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Soil Biology & Biochemistry

Estimation of Gross Nitrogen Transformations and Nitrogen Retention in Grassland Soils Using FLUAZ

^a European Commission, Joint Research Centre DG, Inst. for Reference Materials and Measurements, B-2440 Geel, Belgium
^b Lab. of Applied Physical Chemistry-ISOFYS, Faculty of Biosciences Engineering, Ghent Univ., Coupure Links 653, B-9000 Gent, Belgium
^c Chilean Nuclear Energy Commission, Amunátegui 95, P.O. Box 188-D, Santiago, Chile
^d Dep. of Soil Management and Soil Care, Faculty of Bioscience Engineering, Ghent Univ., Coupure Links 653, B-9000 Gent, Belgium

^* Corresponding author (pascal.boeckx{at}ugent.be)

The flux of N through mineralization–immobilization turnover (MIT) 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 ¹⁵N-tracing model FLUAZ. Differentially ¹⁵N-labeled NH₄NO₃ (at a rate of 100 mg N kg^–1 soil) was added to the soils in paired laboratory incubation experiments. Size and ¹⁵N 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 ¹⁵N retention after addition of ¹⁵NH₄¹⁴NO₃ was approximately five times larger than after addition of ¹⁴NH₄¹⁵NO₃.

Abbreviations: DNRA, dissimilatory nitrate reduction to ammonium • MIT, mineralization-immobilization turnover • MWE, mean weighted error • SOM, soil organic matter