Grassland Precipitation and N Deposition Drive Soil Greenhouse Gas Fluxes.

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. Gas and soil samples were collected bi-weekly to bi-monthly throughout the year from 2011-2012. We hypothesized that the effect of added N and water would be an increase in the soil microbial GHG release of this grassland.

N₂O production was more than twice as high in the increased nitrogen treatment relative to the control. The rate of CH₄ production and consumption was low and highly variable throughout the year. However, in late March the added N plots consumed more CH₄ than the ambient plots, which on average released methane; also, the reduced water treatment increased net CH₄ consumption. We found that soil respiration (CO₂ 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.

Overall, the addition of nitrogen stimulated both the release of N₂O and the consumption of CH₄ in this grassland. In terms of GHG forcing, these opposing effects result in no net increase in climate warming potential. The decreased water treatment caused increased methane consumption and decreased CO₂ release. The impact of drought in this grassland is therefore to decrease the overall GHG release to the atmosphere. Further molecular research is underway to understand the microbial community responses associated with the observed changes in gas fluxes.