Location: Rangeland Resources & Systems ResearchTitle: Importance of early season conditions and grazing on carbon dioxide fluxes in Colorado shortgrass steppe
|MORGAN, JACK - Retired ARS Employee|
|PARTON, WILLIAM - Colorado State University|
|GLIMANOV, TAGIR - Gilmanov Research And Consulting Llp|
Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/30/2019
Publication Date: 9/7/2016
Citation: Morgan, J.A., Parton, W., Derner, J.D., Glimanov, T., Smith, D.P. 2016. Importance of early season conditions and grazing on carbon dioxide fluxes in Colorado shortgrass steppe. Rangeland Ecology and Management. 69:342-350.
Interpretive Summary: With atmospheric carbon dioxide (CO2) concentration continuing to increase in Earth’s atmosphere, contributing to climate change. There is an urgent need to both understand how human activities are contributing to the emission of CO2 and other greenhouse gases, and to use that knowledge to reduce those greenhouse gas emissions, or even enhance the uptake of those gases from the atmosphere. For six years, we used micro-meteorological tools in the shortgrass steppe of northern Colorado to monitor the exchange of CO2 between pastures and the ambient atmosphere. Pastures were subjected to different grazing intensities by cattle so we could evaluate how grazing affected CO2 fluxes. In 4 of 6 years characterized by abundant early-season precipitation and plant biomass, an average 59 g m-2 of CO2-C carbon dioxide were assimilated from the atmosphere to the grassland. However, net emissions of CO2 from the grassland to the atmosphere averaging 45 g CO2-C m-2 occurred in two years with dry springs and little early-season aboveground plant biomass. Pastures subjected to moderate intensity grazing or no grazing at all had similar patterns of seasonal gains (in spring and summer) and losses (in fall and winter) of CO2 to the atmosphere. In one study, a moderately grazed pasture exhibited a net annual gain of 73 g m-2 CO2-C over three years compared to no net change in CO2-C assimilated in the heavily grazed pasture. Results indicate that maintaining a high amount of aboveground biomass and abundant spring soil moisture are most conducive to assimilating CO2 from the atmosphere into the grassland. To enhance C sequestration, management strategies should emphasize flexible and adaptive grazing practices that consider early-season soil moisture and promote vegetation cover during the key early spring growth period.
Technical Abstract: Understanding the influence of environmental and management drivers on fluxes of carbon dioxide (CO2) is essential for optimizing carbon (C) uptake and storage in livestock production systems. Herein, using 15 treatment-years (two three-year experiments, one with three grazing treatments, the other with two), we evaluated the influence of soil water content (SWC) and grazing intensity on net CO2-C ecosystem exchange (NEE) in shortgrass steppe. April SWC and early-season Normalized Difference Vegetation Index were highly correlated with NEE (-.96 and -0.98, respectively) during the critical second quarter of the year, which is the primary growing season (April-June), and also over the entire growing season (April-September; -0.94 and -0.92). Due to the strong effect of early-season SWC on NEE, an average 45 g m-2 CO2-C were lost in two years with dry spring weather and low plant cover, compared to an average 59 g m-2 CO2-C assimilated annually in four other years with more abundant early-season precipitation and plant cover. Grazing intensity effects on NEE were also apparent, but less in magnitude compared to year responses. In one grazing experiment, moderate grazing resulted in an annual C assimilation of 73 g m-2 CO2-C over all three years compared to essentially no net annual CO2-C exchange in the heavily-grazed pasture. However, that treatment difference in annual C assimilation was only about half that experienced between dry (56 g m-2 CO2-C lost annually) and wet (83 g m-2 CO2-C assimilated annually) years. Similar trends were observed in a second grazing experiment, although results were not significant. Results suggest that the recommended moderate grazing intensity for the Colorado shortgrass steppe is near-optimal for sequestering C under season-long continuous grazing, with annual climatic variability being at least as influential on C storage. To enhance C sequestration, management strategies should emphasize flexible and adaptive grazing practices that consider early-season SWC and promote vegetation cover during the key early spring growth period.