Nitrogen mineralization potential of organomineral size separates from soils with annual straw incorporation

With annual incorporation of straw, soil N mineralization is expected to increase whereby requirements for fertilizer N inputs may be reduced. Samples of whole soil, clay (< 2 μm), silt (2–20 μm) and sand (20–2000 μm) sized organomineral separates from three soils with annual additions of straw ranging from 0 to 12 t ha^–1 were leached after 0, 1, 2, 4, 8, 12 and 16 weeks of incubation at 20°C, to determine the content of NH₄ + NO₃. A three-pool model using first order kinetics and fixed rate constants (N₁, k₁ = 0.231 day^–1; N₂, k₂ = 0.00693 day^–1; N₃, k₃ = 0) was fitted to the mineralization data. The mineralizability of whole soil N (mg N g^–1 N) differed among soil types. Straw generally increased the fast N₁ and the passive N₃ pool while the medium-term N₂ pool was reduced in size. The N₁, N₂ and N₃ averaged 0.8, 2.6 and 96.6% of the whole soil N, respectively. The N mineralizability increased in the order: sand < silt < clay. The lability of N in a given size separate was almost similar across soil types and straw managements. The active N pools (N₁ + N₂) averaged 7.1% of the clay N and 2.2% of the silt N. The main difference was related to the N₂ pool, which accounted for 5.5% in clay and 1.2% in silt. Mineral N produced during incubation ranged from 63 to 105 kg N ha^–1. Effects of straw disposal were small (< 11 kg N ha^–1). Maximum response was at 4 t straw ha^–1; adding more straw diminished mineralization of N. Long-term annual incorporation of cereal straw contributes mainly soil N with a slow turnover.