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<br />13 <br /> <br />requirements. As resources become less available, the slower- <br />growing but less resource demanding species begin to dominate. <br />Nitrogen is a major resource required for,the production of <br />plant biomass and the rate of secondary succession is significantly <br />influenced by its availability. In the semiarid system we studied, <br />the rate of secondary succession is inversely related to the <br />availability of soil N and the decomposer subsystem is a primary <br />component in the supply of this resource. The application of <br />sucrose carbon decreased available soil N03-N levels in the field <br />plots of our study. Similar results have been reported in <br />laboratory systems utilizing glucose (Lamb 1980) and starch <br />(Robertson 1982) and have been attributed to greater N use by <br />heterotrophic microorganisms which have increased competitive <br />advantage as C:N ratios increase. A similar reduction in available <br />soil N levels may occur during early secondary succession as large <br />amounts of litter are quickly added to the system. Therefore, <br />decomposers appear to fill an important role in the successional <br />process which is at least two-fold: the replenishment of available <br />soil N through mineralization and the reduction of available soil <br />N through immobilization. The rate of secondary succession may be <br />slowed by an increase in mineralization and accelerated by <br />increased immobilization. We do not suggest that the levels of <br />soil N (available or total) eventually reached during late seral <br />stages are higher or lower than those of early seral stages, but <br />that available soil N decreases early during secondary succession, <br />at least temporarily, because of immobilization and this decrease <br />