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 reduce d. Samples of whole soil, clay (< 2 mu m), silt (2-30 mu m) and sand ( 20-2000 mu m) sized organomineral separates from three soils with annu al 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 degrees C, to dete rmine the content of NH4 + NO3. A three-pool model using first order k inetics 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 da ta. 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, respectivel y. The N mineralizability increased in the order: sand < silt < clay. The lability of N in a given size separate was almost similar across s oil types and straw managements. The active N pools (N-1 + N-2) averag ed 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)). Maxim um response was at 4 t straw ha(-1); adding more straw diminished mine ralization of N. Long-term annual incorporation of cereal straw contri butes mainly soil N with a slow turnover.