Accurately predicting terrestrial carbon (C) and nitrogen (N) storage requires understanding how plant invasions alter cycling and storage. A common, highly successful type of plant invasion occurs when the invasive species is of a distinctly different functional type than the native dominant plant, such as shrub encroachment throughout the US west, annual grass invasions throughout the Great Basin, and in Mediterranean-climate shrublands, as studied here. Such invasions can dramatically transform landscapes and have large potential to alter C and N cycling by influencing storage in multiple pools. We used a 3-year manipulation of non-native annual grass litter within a shrub-dominated habitat of southern California (coastal sage scrub) to study how grass invasion alters ecosystem C and N storage.
Grass litter greatly increased storage in plant litter, aboveground native and non-native biomass, and soil. Aboveground litter storage increased due to a 15-fold increase in NPP, and 2.7 times slower decomposition due to the low decomposition rate of non-native grass litter and reduced photodegradation of all non-native and native litter. Soil C and N pools in areas of high litter increased up to 20% in only two years following manipulation and were consistent sinks for C and N, while areas with low litter cover were sources. We synthesize our results into a C cycle of invaded and uninvaded areas of CSS and link striking changes to associated increases in the soil fungi:bacteria ratio, increased plant inputs, and decreased litter loss. Overall, we show that grass, especially through its litter, controls important abiotic and biotic mechanisms governing C and N storage in multiple pools, with widespread implications for semi-arid systems undergoing grass or shrub invasions.