Soil bacterial community responses to species-specific leaf litter dissolved organic matter additions

Soil microorganisms mediate the mineralization of bulk dissolved organic matter (DOM) in soil, however little is known about which organisms are responsible for the respiration of specific compounds. I ask the question: how do bacteria respond at the phylum and subphylum level to the addition of a labile and a more recalcitrant source of DOM? To address this, tropical soil was incubated with water (as a control), labile leaf litter DOM leached from Schizolobium parahyba, and more recalcitrant leaf litter DOM leached from Qualea paraensis. Incubations were analyzed for soil respiration rates and microbial community composition. The latter was determined with the construction of 16S rRNA gene clone libraries as well as by using group-specific quantitative polymerase chain reaction (qPCR) analyses. My results showed that the highest rates of soil respiration occurred during the first day of the incubation, with the more labile S. parahyba DOM eliciting the highest rate of respiration. All treatments resulted in a significant increase in the relative abundance of bacteria in the soil microbial community by day 10. Likewise, all treatments induced a pronounced increase in the relative abundance of the acidobacteria, actinobacteria, alphaproteobacteria, and betaproteobacteria by day 10. The relative abundance of betaproteobacteria was significantly higher in incubations containing S. parahyba DOM. All treatments and all time points featured a broad phylogenetic distribution of acidobacteria 16S rRNA gene sequences across several orders, while all alphaproteobacterial sequences were related to nitrogen-fixing organisms in the Bradyrhizobiales and Rhizobiales orders. Betaproteobacterial 16S rRNA genes were also widely distributed, but soils treated with S. parahyba DOM featured sequences related to the Ralstoniaceae and Comamonadaceae, which were generally absent from other treatments. These results suggest that these four bacterial groups respond generally to the wetting of soils, and that the betaproteobacteria specifically respond to more labile inputs of native DOM.