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

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SummaryWith 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.