Soil bacteria play a key role in cycling nutrients through ecosystems. The taxonomic composition and functioning of these bacterial communities can have implications for carbon storage and thus climate change. Bacteria are also central mediators of nitrogen cycling in soils. In order to anticipate microbial feedbacks to biogeochemical cycles, it is crucial to understand how soil bacterial communities are affected by environmental change. In particular, we are interested in the impacts of drought and nitrogen pollution, which are increasingly common stressors in this region, on soil bacteria in the grasslands of Southern California. To simulate future conditions of extreme drought and nitrogen amendments, we utilized a long-term ecological research project at Loma Ridge, CA, USA that has been active for 12 years. Within a grassland habitat, plots were set up in a factorial design with reduced precipitation and nitrogen fertilizer additions. We sampled soil in May 2019 in 10-centimeter increments down to 30 centimeters. Bacterial DNA was extracted to compare the quantity of extractable DNA between depths and sites. Additionally, microbial extracellular enzymes were assayed to compare bacterial activity. We hypothesized that bacterial abundance and enzyme activity would differ with depth and with treatment.
Results/Conclusions
We found that the quantity of extractable DNA generally decreased with soil depth, indicating that there may be a gradient of bacterial abundance within the top 30 centimeters of soil. The plots with ambient precipitation and nitrogen additions had the highest concentration of bacterial DNA, followed by the reduced precipitation and ambient nitrogen plots, then reduced precipitation and added nitrogen, and lastly ambient precipitation and ambient nitrogen. Based on these results, nitrogen fertilization may promote bacterial growth. Reduction in precipitation did not affect DNA abundance. These results suggest that nitrogen, but not drought, may have an impact on bacterial abundance. We also measured seven key extracellular enzymes, several of which are involved in soil carbon metabolism, and found that enzyme concentration generally decreased with depth. Most studies of soil microbial activity are limited to the top 10 centimeters. Therefore, our measurements of microbial enzymes in deeper soil layers may help quantify and model carbon cycling in grassland ecosystems.