EFFECT OF A NITRIFICATION INHIBITOR ON N IMMOBILIZATION AND RELEASE OF 15N FROM NONEXCHANGEABLE AMMONIUM AND MICROBIAL BIOMASS

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Abstract
The disposition of 15N-aqua NH3 and 15N-solution urea in the presence and absence of a nitrification inhibitor [4-amino-1,2,4-triazole (ATC)] was measured under field conditions. ATC caused a 15% greater recovery of fertilizer N in the soil-plant system (95 vs. 80%) but no changes in wheat N uptake (37%). The 0- to 15-cm layer of ATC-treated soils contained 52–55% of the fertilizer N. The same layer of the non-ATC-treated soils contained 28–30%. The recovery of fertilizer N in the soil profile was 55–59% in ATC treatments compared to 40–42% in non-ATC treatments. Five to eight percent of fertilizer N was recovered in the non-exchangeable NH4+ fractions of A horizons of ATC-treated soils compared to ~ 1% in non-ATC treatments. Laboratory incubations and isotopic analysis of the 15N-enriched soil, a Dark Gray Chernozem, showed that the nonexchangeable 15NH4+ was released at rates equivalent to a half-life of 38 wk (k = 0.018 wk−1) at 28 ± 1 °C and soil pore water potential of 34 kPa. Particle size and mineralogical analyses showed that the coarse clay fraction composed of mica, vermiculite and smectites contained 49% of the labeled nonexchangeable NH4+; the coarse silt fraction contained 26% of the labeled nonexchangeable NH4+. After growth of wheat fertilized with NH4OH treated with ATC, the microbial biomass accounted for 41% of the organic 15N remaining in soil. Soil samples from the ATC-treated plots contained almost two times the amount of 15N in the microbial biomass compared to non-ATC treatments; this accounted for 46% of the organic 15N remaining in the soil. The average half-life of microbial biomass 15N was 27.6 wk in all the treatments. Thus, ATC caused a greater immobilization of fertilizer 15N but no change in the rate of release of 15N-microbial biomass. The conserved fertilizer would be slowly released over a long period of time. Key words: Nitrogen immobilization, soil biomass, 15N