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<br />io <br />which dominate the site after the third year following disturbance, <br />are less effective (than early-seral annuals) accumulators of soil <br />N at high availability levels, but more effecting as soil N becomes <br />limited. Therefore, as time passes and increasing amounts of soil <br />N become incorporated in perennial biomass and the detritus <br />subsystem, these species begin to have greater competitive <br />advantage. Later mid-seral dominants, shrubs in our system and <br />exemplified by Chrvsothamnus nauseosus, appear to be the most <br />effective competitors for limited soil N since the content of this <br />species did not decrease along the gradient. <br />The relationship between secondary succession and soil N <br />availability indicated by our results (i.e., soil N decreases <br />during early seral stages and characteristic compositional changes <br />take place largely in response to this decrease) is opposite to <br />that reported by Tilman (1987) for a Minnesota sandplain but <br />similar to that reported by Carson and Barrett (1988) for an old- <br />field in Ohio. Tilman (1987) found that (1) as secondary <br />succession proceeded on the sandy sites of his study, soil N <br />increased and (2) that N addition to these soils caused early <br />successional species to decrease in abundance and later <br />successional species to increase. we attribute these primary <br />differences in results to primary differences in response patterns <br />between N-poor soils such as studied by Tilman (1984, 1986, 1987) <br />and soils such as on our study site that are not N-poor. Tilman <br />(1987:212-213) suggested such a distinction and probable difference <br />