Reddy, K. Raja (Mississippi State University, Department of Plant and Soil Sciences, 117 Dorman, Mississippi State, MS, 39762; Phone: 662-325-9463; Fax: 662-325-9461; Email: krreddy@ra.msstate.edu)

Effect of Different Temperatures and Carbon Dioxide Levels on Biomass Accumulation and Partitioning in Big Bluestem (Andropogon gerardii)

K.R. Reddy *, V.G. Kakani, R.L. King

Rangelands occupy 61% of United States and would play a major role in sequestering atmospheric carbon under projected climates with elevated CO2 and temperatures. The objective of this study was to evaluate the effect CO2 and temperature interaction on biomass accumulation and partitioning in Andropogon gerardii, a dominant C4 species in tallgrass prairies. Ten sunlit Soil-Plant-Atmosphere-Research chambers were used to study the effects of two CO2 levels (360 and 720 ppm) and five different temperatures (20/12, 25/17, 30/22, 35/27 and 40/32 ºC; day/night). Grass seeds were sown in 11 rows at 0.2 m spacing and after emergence were thinned down to ten plants per row. A predetermined combination of CO2 and temperature was maintained from sowing to maturity under optimum water and nutrient conditions. At maturity, individual plants were harvested and divided into leaves, stems and panicles. The roots were dug out from the soil bin into 4 layers at 25 cm interval in each of the SPAR units and thoroughly washed to remove sand particles. The samples were then oven dried at 70 ºC for 72 h and dry weights were recorded. Biomass decreased either above or below the optimum temperature of 30/22 ºC. The effect of elevated CO2 on biomass accumulation (12-30% increase) was observed at or less than optimum temperature (30/22 ºC)and vanished at higher temperatures. With increase in temperature, irrespective of the CO2 level, biomass partitioned to leaves increased (35%) where as that to stems decreased (33%). Panicle weight was 6-7% of biomass at 25/17 ºC and fell to 1.6 % at 40/32 ºC. The biomass partitioned to roots, across the temperatures, was constant for plants grown at 360 ppm CO2 but decreased by 7% for those grown at 720 ppm. The study suggests that at above optimum temperatures (above 30/22 ºC) the selected C4 tallgrass prairie species, Big Bluestem, fails to capture higher amounts of carbon under elevated CO2 conditions but partitions more to leaves for later incorporation into soil. The decrease in panicle production at higher temperature would also reduce the selected C4 species population and dominance in tallgrass prairies.