Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/31/2007
Publication Date: 1/24/2008
Citation: Norton, U., Mosier, A., Morgan, J.A., Derner, J.D., Ingram, L.J., Stahl, P. 2008. Moisture pulses, trace gas emissions and soil C and N in cheatgrass and native grass-dominated sagebrush-steppe in Wyoming, USA. Soil Biology and Biochemistry 40:1421-1431. Interpretive Summary: The problem of anthropogenically released greenhouse gases into Earth’s atmosphere and consequences for global climate change has spurred research into understanding sources and sinks for these gases in hopes of developing management strategies to reduce or in some cases even eliminate net losses to the atmosphere. This experiment evaluates how invasion of a western sagebrush rangeland by cheatgrass affects the soil microbiology and the emission of greenhouse gases from the weed-invaded soils. Briefly, areas with substantive cheatgrass infestations emitted more nitrous oxide (N2O), a N-containing greenhouse gas, to the atmosphere compared to adjacent areas with only native grasses. The higher N losses in the cheatgrass areas suggest less efficient cycling of nutrients may occur with weed invasion and may lead to gaseous losses of N to the atmosphere. These results indicate that in addition to the more traditional problems faced by livestock producers and other land managers on rangeland in the western United States due to weed incursions, weeds may also stimulate the emission of some greenhouse gases to the atmosphere, exacerbating the problem of global climate change.
Technical Abstract: Episodic summer precipitation typical of semi-arid areas dominated by shrub-steppe triggers rapid soil C and N transformations followed by losses via greenhouse gas (GHG) emissions. Land use practices and disturbances can have significant impacts on vegetation present and therefore, can control the magnitude of soil response to water pulse. The objective of this study was to investigate the relationship between vegetation types present in shrub-steppe and how they may influence the mechanisms controlling soil C and N transformations and GHG production following the addition of an artificially applied summer water pulse in two Wyoming big sagebrush (Artemisia tridentata Nutt. ssp. wyomingensis) steppe communities: one with an understory dominated by the annual invasive grass, cheatgrass (Bromus tectorum L.) and the other dominated by the native perennial grass, western wheatgrass (Pascopyrum smithii Rydb.). We collected soil samples from beneath shrub canopy and shrub interspace for various C and N indices and monitored GHG production at seven times over a period of 216 hours after wetting up the soil. Regardless of treatments, maximum rates of production for N2O and CO2 occurred within four hours of the water pulse. Overall, B. tectorum sub canopy and interspace soils produced significantly more N2O, emitted less CO2 and consumed less CH4 than P. smithii soils during 216 hours of post wet up monitoring. Microbial biomass in all soils increased as early as 4 hours after wet up but declined shortly after in B. tectorum soils only. This decline coincided with rapid increases in soil dissolved organic C and N. In contrast, P. smithii interspace soils responded to soil wet up with a slower increase in microbial growth with microbial biomass C concentration peaking at 24 hours after the addition of water. This microbial biomass peak declined shortly after, but to levels above, pre wet. No significant changes in labile C and N pools throughout the time soil moisture remained elevated were observed in P smithii soils. Lower N losses to GHG emissions in P smithii soils compared to B. tectorum soils probably resulted in a greater availability of N for microbial uptake following wetting. In conclusion, sagebrush shrub-steppe sites invaded by B. tectorum were prone to greater N losses via gaseous N emissions upon summer moisture pulses in comparison with P. smithii soils. Possible mechanisms include nitrification processes generating greater amounts of N2O and a more rapid turnover of microbial biomass triggered by shortages of microbially available N.