While the invasions of exotic plant species are thought to alter ecosystem functioning, the effects of most exotic plant species on soil microbial function are not well understood. Cynara cardunculus, an exotic herbaceous, perennial thistle, is considered a serious invader of Coastal Sage Scrub (CSS) in southern California. Simultaneous increases in abundance of the native, perennial shrub Baccharis pilularis have also become a threat to CSS diversity. Here we contrast the effects of invasion by woody and non-woody species into CSS on microbial community function, as characterized by soil microbial extracellular enzyme activity. In addition, extracellular enzyme activity was monitored following herbicide-control of C. cardunculus to determine whether soil community function would return to the native state. We compared effects associated with the two invading species with those associated with Artemesia california, a native shrub typical of intact CSS communities.

Results/Conclusions

Activities of soil microbial extracellular enzymes involved in processing organic carbon (β-D-cellobiohydrolase and peroxidase), nitrogen (N-acetyl-β-glucosaminidase), and phosphorous (acid phosphatase) were significantly higher in soils associated with either C. cardunculus or B. pilularis than those associated with A. californica. The higher extracellular enzyme activities found in soils associated with C. cardunculus and B. pilularis may be due to higher relative growth rates or litter inputs of these two species. High terpinoid levels in A. californica’s litter may also be partially responsible for the observed difference in extracellular enzyme activity between species. The activities of β-D-cellobiohydrolase, peroxidase, N-acetyl-β-glucosaminidase, and acid phosphatase significantly increased following application of herbicide to C. cardunculus. This increase in activity was likely a response of the microbial community to the high levels of dead C. cardunculus tissue remaining in the system following the herbicide treatment. The increase in extracellular enzyme activity associated with range expansion of B. pilularis, invasion of C. cardunculus, and herbicide-control of C. cardunculus likely led to increased levels of available nitrogen and phosphorous in the soil. The change in microbial function and increased soil nutrient levels, combined with gaps left in the plant cover by removal of C. cardunculus, may help fuel subsequent invasion of these and other exotic species into Coastal Sage Scrub in the future.