PLANT-SOIL INTERACTIONS IN TEMPERATE GRASSLANDS

We present a conceptual model in which plant-soil interactions in gras slands are characterized by the extent to which water is limiting. Pla nt-soil interactions in dry grasslands, those dominated by water limit ation ('belowground-dominance'), are fundamentally different from plan t-soil interactions in subhumid grasslands, where resource limitations vary in time and space among water, nitrogen, and light ('indetermina te dominance'). In the belowground-dominance grasslands, the strong li mitation of soil water leads to complete (though uneven) occupation of the soil by roots, but insufficient resources to support continuous a boveground plant cover. Discontinuous aboveground plant cover leads to strong biological and physical forces that result in the accumulation of soil materials beneath individual plants in resource islands. The degree of accumulation in these resource islands is strongly influence d by plant functional type (lifespan, growth form, root:shoot ratio, p hotosynthetic pathway), with the largest resource islands accumulating under perennial bunchgrasses. Resource islands develop over decadal t ime scales, but may be reduced to the level of bare ground following d eath of an individual plant in as little as 3 years. These resource is lands may have a great deal of significance as an index of recovery fr om disturbance, an indicator of ecosystem stability or harbinger of de sertification, or may be significant because of possible feedbacks to plant establishment. In the grasslands in which the dominant resource limiting plant community dynamics is indeterminate, plant cover is rel atively continuous, and thus the major force in plant-soil interaction s is related to the feedbacks among plant biomass production, litter q uality and nutrient availability. With increasing precipitation, the o ver-riding importance of water as a limiting factor diminishes, and fo ur other factors become important in determining plant community and e cosystem dynamics: soil nitrogen, herbivory, fire, and light. Thus, se veral different strategies for competing for resources are present in this portion of the gradient. These strategies are represented by diff erent plant traits, for example root:shoot allocation, height and phot osynthetic pathway type (C-3 vs. C-4) and nitrogen fixation, each of w hich has a different influence on litter quality and thus nutrient ava ilability. Recent work has indicated that there are strong feedbacks b etween plant community structure, diversity, and soil attributes inclu ding nitrogen availability and carbon storage. Across both types of gr asslands, there is strong evidence that human forces that alter plant community structure, such as invasions by nonnative annual plants or c hanges in grazing or fire regime, alters the pattern, quantity, and qu ality of soil organic matter in grassland ecosystems. The reverse infl uence of soils on plant communities is also strong; in turn, alteratio ns of soil nutrient supply in grasslands can have major influences on plant species composition, plant diversity, and primary productivity.