for symbiotic fixation in native range without sweetclover. Native legumes are usually sparse on the Great Plains (Coupland 1950; Whitman and Stevens 1952)...