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Carbon and Nitrogen Movement from Surface-Applied Wheat (Triticum aestivum) Straw¹

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%.

¹ Contribution from Agric. Res., Sci. and Ed. Admin., U.S. Dep. of Agric., in cooperation with the College of Agric. Res. Center, Washington State Univ., Pullman, WA 99164. Scientific Paper no. 5570.

² Soil Scientist, Microbiologist, and Soil Scientist, respectively, USDA-SEA/AR, Pullman, WA 99164.

Received for publication January 13, 1980. Accepted for publication June 2, 1980.

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