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 a first study we investigated the relationship between soil organic matter (SOM) content and gross N transformations in three grassland soils of different age (6, 14 and 50 y-old) with sandy loam textures. In a second study, gross N transformation rates and N retention were investigated upon 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. ¹⁵N-labeled NH₄NO₃ 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 days after NH₄NO₃-addition. In general, the accuracy of the simulations of the data using FLUAZ were robust, but tended to decrease (1) with increasing incubation times, (2) with increasing duration of the time intervals considered and (3) with increasing experimental variability.

Gross N mineralization, nitrification, and (long-term) gross N immobilization rates tended to increase with increasing age of the grasslands, and showed strong, positive correlations with the total C and N contents. The calculated gross N mineralization rates (7-d incubations) and net N mineralization rates (70-d incubations) corresponded with a gross N mineralization of 643, 982 and 1876 kg N ha^-1 y^-1, and a net N mineralization of 195, 208 and 274 kg N ha^-1 y^-1 in the upper 20 cm of the 6, 14 and 50 y-old grassland soils, respectively. Linear regression analysis showed that 93% of the variability of the gross N mineralization rates could be explained by variation in total N contents, whereas total N contents together with the C-to-N ratios of the <50 µm SOM fraction explained 84% of the variability of the net N mineralization rates. The relationship between long-term net N mineralization rates and gross N mineralization rates could be fitted by means of a logarithmic equation (net m = 0.24Ln(gross m) + 0.23, R²=0.69, P<0.05). The ratio of gross N immobilization-to-gross N mineralization tended to increase with increasing SOM contents. Microbial demand for N tended to increase with increasing SOM content in the grassland soils, indicating that potential N retention in soils through microbial N immobilization tends to be limited by C availability.

The proportion of the initial N content mineralized upon 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 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.

The results of these studies are published in: 1. Accoe et al. (2004) Soil Biology & Biochemistry 36: 2075-2087; and 2. Accoe et al. (2005) Soil Science Society of America Journal 69: 1967-1976.

Keywords: ¹⁵N, gross N mineralization, immobilization, grasslands, soil organic matter, N transformation, ¹⁵N-labeled NH₄NO₃