N-fixing trees in wetland restoration plantings: effects on nitrogensupply and soil microbial communities

To investigate the impact of an exotic Frankia nodulated tree (Alnus trabeculosa) on soil nitrogen content, soil microbial composition, and the abundance of N turnover-related functional microorganism community, we compared the community structure and abundance of key functional genes (nifH, bacterial/archaeal amoA, and nosZ) in the rhizosphere and nonrhizosphere of monoculture of Phragmites australis and A.trabeculosa-P.australis mixed communities by MiSeq Illumina sequencing and real-time PCR, respectively. The introduction of Frankia nodulated tree to recover degraded wetland was effective in the accumulation of soil organic carbon and nitrogen, which was the key factor to impact on the bacterial community composition revealed by canonical correspondence analysis. Acidobacteria and Proteobacteria were the dominant bacterial phylums while seven rare phyla appeared the most phylogenetically different among the investigated soil of two vegetations, including Chlorobi, Cyanobacteria, OD1, OP11, TM6, TM7, and GN02. The gene copy numbers of nifH were ranged from 2.28 × 10⁸ to 2.96 × 10⁹ copies g^-1 dry soil in the wetland, and which were significantly higher in soil samples from P. australis than that from A.trabeculosa. While the abundance of nosZ in both rhizosphere and nonrhizosphere soils of A.trabeculosa-P.australis mixed communities was significantly lower compared with P.australis monoculture. The potential nitrification (PNA) (0.15-0.41 mg NOx-N kg^-1 dry soil d^-1) in the rhizosphere of A. trabeculosa was significantly higher than that of P. australis, and the soil denitrification enzyme activity (DEA) (0.42-0.90 nmol N₂O-N g^-1 dry soil h^-1) was lower in the mixed community compared with monoculture of P. australis. The introduced planting of Frankia nodulated tree effectively accumulated soil organic carbon and nitrogen and reduce the relative abundance and activity of nitrogen-fixing bacteria and denitrification bacteria.