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The proliferation of non-native plant species in novel habitats has altered plant community diversity, ecosystem carbon (C) and nutrient cycling, and disturbance regimes worldwide. However, the impacts of non-native plants on soil microbial community composition and activity are still poorly known, especially in semi-arid woodlands. Here we utilized next generation sequencing (16S rRNA sequencing of the V3-V4 regions) and measurements of microbial activity (respiration, net N mineralization, and enzyme assays) to determine how soil microbial composition and activity varied between native and non-native vegetation patches in coastal sage scrub (CSS) woodlands. We sampled eight CSS sites and collected topsoil (0–10 cm) samples from six paired native and non-native vegetation patches at each site. Native patches had significantly (p < 0.05) higher vegetation cover, dissolved organic C (DOC), and dissolved inorganic nitrogen (DIN) concentrations than non-native patches. β-glucosidase activity was significantly higher (ca. 10 nM g−2 h−1) and peroxidase activity was significantly lower (ca. 60 nM g−2 h−1) in non-native vegetation, while rates of net N mineralization and nitrification, phosphatase and NAGase activities, and respiration were not significantly different in native and non-native vegetation but were positively correlated with total soil C and N (r > 0.60; p < 0.05). Bacterial alpha- and beta-diversity were not significantly different (p > 0.05) between native and non-native patches; however, bacteria/archaea in the phyla Gemmatimonadota , Planctomycetota , Armatimonadota , Fibrobacterota , Thermoplasmatota , and Entotheonellaeota were over-expressed in non-native patches. Within these phyla there were 20 orders that were differentially expressed in native or non-native vegetation, and more than half (60 %) had significantly higher relative abundance in native vegetation. Our results indicate that the expansion of non-native vegetation in semi-arid woodlands significantly alters soil C and N, which feeds back on microbial activity and community composition. • Microbial composition and activity were measured in native and non-native patches. • Native patches had higher plant cover, dissolved organic C, and inorganic N. • Non-native patches had higher β-glucosidase activity but lower peroxidase activity. • 20 bacteria/archaea orders were over-expressed in native or non-native patches. • Non-native vegetation expansion alters microbial activity and community composition. [ABSTRACT FROM AUTHOR]

Semi-arid shrublands experience chronic anthropogenic nitrogen (N) inputs and episodic pulses of N and carbon (C). Chronic N deposition can cause N saturation, leading to soil acidification and N loss; however, some ecosystems experience acidification and N loss before N saturation while others exhibit an increase in N retention with prolonged N deposition. These divergent responses may be due to variations in soil N and C availability and microbial activity. We tested whether labile C and/or N pulses affected microbial activity (respiration and net N mineralization) in soil exposed to chronic N input. We hypothesized that C pulses would increase microbial activity, but that pulsed and chronic N enrichment would decrease activity. We found that a C pulse stimulated microbial respiration and net N immobilization, while pulsed and/or long-term N enrichment significantly increased N accumulation but not microbial respiration. Nitrate retention was highest in soil exposed to a C pulse and chronic N enrichment, indicating substantial capacity for microbial N retention with sufficient labile C availability. Our data suggest that N retention in semi-arid shrublands is driven more by spatial and temporal variations in labile C availability than exceedance of N storage capacity. [ABSTRACT FROM AUTHOR]

Anthropogenic nitrogen (N) deposition has affected the primary production of terrestrial ecosystems worldwide; however, ecosystem responses often vary over time because of transient responses, interactions between N, precipitation, and/or other nutrients, and changes in plant species composition. Here we report N-induced changes in above- and below-ground standing crop and production over an 11-year period for two semi-arid shrublands, chaparral and coastal sage scrub (CSS), of Southern California. Shrubs were exposed to 50 kgN ha−1 in the fall of each year to simulate the accumulation of dry N deposition, and shoot and root biomass and leaf area index (LAI) were measured every 3 months to assess how biomass production responded to chronic, dry N inputs. N inputs significantly altered above- and below-ground standing crop, production, and LAI; however, N impacts varied over time. For chaparral, N inputs initially increased root production but suppressed shoot production; however, over time biomass partitioning reversed and plants exposed to N had significantly more shoot biomass. In CSS, N inputs caused aboveground production to increase only during wet years, and this interaction between added N and precipitation was due in part to a highly flexible growth response of CSS shrubs to increases in N and water availability and to a shift from slower-growing native shrubs to fast-growing introduced annuals. Together, these results indicate that long-term N inputs will lead to complex, spatially and temporally variable growth responses for these, and similar, Mediterranean-type shrublands. Unlabelled Image • Nitrogen (N) deposition is high in chaparral and coastal scrub (CSS) ecosystems. • Root and biomass production were measured in an N addition experiment for 11 years • N-exposed chaparral initially produced more roots but switched to shoots over time. • N-exposed CSS had higher rates of productivity during wet years only. • N responses are often variable because of interactions with water and nutrients. [ABSTRACT FROM AUTHOR]

Atmospheric nitrogen (N) deposition represents an important input of N into natural ecosystems such as semiarid shrublands of Southern California, which can receive up to 45 kg N ha−1 y−1. These N inputs will presumably alter soil microbial abundance and composition and impact ecosystem N and carbon (C) cycles. We used a 16S rRNA sequencing-based approach to characterize shifts in soil bacterial communities in chaparral and coastal sage scrub (CSS) shrublands that received annual inputs of 50 kg N ha−1 over a period of 14 years. Experimental N addition caused shifts in bacterial taxonomic composition in these shrublands. CSS exposed to N had an increase in Proteobacteria and Bacteriodetes, while N inputs to chaparral caused an increase in Bacteriodetes and Firmicutes and a decrease in Acidobacteria. Canonical correspondence analysis (CCA) indicated that extractable NH 4 and/or NO 3 concentrations were a strong predictor of Proteobacteria and Firmicutes (positive) and Acidobacteria and Verrucomicrobia (negative) abundance. Increases in soil pH were coincident with declines in Proteobacteria but increases in Acidobacteria, while increases in total C were positively correlated with Acidobacteria abundance. These results support the hypothesis that long-term N inputs in semi-arid shrublands promote the growth of copiotrophic taxa, such as Proteobacteria, Bacteroidetes, and Firmicutes, and inhibit the growth of oligotrophic taxa like Acidobacteria. Nitrogen addition failed to affect α-diversity at the phylum level but significantly increased α-diversity of bacterial genera, and indicator species analyses revealed more genera associated with N treatment plots (125) than control plots (91). These results imply that future increases in N deposition will alter soil microbial abundance and community composition, and in turn, affect ecosystem C and nutrient cycling in these semi-arid shrublands. • Chaparral and coastal sage scrub (CSS) were exposed to 50 kg N ha−1 y−1 for 14 years. • 16S rRNA sequencing was used to measure shifts in soil bacterial taxa to N inputs. • Experimental N inputs caused shifts in bacterial composition but not diversity. • N increased copiotrophs (CSS and chaparral) and decreased oligotrophs in chaparral. • Soil inorganic N and pH were strong predictors of bacterial abundance. [ABSTRACT FROM AUTHOR]