Elevated CO2 Good for Soybean, Sorghum Growth

WASHINGTONA long-term study comparing tillage practices under high carbon dioxide (CO2) levels suggests elevated CO2 causes soybean and sorghum plants to increase photosynthesis while reducing the amount of water the plants release. The result is increased water use efficiency whether the crops were grown with no-till or conventional tillage, according to researchers with USDA's Agricultural Research Service (ARS).

The 6-year outdoor study was conducted using open-top growth chambers for exposing the crops to the higher levels of CO2. The crops were monitored for photosynthesis and transpiration during their reproductive growth stages, when water demand is highest. Researchers compared soybean/sorghum rotations with both conventional tillage and no-till. With no-till there is no plowing, only minimal disturbance of the soil while planting seeds.

The scientists also compared current ambient CO2 levelsabout 370 parts per million (ppm)with levels of 720 ppm expected within this century. With the higher level of CO2, regardless of tillage method, soybean photosynthesis increased by about 50 percent, while sorghum photosynthesis rose by only 15 percent. This was expected because crops like soybean, which have a C3 photosynthetic pathway, are known to respond better to high CO2 levels than crops like sorghum and corn that have a C4 photosynthetic pathway. Sorghum's increased water use efficiency was mainly due to less water transpired or lost through the leaf pores.

Although no-till didn't make a difference as far as crops responding to high CO2, it can greatly reduce soil erosion, conserve soil water and increase carbon storage.