Chronic dry nitrogen inputs alter soil microbial abundance of semi-arid shrublands in Southern California

Abstract:

Fossil fuel burning and fertilizer use have increased nitrogen (N) deposition on a global scale, and in dense urban areas like Southern California, semiarid shrublands can be exposed up to 45 kgN ha^-1 yr^-1. These N inputs are likely to cause alterations in microbial abundance and community composition, which will cause changes in N and carbon (C) cycles. We used next generation sequencing and metagenomics to quantify shifts in bacterial abundance of chaparral and coastal sage scrub (CSS) shrublands exposed to 50 kgN ha^-1 yr^-1 for 14 years. Chronic N inputs caused shits in taxonomic composition but not diversity in these shrublands. Semi-deciduous CSS exposed to N exhibited an increase in Proteobacteria, and during the fall, Bacteriodetes, while N inputs to evergreen chaparral caused a significant increase in Bacteriodetes and Firmicutes but a significant decrease in Acidobacteria. Cononical Correspondence Analysis (CCA) indicated that extractable NO₃ concentration was a strong predictor of the abundance of Proteobateria, Bacteroidetes, Actinobacteria, and Frimicutes, pH, soil moisture, and total soil C were the strongest predictors of Acidobacteria, Verrucomicrobia, and Chloroflexi abundance. These results support the hypothesis that N input promotes the growth of copiotrohpic taxa, such as Proteobateria, Bacteroidetes, and Frimicutes, and inhibits the growth of oligotrophic taxa like Acidobacteria. Given the anticipated increase in N deposition with increasing human populations and activity, changes in the microbial abundance and community composition in response to N deposition will have important consequences on ecosystem functionality and stability.