Soil residual nitrogen under various crop rotations and cultural practices

Crop rotation and cultural practice may influence soil residual N available for environmental loss due to crop N uptake and N immobilization. We evaluated the effects of stacked vs. alternate-year crop rotations and cultural practices on soil residual N (NH4-N and NO3-N contents) at the 0-125 cm depth, annualized crop N uptake, and N balance from 2005 to 2011 in the northern Great Plains, USA. Stacked rotations were durum (Triticum turgidum L.)-durum-canola (Brassica napus L.)-pea (Pisum sativum L.) (DDCP) and durum-durum-flax (Linum usitatissimum L.)-pea (DDFP). Alternate-year rotations were durum-canola-durum-pea (DCDP) and durum-flax-durum-pea (DFDP). Both of these are legume-based rotations because they contain legume (pea) in the crop rotation. A continuous durum (CD) was also included for comparison. Cultural practices were traditional (conventional tillage, recommended seeding rate, broadcast N fertilization, and reduced stubble height) and improved (no-tillage, increased seeding rate, banded N fertilization, and increased stubble height) systems. The amount of N fertilizer applied to each crop in the rotation was adjusted to soil NO3-N content to a depth of 60 cm observed in the autumn of the previous year. Compared with other crop rotations, annualized crop biomass N was greater with DCDP and DDCP in 2007 and 2009, but was greater with DDFP than DCDP in 2011. Annualized grain N was greater with DCDP than CD, DFDP, and DDFP and greater in the improved than the traditional practice in 2010 and 2011. Soil NH4-N content was greater with CD than other crop rotations in the traditional practice at 0-5 cm, but was greater with DDCP than CD and DDFP in the improved practice at 50-88 cm. Soil NO3-N content was greater with CD than other crop rotations at 5-10 cm, but was greater with CD and DFDP than DCDP and DDCP at 10-20, 88-125, and 0-125 cm. Nitrate-N content at 88-125 and 0-125 cm was also greater in the traditional than the improved practice. Nitrogen balance based on the difference between N inputs and outputs was greater with crop rotations than CD. Increased N fertilization rate increased soil residual N with CD, but legume N fixation increased N balance with crop rotations. Legume-based crop rotations (all rotations except CD) reduced N input and soil residual N available for environmental loss, especially in the improved practice, by increasing crop N uptake and N immobilization compared with non-legume monocrop.