Microbial Communities and Soil Structure are Affected by Reduced Precipitation, but Not by Elevated Carbon Dioxide

Elevated carbon dioxide (eCO(2)) concentrations in the atmosphere, along with decreased precipitation regimes expected in the next decades, can alter soil microbe functioning and hence soil organic matter (SOM) dynamics causing important impacts on the global C cycle. We investigated the effects of eCO(2) and reduced soil moisture, further referred to as reduced precipitation treatment (RP), on SOM fractions and soil microbial distribution across different soil zones (i.e., rhizosphere vs. bulk soil) and physically-separated SOM fractions (coarse particulate organic matter (cPOM; >250 mm), microaggregate (53-250 mm), and silt-and-clay fraction (<53 mm)) at the soybean free air concentration enrichment (SoyFACE) experiment in Illinois. To quantify the abundance of microorganisms, we used real-time quantitative polymerase chain reaction (PCR) for the total bacterial (16S rRNA) and denitrifier (nosZ) genes. We did not detect any significant effects of eCO(2) on bacterial abundance, soil C, and N concentrations either in the whole soil or in the SOM fractions. These findings corroborate the previously reported absence of soil C responses to eCO(2). The mass of microaggregates was highest in RP treatments in both rhizosphere and bulk soil. Furthermore, the abundance of 16S rRNA and nosZ genes increased in the microaggregates under RP compared to ambient conditions of soil moisture. Hence, RP increased the formation of microhabitats in which the microorganisms are partly protected from the adverse effects of reduced soil moisture.