Nitrification and denitrification in an acidic soil of tea (Camellia sinensisL.) field estimated by δ15N values of leached nitrogen from the soil columns treated with ammonium nitrate in the presence or absence of a nitrification inhibitor and with slow-release fertilizers

Nitrification and denitrification processes of inorganic nitrogen (N) from several fertilizers placed in an acid soil (pH 4.1) of tea fields were examined in two experiments based on the NO3-N leaching pattern from soil columns (20 cm depth) and on the δ15N values of leached N. In the first experiment, ammonium nitrate (AN) was placed in the soil columns with and without dicyandiamide (DCD), a nitrification inhibitor. The NO3-N content in the leachate was more than 10 times higher than the NH4-N content and DCD application decreased the NO3-N content in the leachate. The δ15N values of the leached N with and without DCD initially decreased, with minimum values of − 3.8‰and − 1.5\%, respectively, which were lower than that of applied AN (+ 0.7\%). These results showed that 15N depletion occurred by the conversion of NH4-N to NO3-N through nitrification under acidic soil conditions (pH 4.1) and larger isotopic fractionation in the presence of DCD. In the second experiment, four slow-release fertilizers, isobutyriden diurea (IB), oxamide (Oxa), polyolefin-coated area (100-d type; PCU), and rape seed meal (RSM), were placed in the soil columns and changes in the inorganic N content in the leachates were compared with the non-fertilized treatment. At the end of the experiment, the amounts of leached N were 52% and 44% in the treatment with Oxa and RSM, respectively, suggesting that organic N was strongly mineralized under the acidic soil conditions in the tea field. The δ15N values of the leached N from the applied N were calculated from the isotopic balance as compared with the volumes of soil-derived N, which were asuumed to be the same in the presence or absence of fertilization. The cumulative δ15N values of the leached N and of inorganic N remaining in soil suggested that some immobilization of fertilizer N had occurred in the treatment with PCU. The cumulative δ15N values in the treatments with IB, Oxa, and RSM suggested the presence of high rates of denitrification, probably associated with the emission of nitrous oxide commonly observed in tea fields amended with organic materials like rape seed meal.