Mineralization, nitrification, and denitrification in histosols of northern Minnesota

Water content and temperature are influences on N mineralization, nitrification, and denitrification rates in drained Histosols. To look at possible differences in these rates and to enable comparisons to mineral soils, we collected soil samples from two Minnesota Histosols planted in cultivated wild rice (Zizania palustris L.), incubated amended subsamples at 4, 7, 10, 16, and 22 degrees C, and monitored changes in NH4-N with time. Amendments used were plus or minus NH4 and plus or minus nitrapyrin. Net mineralization rates for Soil 1 (plus-nitrapyrin treatment) increased from 0.30 to 2.0 mg N kg-1 dry soil d-1 from 4 to 22 degrees C. Nitrification and denitrification occurred simultaneously, with lower denitrification than nitrification rates. Nitrification rates (determined from the zero-order plots of the disappearance of NH4-N with time in the plus-NH4 samples) increased from 4 to 22 degrees C (Soil 1, 17-99 mg N kg-1 d-1; Soil 2, 11-40 mg N kg-1 d-1). Soil 1 nitrified faster than most mineral soils; Soil 2 was comparable with many mineral soils. Denitrification rates for Soil 1, determined from the decrease in total inorganic N (NH4-N + NO3-N; plus-NH4 samples), increased from 9 mg N kg-1 d-1 at 4 degrees C to 53 mg N kg-1 d-1 at 22 degrees C. There were no differences between the two experimental water contents (corresponding to soil water pressure heads of -25 and -50 cm) for any of these rates. The high water-holding capacity of Histosols increases the potential for simultaneous nitrification and denitrification and will result in removal of native and applied NH4 from the system, thus affecting N use efficiency of both wild rice and upland crops grown on Histosols.