Distinct response of anammox and denitrifiers to organic or inorganic fertilizers in a paddy soil

Fertilizer application stimulates anammox and denitrification in paddy soils, resulting in increased gaseous nitrogen loss. Understanding how organic and inorganic fertilizers differently regulate microbial mediated nitrogen loss pathways in paddy soils can help improve field nitrogen management. But how microbial abundance and activity respond differentially to various fertilization practices remains unclear, restricting the comprehensive understanding of fertilization-induced nitrogen loss mechanisms. This study applied isotope tracer techniques to quantify soil anammox and denitrification rates under different fertilization treatments at a long-term field experimental site. Anammox and denitrifying microorganisms were investigated at the DNA and RNA levels. Results indicated that all fertilization practices stimulated anammox and denitrification. Rates of anammox for the no-fertilization control (CK) averaged 0.22 nmol N g −1 dry soil h −1 , and organic and inorganic fertilizer application increased rates by 31.8% and 68.2% compared with CK. Denitrification followed the same trend as anammox. The abundance of anammox and denitrifiers for CK were 0.249 ± 0.029 × 10 5 copies·g −1 (dry soil) and 0.530 ± 0.030 × 10 6 copies·g −1 (dry soil), respectively. Application of inorganic fertilizer significantly increased the abundance of these bacteria. Organic fertilizer increased the activity-related responses of anammox and denitrifying microorganisms compared with CK or NPK, suggesting that inorganic fertilizer exerted a stronger influence on microbial abundance, whereas organic fertilizer showed a greater effect on microbial activity-related traits. This study differentiates the responses of microbial abundance and activity to different fertilization treatments, which helps clarify the microbial mechanisms underlying fertilization-regulated soil nitrogen loss. Fertilizers, however, did not significantly alter the community structure of the anammox and denitrifying microorganisms investigated in this study. Candidatus Brocadia is the main group of anammox, and Rhizobiaceae bacterium N50 remained the dominant group among the denitrifiers. This study illustrates the differing microbial regulatory patterns by which organic and inorganic fertilizers affect nitrogen loss in paddy soils, which may provide a theoretical reference for optimizing fertilization practices to mitigate nitrogen gaseous loss and promote nitrogen utilization efficiency.