Optimizing rice yield: evaluating the nitrogen supply characteristics of slow- and controlled-release fertilizers using the leaf nitrogen balance index

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Synchronizing the nitrogen (N) supply of slow- and controlled-release N fertilizers (SCRNFs) with rice N demand is pivotal in substituting multiple urea applications by a single basal application of SCRNFs. Traditional assessment of N supply characteristics focuses mainly on N release patterns, which are only applicable to coated SCRNFs and ignore N transformation mechanisms, thus raising the need for a more universal and reliable index. Based on the ability of crop N status to detect N deficiency or excess, we hypothesized that employing leaf N balance index (NBI) as a measure of N status could provide new insights into assessing N supply characteristics of SCRNFs. We conducted field experiments involving four individual SCRNFs-humic acid urea (HAU), sulfur-coated urea (SCU), urease inhibitor urea (UIU), and polymer-coated urea (PCU)- and their four combined forms, along with the high-yield urea split application as control (CK). The results showed that NBI dynamics relative to CK could reflect the N supply potential of different SCRNFs while classifying them as short-, medium-, and long-acting fertilizers. Combinations that incorporated the long-acting SCRNF (PCU) consistently outperformed others in yield (by 5.5%) and N use efficiency (by 42.8%) by providing a more consistent and efficient N supply throughout the rice growth cycle. Grain yield showed negative correlation with the difference in NBI dynamics between SCRNFs and CK, indicating synchronizing N supply between one-time application of SCRNFs and conventional high-yield fertilization is the key for high yield. Our findings identify the potential of N status diagnosed by leaf NBI to evaluate N supply characteristics of SCRNFs and emphasize the significant role of synchronized N supply for a one-time SCRNF application.