Mineralization of Nitrogen from Nitrogen‐15 Labeled Crop Residues Under Field Conditions

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Abstract
The management of crop residues and their contribution to the supply of plant nutrients is intimately tied with N cycling in the soil. Any attempt to utilize crop residues to the fullest extent requires an understanding of how the system functions. The objective of this research was to determine the extent and rate of N mineralization from 15N‐labeled winter wheat (Triticum aestivum L.) and grain sorghum [Sorghum bicolor (L.) Moench] residues. In addition, the influence of soil texture and fertilizer N on net N mineralization also was evaluated. Nitrogen mineralization from decomposing crop residues was followed by means of plant and soil sampling during subsequent cropping seasons. Plant N uptake and soil inorganic N data showed that 12 to 15% of wheat residue N and 12 to 33% of sorghum residue N were mineralized after one cropping season. Net mineralization differences between crop residues may be attributed to both the C/N ratios and chemical composition of these plant materials. Soil texture and the addition of fertilizer N mainly influenced the initial rate of mineralization, particularly with the wheat residue. There was also evidence of N immobilization of the recently mineralized 15N followed by remineralization later in the growing season for wheat and sorghum residues decomposing in the medium‐textured soils. This immobilization occurred during a period of rapidly increasing soil temperatures. On avg, 79% of the mineralized wheat residue N and 82% of the mineralized sorghum residue N were utilized by wheat and sorghum plants, respectively. Based on these results, the greatest proportion of N mineralized from decomposing wheat and sorghum residues appears to coincide with plant N demand by the subsequent crop, irrespective of differences between C/N ratios of these residues or soil texture.