Drought and N addition control grassland soil GHG flux and microbial composition

Background/Question/Methods

In Southern California, nitrogen deposition is increasing due to agriculture and industry and precipitation rates are predicted to decrease due to climate change. We investigated the relationships between precipitation change, increased nitrogen deposition, soil microbial diversity, and greenhouse gas fluxes in a Southern California grassland. This factorial study consists of 48 large plots, which have been treated over the last 7+ years with increased (+50%) and decreased rainfall (-50%), crossed with increased nitrogen deposition. Hundreds of gas and soil samples were collected bi-weekly to bi-monthly from October 2011-March 2013. Gas samples were analyzed for methane, carbon dioxide and nitrous oxide. Soil sample DNA was extracted, and the V3 and V4 regions of the 16S gene were amplified and sequenced. We hypothesized that the effect of added N and water would be an increase in the net soil microbial GHG release of this grassland. In addition, we hypothesized that N addition and drought would decrease soil microbial diversity.  

Results/Conclusions

We found that soil respiration (carbon dioxide release) declined on average by >33% in the reduced precipitation treatments. The increased precipitation plots showed first increased and subsequently decreased respiration across the rainy season relative to control. Soil respiration did not respond to N treatments. Nitrous oxide production was tripled in the N addition treatment relative to the control. Nitrous oxide and carbon dioxide release were also decreased by greater than 50% in the 24h after a rain event. The rate of methane production and consumption were low and highly variable throughout the year. However, in late March the added N plots consumed more methane than the ambient plots, which on average released methane. Further, the reduced water treatment increased net methane consumption. The drought treatment microbial community was less variable than that of the control and added water treatment. Analysis is ongoing to connect the soil microbial community with the trace gas fluxes observed.