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Atmospheric nitrogen (N) deposition is enriching soils with N across biomes. Soil N enrichment can increase plant productivity and affect microbial activity, thereby increasing soil organic carbon (SOC), but such responses vary across biomes. Drylands cover ~45% of Earth's land area and store ~33% of global SOC contained in the top 1 m of soil. Nitrogen fertilization could, therefore, disproportionately impact carbon (C) cycling, yet whether dryland SOC storage increases with N remains unclear. To understand how N enrichment may change SOC storage, we separated SOC into plant‐derived, particulate organic C (POC), and largely microbially derived, mineral‐associated organic C (MAOC) at four N deposition experimental sites in Southern California. Theory suggests that N enrichment increases the efficiency by which microbes build MAOC (C stabilization efficiency) if soil pH stays constant. But if soils acidify, a common response to N enrichment, then microbial biomass and enzymatic organic matter decay may decrease, increasing POC but not MAOC. We found that N enrichment had no effect on C fractions except for a decrease in MAOC at one site. Specifically, despite reported increases in plant biomass in three sites and decreases in microbial biomass and extracellular enzyme activities in two sites that acidified, POC did not increase. Furthermore, microbial C use and stabilization efficiency increased in a non‐acidified site, but without increasing MAOC. Instead, MAOC decreased by 16% at one of the sites that acidified, likely because it lost 47% of the exchangeable calcium (Ca) relative to controls. Indeed, MAOC was strongly and positively affected by Ca, which directly and, through its positive effect on microbial biomass, explained 58% of variation in MAOC. Long‐term effects of N fertilization on dryland SOC storage appear abiotic in nature, such that drylands where Ca‐stabilization of SOC is prevalent and soils acidify, are most at risk for significant C loss. [ABSTRACT FROM AUTHOR]

Urban landscaping conversions can alter decomposition processes and soil respiration, making it difficult to forecast regional CO2 emissions. Here we explore rates of initial mass loss and net nitrogen (N) mineralization in natural and four common urban land covers (waterwise, waterwise with mulch, shrub, and lawn) from sites across seven colleges in southern California. We found that rates of decomposition and net N mineralization were faster for high-N leaf substrates, and natural habitats exhibited slower rates of decomposition and mineralization than managed urban landcovers, especially lawns and areas with added mulch. These results were consistent across college campuses, suggesting that our findings are robust and can predict decomposition rates across southern California. While mechanisms driving differences in decomposition rates among habitats in the cool-wet spring were difficult to identify, elevated decomposition in urban habitats highlights that conversion of natural areas to urban landscapes enhances greenhouse gas emissions. While perceived as sustainable, elevated decomposition rates in areas with added mulch mean that while these transformations may reduce water inputs, they increase soil carbon (C) flux. Mimicking natural landscapes by reducing water and nutrient (mulch) inputs and planting drought-tolerant native vegetation with recalcitrant litter can slow decomposition and reduce regional C emissions. [ABSTRACT FROM AUTHOR]

Nitrogen (N) deposition in heavily polluted southern Californian shrublands is estimated to be 20-45 kg Nha-1yr-1, but more exposed locales can receive as much as 145 kg Nha-1yr-1. This large anthropogenic N input has the capacity to alter the composition of plant communities. We conducted N-fertilization experiments in chaparral and coastal sage scrub (CSS) stands over a five-year period to test the hypothesis that plant community composition would change in response to dry-season N addition because of an increase in the relative abundance of herbaceous plant species. Our results indicate that dry-season addition of N significantly altered the community composition of CSS but not chaparral. Contrary to our original hypothesis, changes in community composition were due to changes in the relative abundance of dominant shrubs and not herbaceous plant species. Given that community-level responses to changes in resource availability may take years to decades in order to fully materialize, our results suggest that continued dry-season-input of N will cause even larger changes in community composition over time. These results have implications for plant species composition and diversity of mediterranean-type shrublands as N deposition increases with population growth and fossil-fuel use. [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]