Microbial pathways for nitrogen loss in an upland soil

The distribution and importance of anaerobic ammonium oxidation (anammox) and nitrite-dependent anaerobic methane oxidation (n-damo) have been identified in aquatic ecosystems; their role in agricultural upland soils however has not yet been well investigated. In this study, we examined spatio-temporal distributions of anammox and n-damo bacteria in soil profiles (300 cm depth) from an agricultural upland. Monitoring nitrogen (N) conversion activity using isotope-tracing techniques over the course of one year showed denitrification (99.0% N-loss in the winter and 85.0% N-loss in the summer) predominated over anammox (1.0% N-loss in the winter and 14.4% N-loss in the summer) and n-damo (0.6% N-loss in the winter) in surface soils (0-20 cm). While below 20 cm depth, N-loss was dominated by anammox (79.4 ± 14.3% in the winter and 65.4 ± 12.5% in the summer) and n-damo was not detected. Phylogenetic analysis showed that Candidatus Brocadia anammoxidans dominated the anammox community in the surface soil and Candidatus Brocadia fulgida dominated below 20 cm depth. Dissimilatory nitrate reduction to ammonium (DNRA), another nitrite reduction process, was found to play a limited role (4.9 ± 3.5%) in the surface soil compared with denitrification; below 80 cm DNRA rates were much higher than rates of anammox and denitrification. Ammonium oxidation was the main source of NO2- above 80 cm (70.9 ± 23.3%), the key influencing factor on anammox rates, and nitrate reduction (100%) was the main NO2- source below 80 cm. Considering the anammox, n-damo and denitrification rates as a whole in the sampled soil profile, denitrification is still the main N-loss process in upland soils.