Understanding how habitat modifications influence carbon (C) storage in soil is important in determining regional C budgets. California sage scrub (CSS), the native low elevation shrub dominated habitat type of Southern California, is listed as endangered (85-98% lost) by the United States Geological Survey. Urban development, changing disturbance regimes and climate change have reduced CSS to less than 10% of its original distribution. Increased disturbance regimes also facilitate the widespread invasion of non-native grasses and often complete type conversion of CSS to non-native annual grasslands. A recent study suggests that type-conversion of CSS to non-native grasslands negatively impact C storage in surface soil. However, this study was conducted at only one site, failing to account for the multiple environmental factors that may influence nutrient storage across the region. Here we report on an ongoing study that is examining how type-conversion from CSS to non-native grasslands and other environmental factors (e.g., soil properties and temperature humidity regimes) influences regional soil carbon concentrations and microbial communities at ten sites along a coast to inland gradient. We surveyed both habitats (CSS and non-native grasslands) at each site to minimize confounding variable within sites.
Preliminary results from ten sites along a coast to inland gradient (Santa Monica Mountains to Redlands/Yucaipa) reveal that site is the most significant factor determining C levels in surface soils. The highest C concentrations were in soils at the coast. High levels of C at coastal sites were positively correlated with soil properties more conducive to C storage (higher concentrations of silt and clay relative to sand, high cation exchange capacity, and increased concentrations of SOM) and higher abundances of active fungi and bacteria. Percent C was higher in CSS soils with average C concentrations ~ 25% higher than those in non-native grasslands. Since type conversion is widespread throughout Southern California, differences in C concentrations between habitat types may reveal a potentially significant change to regional scale C dynamics. However, total C, calculated by multiplying percent C by soil density, did not differ between habitats. Increased soil densities in the non-native grasslands surveyed drove differences in patterns revealed by percent C and total C analyses. Combined our results highlight the importance of understanding the effects of both plant community and soil properties, particularly soil density, on soil C if we intend to accurately model C storage across the region.