Wednesday, August 6, 2008 - 4:40 PM

COS 75-10: The role of microbes in interspecific plant competition for nitrogen

Stephanie Yelenik1, Benjamin Colman1, Janneke Hille Ris Lambers2, and Jonathan M. Levine1. (1) University of California, Santa Barbara, (2) University of Washington


The domination of California grasslands by Mediterranean annual grasses is a spectacular example of the displacement of a native flora by exotic plant species.  To investigate the role of resource competition in this invasion, we established a manipulative experiment within an R* framework. Tilman’s R* theory states that better competitors within a nitrogen (N) limited community should have lower levels of N in monoculture plots and higher abundance in mixture plots at the end of the growing season. But is plant uptake the underlying mechanism of final N levels, as assumed by the theory? This is a contentious assumption in our system, given that microbes can be better competitors for N than plants in California grasslands.  Perhaps the level of N in monocultures is actually due to plant species effects on microbial communities and N dynamics, and not plant uptake of nitrogen.  To disentangle the relative roles of plant uptake and microbially mediated processes in resource competition, we quantified N dynamics in monoculture and mixture plot soils, as well as N in plant and microbial biomass. 


We found that available N in monoculture plots at the end of the growing season (R*) explains 73% of the variation in aboveground plant biomass in mixture plots, implying that resource competition is playing a large role in structuring these communities.  Positive correlations between microbial biomass N and microbial process rates (net N mineralization, substrate induced respiration, and nitrification potential) suggest that microbial abundance regulates N cycling in these soils. However, the lack of a relationship between microbial biomass N and available N pools strongly implies that while microbes are regulating N turnover, they are not driving final available N pools in monoculture plots.  Indeed, the total amount of N in aboveground plant tissues is a better predictor of R*.  Thus plant uptake of soil N is an important part of the mechanism driving interspecific plant competition in these California grasslands.