Rhizosphere Effect Enhances Belowground Competition of Coastal Invasive <i>Spartina alterniflora</i> With Mangroves

<i>Spartina alterniflora</i> 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 <i>S. alterniflora</i> neighboring mangroves was hypothesized. Here, both rhizosphere soil of <i>S. alterniflora</i> and bulk soil were collected from the center of <i>S. alterniflora</i> marsh and border sites where <i>S. alterniflora</i> was adjacent to <i>Kandelia obovata</i> and <i>Aegiceras corniculatum</i>, 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 <i>S. alterniflora</i> 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 <i>S. alterniflora</i> rhizosphere microbiome. Microbial interkingdom co-occurrence networks were simplified when <i>S. alterniflora</i> neighbored mangroves, while network modularity significantly increased. Metagenomics indicated that genes involved in methanogenesis (<i>ackA</i>, <i>mvhD</i>, etc.) and nitrogen fixation (<i>nifH</i>, <i>nifK</i>, etc.) were significantly enriched in those <i>S. alterniflora</i> neighboring <i>K. obovata</i>, and genes related to phosphate transporter (<i>pstA</i>, <i>pstB</i>, etc.) were significantly enriched in those <i>S. alterniflora</i> neighboring <i>A. corniculatum</i>. These results demonstrated that the rhizosphere effect intensified the belowground interspecific competition of <i>S. alterniflora</i> adjacent to mangroves by altering root exudates, changing the soil microbial composition, and modulating strategies for core nutrient metabolism.