Carbon and nitrogen storage in the topsoils of Inceptisols and Mollisols under native sage scrub and non-native grasslands in southern California

In southern California, native California sage scrub is increasingly being converted to non-native grasslands, a process known as type conversion. Here we present data that was used to test if type conversion influences C and N storage in surface soil (A horizon). To better understand the factors influencing regional nutrient storage, we examined total C and total N concentration (%) and quantity (g/m2), key soil properties, and microbial abundances and assemblages in sage scrub and non-native grassland habitats at three sites. The three sites were along a coast to inland gradient spanning both Los Angeles and San Bernardino counties. The coastal site was in the Santa Monica Mountains while the most interior site was in the Crafton Hills Conservancy, 155 km east. An intermediate site was the Robert J. Bernard Biological Field Station, located 101 km east of the Santa Monica Mountains and 55 km west of Crafton Hills. To compare total C and N, soil properties, and microbial abundances, we collected soil from six sampling locations in both habitat types at each site in the spring of 2016 (March 25 through April 1). At each sampling location, we collected five different types of soil samples. First, we collected an intact soil clod from the mineral soil surface (immediately below the O horizon) to determine bulk density using a modified version of the clod method—a measure of soil density that excludes rock fragments > 2 mm diameter (description of the method is attached). Second, we gathered ~30 ml of loose soil from the top ~10 cm of the soil profile (all within the A horizon) for analyses of percent C and N using an Elementar vario MICRO cube elemental analyzer (Elementar Mt. Laurel, New Jersey). Third, we collected ~250 mL of soil from the A horizon at each sampling location to send to Earthfort Laboratories (Corvallis, Oregon) to determine total and active bacteria and fungi. Total bacteria and fungi (µg/g) were determined through direct enumeration using microscopy. Bacteria were identified using the fluorescein isothiocyante method. Total fungal biomass was determined by converting width and length measurements. Active bacteria and fungi (µg/g) were quantified using direct microscopy after staining samples with fluorescein diacetate, which binds and fluoresces to metabolically active bacteria and fungi. Fourth, a composite soil sample comprised of soil from all six sampling locations within each habitat at each site was sent to the UC Davis Analytical Laboratory to determine organic matter content, CEC, pH, and soil texture. The final type of sample was 20 mL of loose soil from each sampling location for 16S and ITS amplicon metagenomics analysis, which involves the direct sequencing of the microbiomes in an environmental sample. Molecular data can be found on GenBank.