Abstract
Non‐native plant invasions can alter nutrient cycling processes and contribute to global climate change. In southern California, California sage scrub (hereafter sage scrub), a native shrub‐dominated habitat type in lowland areas, has decreased to <10% of its original distribution. Postdisturbance type‐conversion to non‐native annual grassland, and increasingly to mustard‐dominated invasive forbland, is a key contributor to sage scrub loss. To better understand how type‐conversion by common invasive annuals impacts carbon (C) and nitrogen (N) storage in surface soils, we examined how the identity of the invader (non‐native grasses, Bromus spp.; and non‐native forbs, Brassica nigra), microbial concentrations, and soil properties interact to influence soil nutrient storage in adjacent native and invasive habitat types at nine sites along a coast to inland gradient. We found that the impact of type‐conversion on nutrient storage was contingent upon the invasive plant type. Sage scrub soils stored more C and N than non‐native grasslands, whereas non‐native forblands had nutrient storage similar to or higher than sage scrub. We calculate that >940 t C km−2 and >60 t N km−2 are lost when sage scrub converts to grass‐dominated habitat, demonstrating that grass invasions are significant regional contributors to greenhouse gas emissions. We found that sites with greater total C and N storage were associated with high cation exchange capacities and bacterial concentrations. Non‐native grassland habitat type was a predictor of lower total C, and soil pH, which was greatest in invasive habitats, was a predictor of lower total N. We demonstrate that modeling regional nutrient storage requires accurate classification of habitat type and fine‐scale quantification of cation exchange capacity, pH, and bacterial abundance. Our results provide evidence that efforts to restore and conserve sage scrub enhance nutrient storage, a key ecosystem service reducing atmospheric CO2 concentrations.
Highlights
The impacts of non‐native plant invasion on nutrient cycling in soil are understudied, though the effects can be dramatic (Bradley, Houghton, Mustard, & Hamburg, 2006; Ehrenfeld, 2003; Jobbágy & Jackson, 2000)
Throughout low‐eleva‐ tion southern California, the success of invasive grasses has culmi‐ nated in the widespread replacement of the native California sage scrub ecosystem by non‐native grass species in a process known as type‐conversion (Cox et al, 2014; Talluto & Suding, 2008)
Pairwise comparisons between adjacent sage scrub and non‐native forbland habitats revealed greater N storage in non‐native forblands compared to sage scrub at two sites and greater C storage in non‐native for‐ blands at one site. These findings indicate that nutrient storage may be higher in non‐native forblands compared to adjacent sage scrub habitats, but further examination focusing on Brassica spp. invasions is required
Summary
The impacts of non‐native plant invasion on nutrient cycling in soil are understudied, though the effects can be dramatic (Bradley, Houghton, Mustard, & Hamburg, 2006; Ehrenfeld, 2003; Jobbágy & Jackson, 2000). Research by Wheeler et al (2016) and Caspi et al (2018) in southern California suggests that type‐conversion has negatively impacted soil C storage in the region Both studies found decreased soil C concentrations in non‐native grasslands compared to adjacent sage scrub habitats. Because of the competitive ex‐ clusion of grasses and alteration of the soil microbial community, soil C and N storage in areas dominated by invasive mustards could differ from areas dominated by Bromus species Despite this possi‐ bility, previous studies investigating type‐conversion have not dif‐ ferentiated between invasive cover dominated by non‐native forbs like Brassica and invasive habitat composed mostly of non‐native grasses (Caspi et al, 2018; Matsuda et al, 2011; Suarez, Bolger, & Case, 1998; Talluto & Suding, 2008). By surveying surface soil horizons along a represen‐ tative southern California environmental gradient, we identify and model key factors that predict soil C and N storage throughout the region, and we further elucidate the impacts of contemporary large‐ scale type‐conversion on these key ecosystem services
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