Community composition and activity of anaerobic ammonium oxidation bacteria in the rhizosphere of salt-marsh grass Spartina alterniflora

Anaerobic ammonium oxidation (anammox) as an important nitrogen removal pathway has been investigated in intertidal marshes. However, the rhizosphere-driven anammox process in these ecosystems is largely overlooked so far. In this study, the community dynamics and activities of anammox bacteria in the rhizosphere and non-rhizosphere sediments of salt-marsh grass Spartina alterniflora (a widely distributed plant in estuaries and intertidal ecosystems) were investigated using clone library analysis, quantitative PCR assay, and isotope-tracing technique. Phylogenetic analysis showed that anammox bacterial diversity was higher in the non-rhizosphere sediments (Scalindua and Kuenenia) compared with the rhizosphere zone (only Scalindua genus). Higher abundance of anammox bacteria was detected in the rhizosphere (6.46 × 10⁶–1.56 × 10⁷ copies g⁻¹), which was about 1.5-fold higher in comparison with that in the non-rhizosphere zone (4.22 × 10⁶–1.12 × 10⁷ copies g⁻¹). Nitrogen isotope-tracing experiments indicated that the anammox process in the rhizosphere contributed to 12–14 % N₂ generation with rates of 0.43–1.58 nmol N g⁻¹ h⁻¹, while anammox activity in the non-rhizosphere zone contributed to only 4–7 % N₂ production with significantly lower activities (0.28–0.83 nmol N g⁻¹ h⁻¹). Overall, we propose that the rhizosphere microenvironment in intertidal marshes might provide a favorable niche for anammox bacteria and thus plays an important role in nitrogen cycling.