Nitrogen transformation rates and N2O producing pathways in two pasture soils

Purpose: Better understanding of N transformations and the regulation of N2O-related N transformation processes in pasture soil contributes significantly to N fertilizer management and development of targeted mitigation strategies. Materials and methods: 15N tracer technique combined with acetylene (C2H2) method was used to measure gross N transformation rates and to distinguish pathways of N2O production in two Australian pasture soils. The soils were collected from Glenormiston (GN) and Terang (TR), Victoria, Australia, and incubated at a soil moisture content of 60% water-filled pore space (WFPS) and at temperature of 20 °C. Results and discussion: Two tested pasture soils were characterized by high mineralization and immobilization turnover. The average gross N nitrification rate (ntot) was 7.28 mg N kg−1 day−1 in TR soil () and 5.79 mg N kg−1 day−1 in GN soil. Heterotrophic nitrification rates (nh), which accounting for 50.8 and 41.9% of ntot, and 23.4 and 30.1% of N2O emissions in GN and TR soils, respectively, played a role similar with autotrophic nitrification in total nitrification and N2O emission. Denitrification rates in two pasture soils were as low as 0.003–0.004 mg N kg−1 day−1 under selected conditions but contributed more than 30% of N2O emissions. Conclusions: Results demonstrated that two tested pasture soils were characterized by fast N transformation rates of mineralization, immobilization, and nitrification. Heterotrophic nitrification could be an important NO3−–N production transformation process in studied pasture soils. Except for autotrophic nitrification, roles of heterotrophic nitrification and denitrification in N2O emission in two pasture soils should be considered when developing mitigation strategies