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Experiments

Location and site description

BioCON is located at the Cedar Creek Ecoscience Reserve in east central Minnesota, USA about 50 km north of Minneapolis/St. Paul (Lat. 45N, Long. 93W). The site is located on a glacial outwash sandplain and production is nitrogen limited. The experiment was set up in a secondary successional old field after the existing vegetation was cleared. Plots were planted in 1997. BioCON consists of 371 2-meter x 2-meter plots, arranged into 6 circular areas or “rings” (20 meter diameter), each containing 61, 62, or 63 plots (plot maps). Sixteen species of herbaceous perennial prairie species, native or naturalized to the Cedar Creek area, were planted in the experiment (species description).

Design

Main Experiment

BioCON is a split-plot arrangement of treatments in a completely randomized design. CO2 treatment is the whole-plot factor and is replicated three times among the six rings. The subplot factors of species nmber and N treatment were assigned randomly and replicated in individual plots among the six rings. For each of the four combinations of CO2 and N levels, pooled across all rings, there were 32 randomly assigned replicates for the plots plant to 1 species (2 replicates per species), 15 for those planted to 4 species, 15 for 9 species, and 12 for 16 species (Reich et al., 2001). This arrangement applies to the “main” experiment which utilizes 296 plots.

Sub Experiment

There is also a sub experiment within BioCON’s framework in which functional group and species assignments were not completely random; functional group diversity was controlled thereby limiting the choices for species composition. The spatial distribution of plots within the rings was still randomly chosen. See Reich et al., 2004 for a description of design and analysis. The 296 main experiment plots are still utilized in analyses for this part of the study; the species assignments for these plots were necessary to complete the factorial design for functional vs. species diversity analyses. In total there are 371 plots in the BioCON experiment: 296 plots in the main randomly assigned experiment, 63 additional plots for controlling functional group diversity, and 12 bare ground plots, void of any plant species.

Description of Treatments

Initial Seeding and Biodiversity Treatment

Each plot was planted with 12g/m2 of seed, split equally among the planted species. Each plot was seeded with 0, 1, 4, 9, or 16 species randomly selected** (without replication) from a 16 species pool equally divided among 4 functional groups (species descriptions). In 1997 the plots were watered regularly to ensure germination and establishment. The plots were not watered after 1997. Species composition is controlled by hand weeding the experiment 2-4 times per growing season. The 16 species in the experiment were chosen from the following criteria: A species had to (1) be native or naturalized to the area; (2) have a proven track record of establishment in previous experiments at Cedar Creek; (3) belong to 1 of 4 functional groups: leguminous forbs, non-leguminous forbs; C3 grasses, and C4 grasses. The table below lists where the seed for each species was purchased.

Species	Where purchased
Andropogon gerardi, Asclepias tuberosa, Amorpha canescens, Bouteloua gracilis, Schizachyrium scoparium, Sorghastrum nutans, Solidago rigida	Prairie Restorations, Inc. (Princeton location)
Anemone cylindrica, Koeleria cristata, Lespedeza capitata, Lupinus perennis, Petalostemum villosum	Prairie Moon Nursery
Achillea millefolium	Stock Seed Farms
Agropyron repens	V & J Seed Farms P.O. Box 82 Woodstock, IL 60098 Phone# 815-338-4029 Fax# 815-338-4029
Bromus inermis, Poa pratensis	Stock Seed Farms??

**the main experiment species selections were completely random, the sub-experiment species selections were subject to more restrictions (see design above).

Nitrogen Treatment and 15N

In order to investigate the effects of nitrogen enrichment, half of the plots receive a nitrogen addition of 4g/m2/year (this amount approximately doubles current rates of N deposition at the site). This is applied as 34% ammonium nitrate pellets (regular lawn or agricultural fertilizer) 3 times per growing season: mid-May, mid-June, and mid-July. The plots that receive N amendments also receive 15N enrichment at the time of fertilization 3 times per growing season. Each plot receives 0.1058g of 5 atom % 15N (as 15NH415NO3 ) per application. In the other half of the plots, the available nitrogen is the ambient amount present in the soil at the start of the experiment. These plots are not enriched with 15N.

Carbon Dioxide Treatment

CO2 enriched air is applied to 3 of the 6 rings; ring 1 (61 plots), ring 3 (62 plots), and ring 5 (61 plots) receive air with a CO2 concentration of 560 ppm. The other 3 rings: ring 2 (62 plots), ring 4 (62 plots) and ring 6 (63 plots) receive ambient air with a CO2 concentration of approximately 368 ppm. The CO2 enrichment is accomplished using a Free Air CO2 Enrichment (FACE) technology developed at Brookhaven National Laboratory. The vegetation is exposed to elevated CO2 seven days per week, during daylight hours, for the full growing season (roughly May 1 to October 15).

Interactive effects of carbon dioxide, water, and nitrogen on grassland ecosystem processes (Water component)

Data

Summary

In the BioCON experiment, we have manipulated biodiversity, nitrogen, CO2 and water allowing us to examine how each of these factors affect ecosystem processes independently and in combination. This will allow us to determine whether the effects of one of these environmental changes enhances or dampens the effects of the others, and to what degree. The fact that BioCON is being done under field conditions rather than at a growth chamber or greenhouse scale also adds to our ability to interpret the results in the context of global change issues. BioCON is expected to operate for 10 years or longer.

Literature Cited

Reich P. B., J. Knops, D. Tilman, J. Craine, D. Ellsworth, M. Tjoelker, T. Lee, D. Wedin, S. Naeem, D. Bahauddin, G. Hendrey, S. Jose, K. Wrage, J. Goth, and W. Bengston. 2001. Plant diversity enhances ecosystem responses to elevated CO2 and nitrogen deposition.(.pdf) Nature 410:809-812.

Reich P. B., D. Tilman, S. Naeem, D. S. Ellsworth, J. Knops, J. Craine, D. Wedin, and J. Trost. 2004. Species and functional group diversity independently influence biomass accumulation and its response to CO2 and N. (.pdf) Proceedings of the National Academy of Sciences of the United States of America 101:10101-10106.