Impacts of invasive annuals on soil carbon and nitrogen storage in southern California depend on the identity of the invader

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 CO 2 concentrations.

Increasing inputs of invasive N‐fixing Acacia litter decrease litter decomposition and associated microbial activity in streams

“…Although PERMANOVA is used mostly to perform multivariate analysis, when ran on Euclidean distance it holds the same F-statistic value and the equivalent p-value (obtained by permutations) as ANOVA analysis (Anderson et al, 2008). Therefore, PERMANOVA can be used as a nonparametric test to perform univariate ANOVAs (e.g., Ainley & Bishop, 2015;Caspi et al, 2019;Orozco-Aceves et al, 2015;Seena et al, 2017;Seena et al, 2019).…”

“…Although PERMANOVA is used mostly to perform multivariate analysis, when ran on Euclidean distance it holds the same F ‐statistic value and the equivalent p ‐value (obtained by permutations) as ANOVA analysis (Anderson et al., 2008). Therefore, PERMANOVA can be used as a nonparametric test to perform univariate ANOVAs (e.g., Ainley & Bishop, 2015; Caspi et al., 2019; Orozco‐Aceves et al., 2015; Seena et al., 2017; Seena et al., 2019). Univariate PERMANOVAs were run on Euclidean distance, with residuals permutated under a reduced model of 9,999 permutations, followed by pairwise comparison tests when significant differences were found (Anderson et al., 2008).…”

Vegetation‐type conversion of evergreen chaparral shrublands to savannahs dominated by exotic annual herbs: causes and consequences for ecosystem function

“…The fibrous short‐lived roots of annual grasses are less dependent on mycorrhizal fungi than coarser chaparral roots (Chen and Brassard, 2013). Extensive fungal hyphae can aid water and nutrient uptake, participate in hydraulic redistribution, and add carbon to the rhizosphere, and these effects are likely altered in VTC areas (Caspi et al, 2019). Key questions remain about how VTC alters the microbiota and the implications for species interactions, ecosystem function, and plant–soil feedbacks (Treseder et al, 2018; Phillips et al, 2019b).…”

Plant Invasion by Chromolaena Odorata Alters Soil Microbiome and Provides Insight Into the Role of the Fcb Group of Bacteria.

“…Organic matter is important for nutrient storage and turnover, soil structure, moisture retention and availability, pollutant degradation, and carbon sequestration. Sites with greater C and N storage are associated with high bacterial concentrations and high cation exchange capacities (Caspi et al 2019). Thus, these results suggest that acidic pH, along with nutrient cycling, plays an important role in controlling C. odorata invasion.…”