Effect of soil-applied calcium carbide and plant derivatives on nitrification inhibition and plant growth promotion

The aim of this study was to evaluate the relative performance of three nitrification inhibitors (NIs) viz. calcium carbide (CaC₂), and plant derivatives of Pongamia glabra Vent. (karanj) and Melia azedarach (dharek) in regulating N transformations, inhibiting nitrification and improving N recovery in soil–plant systems. In the first experiment under laboratory incubation, soil was amended with N fertilizer diammonium phosphate (NH₄)₂HPO₄] at a rate of 200 mg N kg^?1, N + CaC₂, N + karanjin, and N + M. azedarach and incubated at 22 °C for 56 days period. Changes in total mineral N (TMN), NH₄ ⁺–N and NO₃ ^?–N were examined during the study. A second experiment was conducted in a glasshouse using pots to evaluate the response of wheat to these amendments. Results indicated that more than 92 % of the NH₄ ⁺ initially present had disappeared from the mineral N pool by the end of incubation. Application of NIs i.e., CaC₂, karanjin, and M. azedarach resulted in a significant reduction in the extent of NH₄ ⁺ disappearance by 49, 32, and 13 %, respectively. Accumulation of NO₃ ^?–N was much higher in N amended soil 57 % compared to 11 % in N + CaC₂, 13 % in N + karanjin, and 18 % in N + M. azedarach. Application of NIs significantly increased growth, yield, and N uptake of wheat. The apparent N recovery in N-treated plants was 20 % that was significantly increased to 38, 34, and 37 % with N + CaC₂, N + karanjin, and N + M. azedarach, respectively. Among the three NIs tested, CaC₂ and karanjin proved highly effective in inhibiting nitrification and retaining NH₄ ⁺–N in the mineral pool for a longer period.