Uniformity of Nitrogen‐15 Enrichment in Different Plant Parts and Subsequent Decomposition Monitoring of Labeled Crop Residues

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Abstract
In order to accurately evaluate the N mineralization from decomposing crop residue, residue should be uniformly labeled, i.e., it should contain a relatively constant atom % 15N in all N compounds within the plant. Our objective in this study was to estimate the uniformity of 15N labeling by comparing the 15N content of different plant parts of winter wheat (Triticum aestivum L.) and grain sorghum [Sorghum bicolor (L.) Moench]. Labeling with 15N was accomplished by applying a single, highly enriched but low rate of N to the soil early in the growth stage of the respective crops. Small enclosed microplots were used for incorporating the 15N enriched fertilizer into the initial crop and for following decomposition in subsequent years. At crop maturity, recovery of the initial K15NO3 application avg. 71.9% for all soil texture‐crop combinations. Data from the 15N labeling study indicated that relatively uniform enrichment of various plant parts was obtained when K15NO3 was applied to the soil at the jointing and eight‐leaf stages for wheat and sorghum, respectively. On average, atom % 15N values for all plant parts ranged from 6.962 to 8.171 and 2.801 to 3.274 for winter wheat and sorghum, respectively. These results imply that segregation of 15N‐labeled plant material into various plant parts for use in subsequent crop residue decomposition experiments is unnecessary.

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