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

Abstract

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.