The Contribution of Anammox to Nitrogen Removal Is Greater in Bulk Soils Than in Rhizosphere Soils in Riparian Wetlands Along the Yangtze River

Abstract

The importance of denitrification and anaerobic ammonium oxidation (anammox) in removing nitrogen (N) from upland runoff has been well documented in riparian wetlands. However, the relative contributions of denitrification and anammox to N removal in the rhizosphere and non-rhizosphere soils of riparian zones remain unclear. Here, we explored the denitrification and anammox rates in the rhizosphere and bulk soils (0–5 and 10–15 cm) of 39 riparian wetlands along the Yangtze River using the 15N isotope pairing technique. Additionally, we used a quantitative polymerase chain reaction assay to determine the abundance of soil denitrifying and anammox bacteria using nosZ and hzsB genes, respectively. The results showed that both denitrification and anammox rates were significantly higher in rhizosphere soils than in bulk soils, suggesting that the rhizosphere environment is favorable for N removal. The contribution of anammox constituted over half (62.53% ± 1.49%) of the N loss and was greater in bulk soils (68.57% ± 1.42%) than in rhizosphere soils (55.64% ± 2.42%). Higher nosZ and hzsB gene abundances were also observed in rhizosphere soils than in bulk soils. Denitrification and anammox rates were significantly regulated by edaphic properties, microbial abundance, and plant biomass. The structural equation model further revealed that soil pH and N availability could affect denitrification and anammox rates both directly and indirectly by altering nosZ and hzsB gene abundance. Overall, this study highlights that the rhizosphere is a control point for N removal and harbors more functional microbes than bulk soils. Therefore, revegetation may effectively enhance the N removal function of riparian wetlands along the Yangtze River.