Drought and Elevated CO2 Impacts On Soil Structure and N-Cycling Microorganisms in a Soybean Agroecosystem.

The current atmospheric carbon dioxide (CO₂) concentration is about 370 ppm and it is projected to increase to about 550 ppm by the middle of this century due to the burning of fossil fuels and changes in land use (IPCC, 2007). Drought-affected areas have increased in some drier regions since the 1970s and the droughts are projected to increase in severity (IPCC, 2007). Soil water availability, and soil C and N are primary factors influencing the activity and abundance of soil microorganisms and elevated CO₂ has been found to affect the plant-soil microbial interactions through increasing soil C input and plant water use efficiency. Thus, the objective of this study was to investigate the responses of soil C and N dynamics and soil bacteria, especially nitrifiers and denitrifiers, under elevated CO₂ and drought. The field experiment was located at the SoyFACE (www.soyface.uiuc.edu) facility in Urbana-Champaign, which consisted of eight 20 m diameter experimental plots. Four rings under ambient CO₂ and four rings under elevated CO₂ with subplots subjected to the drought treatment. We studied the impacts of elevated CO₂ and drought on the whole soil and in the soil fractions (microaggregated, particulate organic matter (POM), silt and clay). In those environments, total C and N, and abundance of the N-cycling microorganisms target genes were quantified. Furthermore, 16S rRNA gene copy number was used as an indicator of the total bacteria. Under elevated CO₂ and drought, the total bacteria were 25% and 22.6% higher than in the drought plots in the rhizosphere and bulk soils, respectively. Total C and N were found to be 13.9% and 12.6%, higher under CO₂ and drought than in the drought plots, respectively. Our findings suggest that CO₂ fertilization mitigates the stress caused by drought.