Estimation of gross nitrogen transformations and nitrogen retention in grassland soils using FLUAZ

The flux of N through mineralization-immobilization turnover (MM in grassland soils is a major determinant for plant N uptake and for N loss processes. In this study we investigated the dynamics of gross N transformation rates and potential N retention on mineral fertilizer addition in three permanent grassland soils of varying texture (loamy sand, loam, and clay loam). Gross N transformation rates were calculated with the N-15-tracingmodel FLUAZ. Differentially N-15-labeled NH4NO3 (at a rate of 100 mg N kg(-1) soil) was added to the soils in paired laboratory incubation experiments. Size and N-15 enrichment of the NH4+, NO3-, and soil organic N pools were measured at 0, 1, 3, 7, 14, and 30 d after NH4NO3 addition. The accuracy of the simulations of the data using FLUAZ were robust, but tended to decrease (i) with increasing incubation times, (ii) with increasing duration of the time intervals considered, and (iii) with increasing experimental variability. The proportion of the initial N content mineralized on incubation was largest in the loamy sand soil (2.5%), followed by the clay loam soil (1.2%) and the loam soil (0.8%). The actual gross nitrification and N immobilization activity followed the same trend. The loam soil showed the lowest relative N retention (ratio N immobilization over [gross N mineralization + gross nitrification]), which was attributed to its low C availability. In general, the N-15 retention after addition of (NH4NO3)-N-15-N-14 was approximately five times larger than after addition of (NH4NO3)-N-14-N-15.