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. 

Results/Conclusions

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.