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

Abstract

Understanding how invasive plants influence terrestrial carbon (C) and nitrogen (N) budgets is important in the context of global climate change. In southern California, type-conversion, a process in which native California sage scrub is type-converted to non-native grassland, is thought to negatively impact total C and N storage in surface soil horizons. To better understand the extent to which type-conversion influences regional nutrient storage, we examined C and 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 that represent varying environmental conditions. Type-conversion decreased soil C concentration, but did not influence C quantity. Differences between these two metrics were driven by a higher aggregate soil density in the non-native grassland habitat compared to the sage scrub habitat at one site. Contrary to previous studies, we found that type-conversion did not impact total N storage, even in a site previously found to have increased soil N quantities under sage scrub. Sage scrub habitats contained more active fungi, and differences in microbial assemblages were found between habitat types. Despite the vast number of microbial OTUs, habitats harbored unique communities of microbial taxa with some species consistently more abundant in one habitat type across sites. Our results demonstrate that type-conversion negatively impacts topsoil C concentrations, but accurate modeling of nutrient stocks requires consideration of the links between vegetation structure, soil properties such as soil density, and microbial communities that vary significantly across small spatial scales. Collectively, we demonstrate that invasive grasses alter microbial communities and reduce soil C storage capacity in the region.