Nitrogen Availability from Surface‐Applied and Soil‐Incorporated Crop Residues

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Abstract
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 15N tagged alfalfa (Medicago sativa L.), sorghum [Sorghum bicolor (L.) Moench], 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 0.25‐mm 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 −1 for sorghum and wheat (−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.
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