Rhizosphere Effect Enhances Belowground Competition of Coastal Invasive Spartina alterniflora With Mangroves

Spartina alterniflora has severely invaded mangroves in China. In order to explore the possible belowground interspecific interaction along with its invasion, the rhizosphere effect enhancing the competition of S. alterniflora neighboring mangroves was hypothesized. Here, both rhizosphere soil of S. alterniflora and bulk soil were collected from the center of S. alterniflora marsh and border sites where S. alterniflora was adjacent to Kandelia obovata and Aegiceras corniculatum, respectively, in both vigorous growth and senescent periods. Soil nutrient properties, rhizospheric low-molecular-weight organic acids (LMWOAs), soil microbiomes, and microbial functional genes were analyzed. Soil total carbon and total nitrogen contents of S. alterniflora neighboring mangroves were increased, and its LMWOAs were altered when adjacent to mangroves in both vigorous growth and senescent periods. These changes were significantly correlated with variation in the composition of S. alterniflora rhizosphere microbiome. Microbial interkingdom co-occurrence networks were simplified when S. alterniflora neighbored mangroves, while network modularity significantly increased. Metagenomics indicated that genes involved in methanogenesis (ackA, mvhD, etc.) and nitrogen fixation (nifH, nifK, etc.) were significantly enriched in those S. alterniflora neighboring K. obovata, and genes related to phosphate transporter (pstA, pstB, etc.) were significantly enriched in those S. alterniflora neighboring A. corniculatum. These results demonstrated that the rhizosphere effect intensified the belowground interspecific competition of S. alterniflora adjacent to mangroves by altering root exudates, changing the soil microbial composition, and modulating strategies for core nutrient metabolism.