This research will investigate the possibility that plants may influence the availability of nitrogen (N) in soil surrounding their roots. This came to light when an unexpected alternation between time of maximum N uptake by plants (during the first half of the growing season) and maximum uptake by soil microbes (in the later half) was observed under alpine conditions, despite the fact that moist conditions would seem more amenable to microbial growth early in the season. Exploring possible explanations for the observed pattern led to two hypotheses. First, as a null hypothesis plant-microbe competition for N may be minimized by growth constraints on both plants and microbes. Plants may assimilate N when it is required for growth in the spring, at which time soil microbes lack the carbon to engage in N assimilation and growth, and so remain quiescent. Later, when plant requirement for carbon (C) and N decreases, and C becomes available to microbes through increased exudates, microbes begin to assimilate N and grow. Second, as an alternate hypothesis direct biotic competition for N may be important. Plants may acquire soil N during times of predicted high microbial activity by direct manipulation of N cycling in the rhizosphere through plant exudates. %%% Nitrogen cycling in the rhizosphere is an important focus for study because long term productivity in an ecosystem is determined by how effectively fixed nitrogen is conserved within the system. If at any given time, soil available N is not being maximally assimilated by either plants or microbes, that N is susceptible to loss via leaching or denitrification. Such loss can result in nitrate pollution of surface waters or groundwaters, denitrification of NO3+ into N20 (a greenhouse gas and a reactant in the degradation of ozone in the ionosphere), and depletion of ecosystem N with consequent loss of productivity. The approach taken here should make it possible to evaluate nutrient status of an ecosystem such as California grassland and assess how it differs with change in species composition. Future ecosystem function might also be predictable with this knowledge, particularly as a function of shifts in rainfall, temperature, and nitrogen deposition in the context of global change models.
