Abstract:

【Objective】 Among the various factors that affect crop yield, N availability is one of the most important limiting factors impeding crop yield increase. 【Method】 In this study, the recovery of a single application of 15N-labeled fertilizer or residues in rice-wheat cropping system was determined for up to 5/6 consecutive growing seasons. The crop residues from the previous season were either incorporated or removed as two different treatments. 【Result】 Results showed that, on average, 17.17% (16.55%-17.79%) of crop-N was derived from N fertilizer during the first growing season, suggesting that approximately 80% of crop N was not directly from the N fertilizer. When 15N-labeled residues were applied, 4.48% of the crop-N was derived from residues in the first growing season. Soil organic matter N contributed 82.83% of the N in the crop when 15N-fertilizer was applied or 87.99% when organic residues were applied. The average recovery of fertilizer-N and residue-N in the soil after the first growing season was 33.46% and 85.64%, respectively. There was a larger difference in the total 15N recovery in plant and soil when N was applied as fertilizer or residues. Incorporation of crop residues following the 15N-fertilizer application did not significantly promote 15N recovery in the crop or soil. On average, only additional 1.46% of N for the fertilizer-applied field or 5.97% of N in the residue-applied field was recovered by the crops during the 2nd and 3rd growing season. After 6 growing seasons, the total recoveries of 15N in crop and soil were approximately 64.38% for fertilizer-applied field and 79.11% for residue-applied field. Although fertilizer-N appeared to be more readily available to crops than residue-N, residue-N replenished soil N pool, especially N in soil organic matter, much more than fertilizer-N after 6 growing seasons. Therefore, residue-N is a better source for sustaining soil organic matter N content. 【Conclusion】 Results of experiment suggest that the long-term effects of fertilizer or residues in the crop and soil are different. However, there was a little difference between the practices of residue incorporation and residue removal following the N fertilizer application.

Key words: crop residue, fertilizer fate, 15N isotope, nitrogen fertilizer, rice field