Controls of Plant and Soil Carbon in a Semihumid Temperate Grassland

A modeling study evaluated the importance of photosynthetic pathways (C₃, C₄, or both) and management strategies to the foliage productivity and soil carbon characteristics of a semihumid temperate grassland subjected to various combinations of climate change. Model values for plant and soil characteristics were obtained at sites near Manhattan, Kansas, and the Manhattan climate record provided the nominal climatic drivers. Model runs used both actual monthly temperature and precipitation data for a 100—yr interval and average weather conditions generated from this record. Monthly temperatures were increased 2°C, left unchanged, or decreased 2°C; annual precipitation was increased 6 cm, left unchanged, or decreased 6 cm. All possible combinations of temperature and precipitation were then used in 100—yr simulations. Regardless of the specific climate scenario, plant production was lowest for C₃ grasses and highest for the mixed C₃—C₄ community. The nominal seasonal pattern of precipitation favored an active C₃ plant community in early to late spring, prior to the emergence of the C₄ vegetation. However, the higher growth and water use efficiencies of C₄ vegetation during summer contributed to the maximization response of the grasslands containing both C₃ and C₄ grasses. An analysis of variance of annual average values observed from 100—yr simulations was used to evaluate the relative importance of climate, photosynthetic pathways, and management activities (annually burned, burned every 4 yr, unburned, or lightly grazed) to plant production and soil carbon values. Photosynthetic pathway and precipitation were identified as the most significant single variables affecting foliage production; the interaction between photosynthetic and temperature was the most significant interaction term. Management treatments were by far the most important variables affecting soil carbon values, but 2°C warming did produce substantial soil carbon losses from C₃ grasslands. Enhanced carbon fixation by the C₄ and C₃—C₄ plant communities negated the losses of soil carbon caused by enhanced soil respiration at warmer temperatures.