Contributions of Different N Sources to Crop N Nutrition in a Chinese Rice Field

N availability is one of the most important factors limiting crop yield enhancement. The recovery of applications of ^15N-labeled fertilizer and crop residues in a rice-wheat cropping system was determined for up to 6 consecutive growing seasons. The crop residues from the previous season were either incorporated or removed as two different treatments. Our results showed that 16.55%-17.79% （17.17% on average） of the fertilizer N was recovered in the crop during the first growing season, suggesting that more than 80% of crop N was not directly from the N fertilizer. When ^15N-labeled residues were applied, 12.01% was recovered in the crop in the first growing season. The average recoveries of fertilizer N and crop residue N in the soil after the first growing season were 33.46% and 85.64%, respectively. N from soil organic matter contributed approximately 83% of the N in the crop when ^15N fertilizer was applied or 88% when crop residues were applied. There was a larger difference in the total ^15N recovery in plant and soil between N applications in the forms of fertilizer and crop 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 1.94% of N for the fertilizer-applied field or 5.97% of N for the crop residue-applied field was recovered by the crops during the 2nd and 3rd growing seasons. The total recoveries of ^15N in crop and soil were approximately 64.38% for the fertilizer-applied field after 6 growing seasons and 79.11% for the crop residue-applied field after 5 growing seasons. Although fertilizer N appeared to be more readily available to crops than crop residue N, crop residue N replenished soil N pool, especially N from soil organic matter, much more than fertilizer N. Therefore, crop residue N was a better source for sustaining soil organic matter. Our results suggested that the long-term effect of fertilizer or crop residues on N recovery were different in the crop and soil. However, there was little difference between the practices of crop residue incorporation and residue removal following the N fertilizer application.