Long-term response of a bahaigrass pasture to elevated atmospheric CO2 and nitrogen management

Authors: Prior, Stephen - Steve; Runion, George; Torbert, Henry - Allen

Submitted to: International Soil Tillage Research Organization Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 6/4/2018
Publication Date: 10/5/2018
Citation: Prior, S.A., Runion, G.B., Torbert III, H.A. 2018. Long-term response of a bahaigrass pasture to elevated atmospheric CO2 and nitrogen management. In: Proceedings of the International Soil Tillage Research Organization, September 24-27, 2018, Paris, France. p. 132-133.

Interpretive Summary: Atmospheric CO2 is increasing due primarily to fossil fuel burning and land use change. Elevated CO2 can increase plant growth when soil nitrogen is not limiting. Understanding CO2 effects is important in developing profitable and environmentally sound agricultural systems. Pastures occupy 80 million acres in the southeastern U.S.(~75% of total pasture acreage in the eastern U.S.) A long-term experiment examined a southeastern pasture system (bahiagrass, Paspalum notatum) under ambient and elevated CO2 levels and soil N management treatment (no N=unmanaged and added N=managed). Elevated CO2 increased forage production only when soil N was added with a small decline in forage quality (in terms of C:N ratio). Ongoing efforts are assessing the ability of this pasture system to sequester CO2 as soil C and influences on greenhouse gas emissions (CO2, CH4, and N2O).

Technical Abstract: Both managed and unmanaged pasture systems in the Southeastern US remain understudied agro-ecosystems in terms of the effects of elevated atmospheric CO2 concentration. A long-term study of bahiagrass (Paspalum notatum Flüggé) response to elevated CO2 using open top field chambers in 2005 on a Blanton loamy sand (loamy siliceous, thermic, Grossarenic Paleudults) was run for 10 years to assess biomass production and tissue quality (carbon and nitrogen). Plants were exposed to ambient or elevated (ambient plus 200 ppm) CO2. After a one-year establishment period, an N treatment was applied where half of all plots received N [(NH4)2SO4] at 90 kg ha-1 three times yearly; the remaining plots received no N fertilization. These two treatments represent managed and unmanaged pastures, both of which are common in the Southeast. Prior to N treatment initiation (establishment), forage biomass was unaffected by CO2 treatment. The main effect of CO2 was significant in all 10 years; the average increase under elevated CO2 was 14%. The main effect of N was highly significant in all 10 years; the average increase from added N was 232%. The interaction of CO2 x N was significant in all years except 2009; generally, forage biomass was increased by elevated CO2 only with added N (15%). Tissue analysis indicated that high CO2 lowered the C:N ratio in the no N treatment, but the opposite was observed with N fertilization. Results show that N fertilization can significantly increase forage biomass production regardless of CO2 level. Elevated CO2 increased forage biomass only when soil N was added; however, forage quality (in terms of C:N ratio) may decline slightly under these conditions. Ongoing efforts are examining changes in soil organic C and N, including assessing the potential of this pasture system to sequester CO2 as soil C and the influence on trace gas emissions (CO2, CH4, and N2O).