Effects of invasive plants on coastal sage scrub soil and microbial communities of Mission Trails regional park

Background/Question/Methods
The coastal sage scrub (CSS) ecosystem is an endangered ecosystem.  It once ranged along the western coast of the United States, from the central California coast into Baja California. Due to urbanization and invasive species, it now only comprises 10%-15% of its previous habitat.  It is characterized by aromatic and drought-deciduous shrub covered lands consisting of black sage (Salvia mellifera), white sage (Salvia apiana), and lemonade-berry (Rhus integrifolia) among other shrubs succulents and cacti. This study looked at the effect of invasive European Grasses on the soil and microbial communities of the Coastal Sage Scrub ecosystem of Mission Trails Regional Park.  Soil samples were collected on a monthly basis under shrubs and from gaps between shrubs in two sites, which differed in the presence of invasive grasses, particularly Bromus madritensis.  Soil temperature, respiration rate, organic matter content, water content and microbial biomass C (by chloroform fumigation-extraction) were measured on each sample. Substrate induced respiration (SIR), using glucose and salicylic acid as substrates, was performed to determine the effect of invasive plants on the metabolism of carbon by microbial communities.

Results/Conclusions

Controlling for temperature and soil moisture, there were no significant effects of plant type or site on soil respiration, measured in situ. Significantly lower water content was observed in the highly invaded site, especially in gaps between shrubs where invasive grasses flourished. Glucose and salicylate SIR were not significantly affected by site or vegetation type.  However, microbial biomass C was found to be lower in the highly invaded site relative to the less invaded site. Interestingly, this reduction in microbial biomass C was compensated by an increase in the biomass-specific activity of the biomass, as shown by a higher ratio of glucose SIR to microbial biomass C in the invaded site. These results show evidence for the alteration of the habitat by invasive plants, particularly in terms of the water cycle, the microbial community and how it processes plant litter. These alterations could have important implications for how the plant community and C balance of CSS ecosystems interact with climate change in the future.