Kinetics of nutrient uptake by roots: responses to global change

There is a growing recognition that accurate predictions of plant and ecosy stem responses to global change require a better understanding of the mecha nisms that control acquisition of growth-limiting resources. One such key m echanism is root physiological capacity to acquire nutrients. Changes in ki netics of root nitrogen (N) uptake might influence the extent to which terr estrial ecosystems will be able to sequester excesses in carbon (C) and N l oads. Despite its significant role in determining plant and ecosystem cycli ng of C and N, there is little information on whether, or how, root nutrien t uptake responds to global change. In this review various components of gl obal change, namely increased CO2 concentration, increased soil temperature and increased atmospheric N deposition and their effects on kinetics of ro ot nutrient uptake are examined. The response of root nutrient uptake kinet ics to high CO2 is highly variable. Most of this variability might be attri butable to differences in experimental protocols, but more recent evidence suggests that kinetic responses to high CO2 are also species-specific. This raises the possibility that elevated CO2 might alter community composition by shifting the competitive interaction of co-occurring species. Uptake of NH4+ and NO3- seem to be differentially sensitive to high CO2, which could influence ecosystem trajectory toward N saturation. Increased soil tempera ture might increase N and P uptake capacity to a greater extent in species from warm and fluctuating soil habitats than in species from cold and stabl e soil environments. The few available data also indicate that increased so il temperature elicits a differential effect on uptake of NH4+ versus NO3-. Root uptake kinetics are generally down-regulated in response to long-term exposure to atmospheric N deposition. The extent of this down-regulation m ight, however, vary among species, stages of succession, land-use history a nd plant demand. Nonetheless, it is suggested that root N uptake kinetics m ight be an accurate biological indicator of the ecosystem capacity to retai n N. The results reviewed here clearly highlight the scanty nature of the l iterature in the area of root nutrient absorption responses to global chang e. It is also clear that effects of one component of global change on root nutrient absorption capacity might be counterbalanced by another. Therefore , the generalizations offered here must be viewed with caution and more eff ort should be directed to rigorously test these initial observations in fut ure research.