Vertical characteristics of anaerobic oxidation of ammonium (anammox) in a coastal saline-alkali field

Anammox contributes significant nitrogen (N) loss in agricultural ecosystems, yet its role in the saline-alkali field is not well predicted. Here, we investigate the activity, functional gene abundance and community composition of anammox bacteria in a coastal saline-alkali soil along depth gradient with isotope tracing technique, quantitative PCR assay, and high-throughput sequencing. The anammox activities ranged from 0.09 to 1.32 nmol N g−1 h−1 but were undetectable below 70 cm. The anammox bacteria were present through the soil with the abundance varying from 2.5 × 104 to 5.3 × 105 copies g−1 soil. The Illumina sequencing targeting anammox 16S rRNA gene identified three anammox genera with Candidatus Scalindua dominating. Anammox contribute 40–87.5% of total N2 at 0−50 cm depth whereas the percentage declined sharply below 50 cm. We conclude that N loss in upper soil is mainly linked to anammox while dominated by denitrification in the deeper coastal saline-alkali field.