The study looked at how different nitrogen sources affect crop nutrition in a Chinese rice field. They used labeled fertilizer and crop residues to track nitrogen uptake by the crops over multiple growing seasons. The results showed that most of the nitrogen in the crops did not come directly from the fertilizer, but from soil organic matter. Crop residues were found to be a better source for replenishing soil nitrogen than fertilizer. Incorporating crop residues did not significantly increase nitrogen uptake by the crops. Overall, crop residues contributed more to sustaining soil organic matter than fertilizer, showing that they play a crucial role in long-term soil health and crop nutrition.