This series of graphs is from Tilman et al. (1996) in the journal Nature. In this experiment at Cedar Creek*, the researchers were asking the question, what is the effect of species diversity on ecosystem productivity and sustainability in a grassland ecosystem. The researchers created 147 diversity plots ranging from random mixtures of 1-24 species. Plant productivity, nutrient capture, and nutrient leaching were measured during the second year of growth. Graph (a) was omitted for simplicity. All three graphs have the same x-axis: species richness (# of species in an area). The y-axis of graph (b) has total plant cover as a percent — the metric used to estimate plant productivity. Graph (b) shows a positive relationship between species richness and plant cover.. Graph (c) shows the relationship between species richness and nitrate (NO3-) in the rooting zone. Nitrate is an important source of nitrogen for plants. This negative relationship indicates that species rich plots can use nitrate more effectively than species poor plots. This is important as nitrogen is often a limiting nutrient. Graph (d) shows the relationship between species richness and nitrate below the rooting zone. Again a negative relationship is observed. This indicates that plots with lower species richness experience greater leaching of soil nitrogen. This loss of nitrogen via leaching has important implications for soil fertility and nitrogen emissions due to the denitrificaiton process. This science is strengthened by a concurrent study by the same group that looked for these patterns in a native grassland that was not under experimental control. All three patterns were observed in the native grasslands. (This aspects of Tilman et al. is worth emphasizing to the students regarding the attributes of high quality science – experimental observation backed by similar patterns found in nature).
**For more information about Cedar Creek: http://www.cedarcreek.umn.edu/research/
1. Using the diagram of the nitrogen cycle below highlight to students exactly where the nitrogen cycle in regulated by species diversity as suggest by Tilman et al. (i.e. lower soil concentrations of nitrate should slow rates of denitrification). What lose of nitrogen mean for soil fertility?
2. What are the implications for rates of denitrification and gaseous nitrogen emissions? How does this connect to global climate change?
3. How might higher rates of nitrate leaching affect local streams and sources of freshwater?
4. How is the above research applicable to farmers and agriculture?