Soil CO2 Concentrations in Biofuel Cropping Systems of the Columbia Basin.

Recent advances in sensor technology provide a robust capability for continuous measurement of soil gases at high data densities. This study assessed seasonal CO2 concentrations in soil as affected by the production of cellulosic biofuel crops managed under three levels of net primary productivity (NPP) induced by varying irrigation, fertilizer, and seeding rates. Soil CO2 was measured continuously at two soil depths using Vaisala GMM220 series CO2 sensors (Vaisala, Finland). Sensors were installed in bare soil and in four crops - continuous corn (Zea mays L.), continuous wheat (Triticum aestivum L.), a warm season grass monoculture of switchgrass (Panicum virgatum L.), and a switchgrass dominated warm season grass polyculture managed at three levels of NPP. Measurements were obtained very 10 s and 1 min averages recorded. In 2008, bare soil CO2 concentrations were low (200 to 1,200 µL L-1) late in the growing season and higher (25-60%) at the 38 cm depth than the 15 cm depth. In the crops, soil CO2 increased during the growing season reaching concentrations exceeding the 10,000 µL L-1 upper limit of the GMM222 sensor mid season in wheat and in the grasses while corn stayed below that limit. The GMM221 sensor, with a higher detection limit, measured maximum soil CO2 concentrations in wheat of 16,000 µL L-1 late in the growing season. Soil CO2 concentrations declined at the end of the growing season in all crops and were consistently higher at the 38 cm than the 15 cm depth.