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Fire is a natural disturbance in many ecosystems such as semi-arid chaparral shrublands, but rates of vegetation regeneration may be slow after intense fires. Thus, land managers may resort to practices such as mulching or seeding in areas that are prone to soil erosion and nutrient loss. These practices, in particular seeding with annual grasses, are controversial because they may inhibit rates of natural vegetation regeneration, introduce exotic species, and be ineffective at soil and/or nutrient retention. We assessed how hydroseeding affected rates of chaparral vegetation regrowth and ecosystem nitrogen (N) storage during the first 2 years of post-fire recovery. We selected three north-facing slopes that were within 500 m of each other: one slope was unburned while the other slopes burned during the “Cocos” fire in May 2014 and were either hydroseeded with a mix of native grasses and suffrutescent shrubs (seeded) or left to naturally regenerate (control). We hypothesized that seeding would reduce native woody shrub cover and production but that ecosystem N storage would be higher in the seeded stand. Seeding significantly reduced woody shrub cover, aboveground biomass, shrub and total species richness, and rates of shrub growth. However, herbaceous biomass, dominated by grasses that were absent on the naturally regenerating slope, was significantly higher in the seeded areas, causing significantly higher total vegetation cover in the seeded area. The higher herbaceous cover led to an increase in plant N storage and a reduction in soil extractable NO 3 . Hydroseeding also caused a reduction in potential N mineralization, and during the growing season, an increase in soil microbial biomass. Our results suggest that seeding recently burned chaparral slopes reduces nutrient loss but at the expense of native plant abundance and diversity. While the long-term effects of seeding on vegetation composition are unknown, changes in vegetation from a shrub- to a grass-dominated system may fundamentally alter future fire regimes. [ABSTRACT FROM AUTHOR]

High atmospheric N inputs to southern Californian shrublands can alter organic matter quantity and quality and influence decomposition and N mineralization rates. We conducted a laboratory experiment to test the hypothesis that N addition would increase microbial respiration and net N mineralization. Intact soil cores and litter from Artemisia californica Less, were collected from field plots exposed to either ambient or elevated N (50 kg N ha-1 yr-1) over a 7-yr period. Microbial respiration was significantly higher in control soil, and litter amendment, regardless of origin, significantly increased respiration. In contrast, net N mineralization was significantly higher in added N soil, and litter addition significantly depressed net N mineralization. Our results suggest that chronic N deposition causes a decline in litter decomposition and microbial N demand. If indicative of the longer-term decomposition kinetics, these results suggest that chronic N inputs may increase soil C storage and N availability in these semiarid shrublands. [ABSTRACT FROM AUTHOR]

Semiarid chaparral and coastal sage shrublands of southern California have been exposed to high levels of atmospheric N for decades, which has the capacity to increase both N and C storage and cycling in these N-limited systems. Thus we hypothesize that soil C and N mineralization will be higher in areas that have been exposed to high atmospheric N deposition. This hypothesis was tested in a 50-wk laboratoiy incubation experiment where the inorganic N (NH4 + NO3) and CO2 production of chaparral and coastal sage soils were repeatedly measured. Soil was incubated in the dark at a constant temperature of 25°C and a soil moisture of 0.25 kg H2O kg-1 dry soil (65% water-filled pore space). Relative differences in N deposition exposure between the study sites were quantified by repeatedly rinsing and collecting the N accumulated on branch surfaces during 1 yr. Temporal trends in cumulative C and N mineralization were best described by single-pool first-order and zero-order models, respectively. Total N mineralization, but not C mineralization, increased linearly with relative N deposition, and NO3 accounted for 95% of the total inorganic N accumulated during the 50-wk incubation. The soil δ15N natural abundance increased with relative N deposition (r = 0.85, P < 0.05) and the soil C/N ratio declined with relative N deposition (r = -0.74, P < 0.05), suggesting that N deposition exposure enhanced N mineralization in part because of increases in the soil organic matter quality (i.e., lower C/N ratio). Furthermore, soil C storage declined as a function of relative N deposition exposure, indicating that high atmospheric N inputs are not likely to stimulate soil C storage in these semiarid ecosystems. [ABSTRACT FROM AUTHOR]

Large inputs of atmospheric N from dry deposition accumulate on vegetation and soil surfaces of southern Californian chaparral and coastal sage scrub (CSS) ecosystems during the late-summer and early-fall and become available as a pulse following winter rainfall; however, the fate of this dry season atmospheric N addition is unknown. To assess the potential for dry season atmospheric N inputs to be incorporated into soil and/or vegetation N pools, an in situ N addition experiment was initiated in a post-fire chaparral and a mature CSS stand where 10 × 10 m plots were exposed to either ambient N deposition (control) or ambient +50 kg N ha−1 (added N) added as NH4NO3 during a single application in October 2003. After 1 year of N addition, plots exposed to added N had significantly higher accumulation of extractable inorganic N (NH4−N + NO3−N) on ion exchange resins deployed in the 0–10 cm mineral soil layer and higher soil extractable N in the subsurface (30–40 cm) mineral soil than plots exposed to ambient N. Chaparral and CSS shrubs exposed to added N also exhibited a significant increase in tissue N concentration and a decline in the tissue C:N ratio, and added N significantly altered the shrub tissue δ 15N natural abundance. Leaching of inorganic N to 1 m below the soil surface was on average 2–3 times higher in the added N plots, but large within treatment variability cause these differences to be statistically insignificant. Although a large fraction of the added N could not be accounted for in the shrub and soil N pools investigated, these observations suggest that dry season N inputs can significantly and rapidly alter N availability and shrub tissue chemistry in Mediterranean-type chaparral and CSS shrublands of southern California. [ABSTRACT FROM AUTHOR]

Semi-arid shrublands in southern California are exposed to high levels of atmospheric dry nitrogen (N) deposition, which can alter soil microbial activity, and thus, ecosystem carbon (C) and N cycling and storage. We used archived soil samples to assess the relative effects of experimental N enrichment on the potential activity of key C, N, and phosphorus (P) cycling enzymes in chaparral and coastal sage scrub (CSS) shrublands over a 14 year period. Chaparral plots exposed to N exhibited a significant decline in β-glucosidase and phosphatase activity and an increase in peroxidase activity over time, while N exposure had no consistent effect on N -acetylglucosaminidase (NAGase) activity. In contrast, N exposure did not significantly affect the activity of any of the measured enzymes in CSS. However, ANCOVA results revealed that the potential response of some enzymes to cumulative N input was affected by annual rainfall, because significantly more N accumulated in the soil during times of low rainfall. These results indicate that chronic dry season N inputs to semi-arid shrublands can fundamentally alter potential enzyme activity, but the impact depends on climate and vegetation type. • N responses of enzymes were measured in a long-term (14 year) field experiment. • Chaparral β-glucosidase and phosphatase declined and peroxidase increased with N. • Coastal sage scrub enzyme activity was not affected by N. • N responses of some enzymes in both shrublands depended on annual rainfall. [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]