Carbon and Nitrogen Movement from Surface‐Applied Wheat (Triticum aestivum) Straw

Abstract

Abstract The N immobilization potential of surface‐applied wheat ( Triticum aestivum L.) straw as compared with incorporated straw was evaluated in the laboratory with soil columns. The columns were leached weekly and C and N content of the leachate determined. Leachate C/N ratios for straw alone exceeded 20:1 on several occasions and reached a maximum of 56, indicating a potential for N immobilization. Less than 5% of the total C in the straw was recovered in the leachates, providing an immobilization potential of <5 kg N ha −1 . Leachate C/N ratios from 1‐, 2‐, and 4‐cm deep soil columns with surface‐applied wheat straw and no fertilizer N ranged up to 55:1, 30:1, and 22:1, respectively, while the highest leachate C/N ratio from the 4‐cm mixed straw treatment was 30:1. A significant percentage of mineralized N was immobilized in the top 1 and 2 cm of soil by surface residues. Much less N was immobilized in the 4‐cm soil columns. Thus, placement of fertilizer N several centimeters below the soil surface would alleviate possible N immobilization from organic C leached from surface crop residues. The amount of applied N recovered in the leachate during 9 weeks of incubation ranged from 60 to 70% for all soil column treatments with or without surface straw. There was no significant differences between treatments. In contrast, the recovery of applied N from the mixed straw treatment was only 36%, indicating a much greater potential for N immobilization with mixed than with surface straw. The quantity of the fertilizer N added probably masked the immobilization potential of the surface residues. Fertilizer N stimulated early release of C from the straw alone treatment, but after 9 weeks of incubation the overall C loss from both fertilized and unfertilized straw was about 30%.