Field and laboratory study of nitrogen mineralization dynamics in four Tunisian soils

Prediction of nitrogen mineralization from soil organic matter is important when we try to optimize nitrogen fertilization and assess adverse nitrogen impacts on the environment. Thus, four non-fertilized soils were selected in Northern Tunisia to investigate, throughout a field and laboratory approach, the influence of soil properties on nitrogen mineralization and to classify them according to their potentially mineralizable nitrogen and kinetics. Our field results showed an enrichment in inorganic nitrogen contents mainly in surface horizons. These contents decreased with depth following different patterns depending on the soil type. The highest content of inorganic nitrogen was recorded in the Calcisol (232.09 mg kg(-1)) due to high organic carbon and nitrogen amounts and low C:N ratio throughout the profile. The lowest content was recorded in the Luvisol (< 30 mg kg(-1)) because of its large clay-silt fraction and low pH level (4.8 +/- 0.1) especially in depth. The vertical distribution of ammonium and nitrate contents showed marked monthly variations. During January and February, the decrease in inorganic nitrogen contents was attributed to lower soil temperature and higher moisture causing the nitrate to be more leached. In March, inorganic nitrogen contents reached their maximum in surface horizons. These contents started to decrease in April and regressed in May and June except for the Vertisol, which still recorded high-inorganic nitrogen content. The laboratory results presented the following decreasing order of potentially mineralizable nitrogen and kinetics: Calcisol > Vertisol > Cambisol > Luvisol, and revealed two fractions constituting organic nitrogen supplies. An active fraction with a rapid mineralization and a passive fraction slowed down by clays and resistant to biodegradation.