University of California, Irvine Irvine, CA, United States
Background/Question/Methods
Microbial communities are key regulators in soil processes including carbon sequestration and nutrient cycling. In California, severe droughts from climate change and nitrogen pollution from urbanization are predicted to influence microbial functioning. Previous research at the Loma Ridge Global Change Experiment, an experimental grassland and coastal sage scrub ecosystem, has documented drought and nitrogen effects on plant communities and leaf litter microbiomes. However, less attention has been paid to the microbial communities in deeper soil. Rhizosphere and bulk soils contain diverse and functionally active microbiomes which can contribute to ecosystem carbon storage and nutrient availability. For this study, we characterize the microbial community of grassland soils down to a depth of 30 cm. Specifically, we determine the effect of drought and nitrogen on microbial abundance, community composition, and function estimated through extracellular enzyme activity (EEA) and functional gene abundance from metagenomic data. We test three hypotheses: 1. Drought reduces microbial abundance, EEA, and the ratio of nutrient-cycling to stress-tolerance genes. 2. Nitrogen increases microbial abundance, EEA, and the ratio of nutrient-cycling to stress-tolerance genes. 3. Community composition is altered by drought and nitrogen, which may constrain the ability for communities to respond quickly to future environmental disturbances.
Results/Conclusions
Extracellular enzyme results show that the grassland soil communities are functionally resilient to drought and nitrogen. Enzyme activity significantly decreased with depth, but there was no effect of drought or nitrogen. Likewise, microbial abundance (measured as the concentration of extractable DNA) was not affected by global change treatments in the top 10 centimeters. Samples were collected after a particularly rainy wet season, and it is therefore possible that even in the drought plots, water levels did not limit microbial growth or activity near the surface. However, at 10-30 cm, drought and added nitrogen interacted to significantly reduce microbial abundance. Metagenomic analyses of relative taxonomic abundance do demonstrate compositional differences between the communities. This indicates that while communities may be functionally similar, taxonomic shifts were required in order to achieve similar functional potentials. Further metagenomic analysis of specific functional genes and the taxa to which they belong will further elucidate how microbial communities are impacted by drought and fertilization. We find in this study that soil below the litter layer contains abundant, active, and resilient microbial communities that should be considered in addition to litter communities when quantifying microbial contributions to ecosystem nutrient cycling.
