Clonal integration in Phragmites australis alters soil microbial communities in an oil-contaminated wetland

Clonal plants can share information and resources among connected ramets (asexual individuals). Such clonal integration can promote ramet growth, which may further influence soil microbial communities in the rooting zone. Crude oil contamination can negatively affect plant growth and alter soil microbial community composition. However, we still know little about how clonal integration affects soil microbial communities, especially under crude oil contamination. In a coastal wetland, ramets of the rhizomatous plant Phragmites australis in circular plots (60 cm in diameter) were subjected to 0, 5 and 10 mm depth of crude oil, and the rhizomes at the edge of the plots were either severed (preventing clonal integration) or left intact (allowing clonal integration). After three years of treatment, we analysed in each plot soil physiochemical properties and soil microbial community composition. The alpha-diversity of the soil microbial communities did not differ between intact and severed plots, but was overall lower in 10-mm than in 0-mm and 5-mm oil plots. Considering all three oil treatments together, soil microbial community dissimilarity (beta-diversity) was positively correlated with soil property distance in both severed and intact plots. Considering the three oil treatments separately, this pattern was also observed in 10-mm oil plots, but not in 0-mm or 5-mm oil plots. The soil microbial community composition was more sensitive to the oil addition than to the clonal integration. Moreover, the relative abundance of the nitrogen-cycling bacterial taxa was lower in intact than in severed plots, and that of the oil-degrading bacterial taxa increased with increasing oil-addition levels. Our results indicate that clonal integration and oil contamination can influence soil microbial communities independently through changing the relative abundance of the component bacteria taxa, which has important implications for ecosystem functions of the soil food web mediated by clonal plants.