Restoration Ecology

Determination of the importance of soil nutrient availability on community composition and the success of prairie ecological restoration

Native prairie soils have limited available soil nitrogen, and prairie plants have evolved to do well under these conditions (Morgan 1997) . However, agricultural fertilization, including the addition of calcium as lime or gypsum to raise soil pH, as well as agronomic and atmospheric nitrogen deposition, have elevated inputs of inorganic nitrogen and calcium into many grassland soils. These new inputs have altered nitrogen cycling processes and facilitated the success of nitrophilic, rapidly growing plant species exotic to native grasslands (Wedin and Tilman 1996, Baer et al. 2003) . There is also evidence for altered calcium uptake and utilization in ways that could amplify the advantage of prairie-invasives over prairie-natives (McLaughlin and Wimmer 1999) . As a consequence, restoration of tallgrass prairies is often hindered by the competitive dominance of invasive old-field species (Maron and Jeffries 2001) .

One strategy for minimizing this problem has been through deliberate soil impoverishment, or reverse fertilization (Averett et al. 2002, Averett et al. 2004, Morgan 1994, Zink and Allen 1998, Baer et al. 2003) . This involves incorporating large amounts of carbon-rich organic matter into the soil, promoting an increase in soil microbial activity that leads to nitrogen immobilization. The reduction in available nitrogen is generally regarded as a temporary, albeit crucial, effect on soil fertility acting over only one to two years (Morgan 1997). This effect may be longer-term, however, as prairie plants grown in low fertility soil have lower tissue nitrogen content, reducing nitrogen cycling through leaf litter fall and fine root turnover for an extended period following the original soil carbon amendment (Wedin and Tilman 1990) .

Prairie restorations and management strategies that target soil nutrient availability can provide a rigorous test environment for ecological understanding of plant competitive dynamics and their influence on population trajectories and community structure. The contrasting responses of prairie-natives and prairie-invasives to soil resources, which hinder prairie restoration, offer a valuable opportunity to apply and test ecological understanding of competitive mechanisms and demographic controls on population growth and community composition.

Our study consists of complementary garden, restoration, and greenhouse experiments running in parallel. Each experiment will target plant responses to six soil fertility treatments consisting of combinations of increased, ambient and decreased nitrogen, and increased and ambient calcium.

•  Garden Experiment. To test demographic controls on population-level competitive outcomes, whole-plant, leaf, and root demographic responses of related prairie-native and prairie-invasive species to soil fertility are being measured. Seedlings of selected focal species are being planted into monospecific field plots. Results will be used in population growth models, which will examine life-history and allocational strategies that promote high population growth rates and in turn allow inference of long-term successional trajectories and restoration success in response to soil nutrient management (reverse fertilization).

•  Restoration Experiment. As a large-scale field test of the influence of population-level controls on community composition, we will measure community-level responses to soil nutrient levels in an experimental tallgrass prairie restoration that includes our focal study species. This experiment is being done at a scale relevant to restorationists and will delineate plant and soil characteristics that promote or retard restoration success.

•  Greenhouse Experiment. To test the influence of specific leaf characteristics and root-shoot allocation as mechanisms promoting population success, focal species used in the garden and restoration experiments are being grown in the greenhouse under varying soil nutrient conditions. By examining variation in leaf traits and plant mass allocation along this resource gradient, we will be able to quantify plant performance mechanisms that promote population success and can be used to inform species selection for ecological restoration.

References

Averett, J.M., R.A. Klips, and P.S. Curtis. 2002. Reverse fertilization reduces cover of weedy species in tallgrass prairie restoration (Ohio). Ecological Restoration 20: 127.

Averett, J.M., R.A. Klips, L. Nave, S.D. Frey, and P.S. Curtis. 2004. The effects of soil carbon amendment on nitrogen availability and plant growth in an experimental tallgrass prairie restoration. Restoration Ecology 12: 567-573.

Baer, S. G., J. M. Blair, S. L. Collins, and A. K. Knapp. 2003. Soil resources regulate productivity and diversity in newly established tallgrass prairie. Ecology 84 :724-735.

Maron, J. L., and R. L. Jeffries. 2001. Restoring enriched grasslands: Effects of mowing on species richness, productivity, and nitrogen retention. Ecological Applications 11 :1088-1100.

McLaughlin, S. B., and R. Wimmer. 1999. Tansley Review No. 104 Calcium physiology and terrestrial ecosystem processes. New Phytologist 142 :373-417.

Morgan, J. P. 1994. Soil Impoverishment: A little-known technique holds potential for establishing prairie. Restoration and Management Notes 12 :55-56.

Morgan, J. P. 1997. Plowing and seeding. Pages 193-215 in S. Packard and C. F. Mutel, editors. The Tallgrass Restoration Handbook, for prairies, savannahs, and woodlands. Island Press, Washington, D.C.

Wedin, D. A., and D. Tilman. 1990. Nitrogen cycling, plant competition, and the stability of tallgrass prairie. Pages 5-8 in D. D. Smith and C. A. Jacobs, editors. Proceedings of the Twelfth North American Prairie Conference. University of Northern Iowa, Cedar Falls, Iowa, USA.

Wedin, D. A., and D. Tilman. 1996. Influence of nitrogen loading and species composition on the carbon balance of grasslands. Science 274 :1720-1723.

Zink, T. A., and M. F. Allen. 1998. The effects of organic amendments on the restoration of a disturbed coastal sage scrub habitat. Restoration Ecology 6 :52-58.