Deep Placement of Nitrogen Fertilizers Increases Nitrogen Utilization Efficiency in No-tillage Paddy Fields Mainly by Inhibiting Nitrification

Deep placement of nitrogen (N) fertilizers can effectively increase N utilization efficiency (NUE) in no-tillage (NT) paddy fields. Nitrification and denitrification are potential ways for affecting NUE by affecting inorganic N supply in crop. However, the effect of nitrification and denitrification on NUE still remains unknown under NT paddy fields with deep placement of N fertilizers. This study investigated the changes in nitrification and denitrification, and their possible effects on NUE with different application methods of N fertilizers [no N (CK), N broadcasting (BC), and point deep placed at 10 cm depth (DP) of N fertilizers] under NT paddy fields. The abundances of ammonium oxidation archaeal (AOA), ammonium oxidation bacteria (AOB), and nirK / S-type denitrifiers were measured by absolute real-time qPCR. Inorganic nitrogen (NH ₄ ⁺ and NO ₃ ⁻ ) transport rates from rhizospheric soil to root surface were measured by a transport model with ¹⁵ NH ₄ ⁺ and ¹⁵ NO ₃ ⁻ isotope. Structural equation modeling was constructed to analyze the data. In both the 2022 and 2023 rice growing seasons, DP lowered nitrification rate and increased denitrification rate with functional microorganisms significantly relative to BC in the non-rhizosphere and rhizosphere. DP obviously enhanced the mean rhizospheric NH ₄ ⁺ transport rate by 21.7% and cut down rhizospheric NO ₃ ⁻ transport rate by 17.9% versus BC. The strengthened root system in DP advanced NUE by 26.5%-27.9% compared with BC. Structural equation modeling shows that DP raises NUE in NT paddy fields mainly by regulating rhizospheric inorganic nitrogen transport rate through inhibiting nitrification.