Long-term response of bahiagrass pastures to elevated atmospheric CO2 and fertility management: Forage quality

Southeastern US pasture systems remain understudied agro-ecosystems as affected by elevated atmospheric CO₂ concentration. A long-term study of bahiagrass (Paspalum notatum Flüggé) assessed responses to elevated CO₂ on a Blanton loamy sand (loamy siliceous, thermic, Grossarenic Paleudults). Pastures were exposed to ambient or elevated (ambient plus 200 μmol mol⁻¹) CO₂ using open-top field chambers. Based on extension soil test recommendations, half of all plots received N [(NH₄)₂SO₄] at 90 kg ha⁻¹ three times yearly plus K, P, and lime (managed) while remaining plots received no fertilization (unmanaged). For 10 years, bahiagrass from all treatments were assessed for impacts on forage quality using forage from yearly August harvests. These routine analyses included, crude protein, acid detergent fiber, neutral detergent fiber, lignin, total digestible nutrient, relative forage quality, NO₃-N, digestible protein, crude fiber, net energy calculations, metabolizable energy and elemental concentrations (C, N, P, K, Ca, Mg, B, Cu, Fe, Mn, Na, and Zn). Fertilization clearly increased productivity and forage quality (e.g., forage N, crude protein, digestible protein) indicating that southeastern US bahiagrass pastures would benefit from proper soil fertility management. Despite reductions in some forage elemental concentrations under managed conditions, nutrients expressed on a land area basis (content) were increased. Effects of elevated atmospheric CO₂ were much smaller and less consistent which may be reflective of a long-term C₄ plant response. Additional research is required to determine if our findings are applicable to other southeastern pasture systems.