NITROGEN AVAILABILITY FROM SURFACE-APPLIED AND SOIL-INCORPORATED CROPRESIDUES

Management systems which maintain crop residues on the soil surface ha ve prompted a need for more detailed information regarding crop residu e N availability. Such information is needed from both agronomic and e nvironmental 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 represe nting five soil orders. The experiment was conducted under aerobic lab oratory conditions, and involved 0.25mm crop residue materials and 2-m m soil particles. Crop residue N availability was determined on the ba sis of N mineralized during short-term (14 d) and long-term (168 d) in cubation. Mean amounts of crop residue N mineralized ranged from <18 k g 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 differ ences were only equivalent to 1 to 7 kg N ha-1. Relative to residue ty pe, the influence of soil type on crop residue N mineralization was se condary. Overall, differences in crop residue N availability associate d with residue placement should have minor agronomic and environmental impact.