NITROGEN AVAILABILITY FROM SURFACE-APPLIED AND SOIL-INCORPORATED CROP RESIDUES

Management systems which maintain crop residues on the soil surface have prompted a need for more detailed information regarding crop residue N availability. Such information is needed from both agronomic and environmental standpoints. Our study examined the placement effects of N-15 tagged alfalfa (Medicago sativa L.), sorghum [Sorghum bicolor (L.) Moenchl, and wheat (Triticum aestivum L.) residues on crop residue N availability in association with eight Oklahoma surface soils representing five soil orders. The experiment was conducted under aerobic laboratory conditions, and involved 0.25mm crop residue materials and 2-mm soil particles. Crop residue N availability was determined on the basis of N mineralized during short-term (14 d) and long-term (168 d) incubation. Mean amounts of crop residue N mineralized ranged from <18 kg N ha-1 for sorghum and wheat (<28% of residue N) to 55 kg N ha-1 for alfalfa (35% of residue N). Surface placement of crop residue reduced residue N availability compared to soil incorporation, but the differences were only equivalent to 1 to 7 kg N ha-1. Relative to residue type, the influence of soil type on crop residue N mineralization was secondary. Overall, differences in crop residue N availability associated with residue placement should have minor agronomic and environmental impact.