UC Riverside

Microbial nitrogen cycling in hot desert soils is dominated by bursts of biogeochemical metabolic activity during short periods of water availability. Both edaphic traits of the soil ecosystem and the functional traits of the soil microbiome influence the transformation of nitrogen during this process. The aim of this research is to determine how soil bacteri and fungi transform mineral N and how air pollution in the form of atmospheric nitrogen deposition (N-dep) impacts nitrogen cycling. I used a set of desert field sites across a nitrogen deposition gradient in order to investigate soil microbiomes with increasing exposure to chronic atmospheric N-dep. In Chapter 1, I track bacterial and fungal communities over time in response to N addition to show patterns of community change corresponding to NO emissions. In Chapter 2, dry soils were taken from across the N-dep gradient and incubated with 15NH4 in the lab in order to stimulate nitrogen assimilation by soil bacteria. Using this heavy isotope substrate enriched nitrogen assimilating bacterial DNA with 15N, allowing me to determine the degree of growth using DNA-quantitative stable isotope probing (DNA-qSIP). I then sequenced the bacterial 16S ribosomal gene from this qSIP-processed DNA and analyzed the sequence data to calculate isotope incorporation by each bacterial species. I found that with increasing exposure to N-dep, there was a more phylogenetically diverse group of assimilators. Furthermore, assimilation of NH4 across the N-dep gradient was dominated by 5 bacterial orders: Rhizobiales, Burkholderiales, Frankiales, Cytophagales, and Sphingomonadales. In Chapter 3, rewetting experiments were performed at 4 desert field sites with increasing annual loads of N-dep in order to stimulate bacterial metabolic activity. Metagenomes were sequenced in order to determine which nitrogen cycling functional genes were present in the soil, and which genes increase in abundance during rewetting. I found that N-dep significantly altered the composition of genes related to denitrification and organic matter decomposition, although I did not find any significant patterns of gene abundance before and after rewetting. Overall, these experiments demonstrate that nitrogen deposition alters several aspects of the desert nitrogen cycle by changing the composition and gene inventory of soil bacteria.