Rhizospheric Denitrification Potential and Related Microbial Characteristics Affected by Secondary Salinization in a Riparian Soil

Abstract

A pot experiment with simulated saltwater incursion was conducted to study the effects of secondary salinization on the denitrification potential and related microbiological activities and size of the denitrifier community by real-time PCR quantification of denitrifying genes in a rhizospheric soil vegetated with different riparian plants. The results showed that saltwater incursion significantly inhibited denitrification potential in the riparian soil, with the average denitrification rate decreased by 34.9%. The inhibitory effects differed with the vegetation types. The rhizosphere soils of Acorus calamus and Phragmites australis showed a significantly lower inhibitory percent of the denitrification rate compared with the corresponding monoculture. Secondary salinization resulted in a decrease in the number of N-cycling microorganisms and in the activities of soil N-transforming enzymes, especially the number of denitrifiers and nitrite reductase activity, which exhibited declines of 51.5% and 43.5%, respectively. Real-time PCR quantification showed that the saltwater incursion significantly changed the abundance and composition of denitrifying genes in the riparian soil despite the vegetation types. The community size of narG was the most sensitive, while nosZ was the least affected by secondary salinization. There were significant differences in the responses of N-transforming microbiological activities and denitrifying gene abundance to secondary salinization in riparian soils with different vegetation types. The inhibition percents of N transforming-related enzyme activities and the abundances of denitrifying genes in the rhizospheric soil of A. calamus and P. australis were significantly lower compared with the pure plant community, indicating that mixed vegetation provided a significant alleviation of the inhibition of microbiological parameters and denitrification processes due to saltwater incursion in riparian soil. Principal component analysis (PCA) analysis indicated that the denitrification rate correlated significantly with the activities of N transform-related enzymes and the abundance of narG, nirK and nirS rather than nosZ.