Norton, Urszula (Univ. of California Davis, 14571 Stone Lane, Sonora, CA, 95370; Phone: 209-588-2785; Fax: 209-532-8978; Email: UNorton@ucdavis.edu)
Trace Gas Emissions and Soil C and N dynamics in Sagebrush and Chaparral Shrublands: Methods, Inventories and Field Experiments
U.Norton *, W.R.Horwath, A.R.Mosier, J.A.Morgan
Sagebrush and chaparral shrubland biomes span a wide geoclimatic range and cover extensive areas of the United States. Historically, fire played a critical role in shaping autosuccession of these fire-adapted ecosystems, returning every 30-40 years. However, as these ecosystems undergo rapid human-induced changes, there is need for better understanding of how these practices impact ecosystem resiliency, sustainability and ability to withstand exotic weed invasion. Unlike many other biomes, C and N dynamics in shrubland ecosystems are poorly characterized. Specifically, information on the contribution of GHG to the atmosphere is lacking. For example, the chaparral biome has one of the largest estimates of nitric oxide and considerable amounts of nitrous oxide emissions. Moreover, over a quarter of the estimated global sum of nitric oxide emissions comes from this biome. Unfortunately, these estimates are based on a very limited amount of scientific data and there are no estimates for other greenhouse gases such as carbon dioxide and methane. We hypothesize that shrubland ecosystems are very prone to disturbance and therefore, any restoration practice that would provide ecological benefits would also improve soil quality and reduce GHG flux to the atmosphere. Simulating water pulses is an important tool for understanding biogeochemical processes in semi arid environments. A single summer rainfall event in sagebrush can contribute as much as 20 or 30 percent of the nitrous oxide in annual GHG budget estimates. Therefore, the purpose of this on-going research is to inventory trace gas emissions from sagebrush and chaparral shrublands ecosystems. Here we present the effects of water additions on GHG emissions and soil C and N in sagebrush soils, both canopy and shrub interspace, on sites dominated by either, western wheatgrass, a native perennial, or cheatgrass, an exotic annual. In August 2003 we simulated a single summer rainfall event in a Wyoming big sagebrush stand located outside Lander, WY. We used static chambers deployed on the soil surface to monitor GHG production, and collected soil samples for various soil C and N indices at 7 times during 216 hours after wet up. Our results indicate that long-term cheatgrass establishment affects not only soil under its own thatch, but also under shrubs within the cheatgrass stand. Overall, soil TN and TOC content on cheatgrass sites were lower than those of western wheatgrass. Sites dominated by native perennial grasses (both shrub interspaces and under shrub canopies) were less likely to contribute as much N gas to the atmosphere as cheatgrass soils upon soil dry down. These soils were also more efficient in methane consumption and effective in coupling of N and C biochemical transformations. Upon water pulse, cheatgrass soils demonstrated greater carbon dioxide production rates relative to pre-wet conditions and greater nitrous oxide flux per unit soil TN. Cheatgrass soils, both interspace and under shrub canopy, had greater nitrate concentrations that became a substrate for microbial immobilization or rapid nitrous oxide production. Cheatgrass soils also showed more rapid initial increase in soil microbial biomass followed by its immediate mortality, and greater concentrations of soil dissolved organic C compared to western wheatgrass. In conclusion, sagebrush sites invaded by cheatgrass were prone to contribute nitrous oxide and methane to the atmosphere upon summer moisture availability. Possible mechanisms include greater nitrification potential and faster turnover of microbial biomass triggered by simultaneous shortages of microbially available C or N.