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

Summary 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 NH 4 + NO 3 . A three‐pool model using first order kinetics and fixed rate constants (N 1 , k 1 = 0.231 day –1 ; N 2 , k 2 = 0.00693 day –1 ; N 3 , k 3 = 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 N 1 and the passive N 3 pool while the medium‐term N 2 pool was reduced in size. The N 1 , N 2 and N 3 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 (N 1 + N 2 ) averaged 7.1% of the clay N and 2.2% of the silt N. The main difference was related to the N 2 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.