Molecular changes of soil organic matter induced by root exudates in a rice paddy under CO2 enrichment and warming of canopy air

While crop productivity and carbon flux/partitioning in agroecosystem have been widely addressed, potential changes in organic matter composition through root deposition are still unknown under climate change. In this study, root exudates of rice and rhizospheric topsoil were collected in a paddy field under 6 years of simulated climate changes respective of CO2 enrichment, canopy air warming, and their combination as compared to the control. The molecular composition of root exudates was analyzed with liquid chromatography/mass spectrometry (LC/MS) while soil organic matter (SOM) was analyzed with pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS), along with a phospholipid fatty acid analysis (PLFA) for characterizing soil microbial community. Compared to control, root exudation under elevated CO2 was enhanced but the exudate composition remained unchanged. On the other hand, root exudation rate under warming was unaffected, but the exudate composition was changed. Relative abundance of phenolic compounds in root exudates under warming, was decreased by 26% but that of amino acids was increased by 53%. However, neither root exudation nor the exudate composition was altered under combined CO2 and temperature elevation. Furthermore, increases in abundance of heterocyclic nitrogen compounds, phenolic acids and phenols (by 37%, 65%, and 26%, respectively) in SOM were relevant to the increase in bacterial/fungal (B/F) ratio under elevated CO2. This accumulation was possibly attributed to preferential utilization of the increased root exudates by bacteria. However, the overall reduction of some highly hydrophobic SOM compounds (fatty acids, phenols, and phenolic acids) by 14%, could be induced by accelerated decomposition in line with significant decrease in B/F ratio under warming. Indeed, the decreased relative abundance of phenolic compounds such as p-coumaric acid (by 49%) in root exudates lead to fungal increase, which accelerated SOM decomposition. Yet, the molecular composition of SOM was hardly changed under the combination of elevated CO2 and temperature. Overall, our findings suggested that in a rice paddy CO2 enrichment increased root exudation and B/F ratio, while air warming altered the root exudate composition and decreased B/F ratio, resulting in changes of SOM composition. These results indicate that root exudates are a key component for the regulation of SOM dynamics under climate change scenarios.