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 su bjected to various combinations of climate change. Model values for pl ant and soil characteristics were obtained at sites near Manhattan, Ka nsas, and the Manhattan climate record provided the nominal climatic d rivers. Model runs used both actual monthly temperature and precipitat ion data for a 100-yr interval and average weather conditions generate d from this record. Monthly temperatures were increased 2-degrees-C, l eft unchanged, or decreased 2-degrees-C; annual precipitation was incr eased 6 cm, left unchanged, or decreased 6 cm. All possible combinatio ns of temperature and precipitation were then used in 100-yr simulatio ns. Regardless of the specific climate scenario, plant production was lowest for C3 grasses and highest for the mixed C3-C4 community. The n ominal seasonal pattern of precipitation favored an active C3 plant co mmunity in early to late spring, prior to the emergence of the C4 vege tation. However, the higher growth and water use efficiencies of C4 ve getation 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, unburn ed, or lightly grazed) to plant production and soil carbon values. Pho tosynthetic pathway and precipitation were identified as the most sign ificant single variables affecting foliage production; the interaction between photosynthetic pathway and temperature was the most significa nt interaction term. Management treatments were by far the most import ant variables affecting soil carbon values, but 2-degrees-C warming di d produce substantial soil carbon losses from C3 grasslands. Enhanced carbon fixation by the C4 and C3-C4 plant communities negated the loss es of soil carbon caused by enhanced soil respiration at warmer temper atures.