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

Synchronizing the nitrogen (N) supply of slow- and controlled-release N fertilizers (SCRNFs) with rice N demand is essential in replacing multiple urea applications with a single basal application of SCRNFs. Traditional assessment of N supply characteristics primarily examines N release patterns, which are limited to coated SCRNFs and disregard N transformation mechanisms, necessitating a more universal and reliable index. Based on the capacity of crop N status to detect N deficiency or excess, we hypothesized that utilizing leaf N balance index (NBI) as a measure of N status could offer novel insights into assessing N supply characteristics of SCRNFs. Field experiments were conducted with 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, alongside high-yield urea split application as control (CK). The results revealed that NBI dynamics relative to CK reflected the N supply potential of different SCRNFs while categorizing them as short-, medium-, and long-acting fertilizers. Combinations incorporating the long-acting SCRNF (PCU) consistently demonstrated superior performance in yield (by 5.5%) and N use efficiency (by 42.8%) through providing more consistent and efficient N supply throughout the rice growth cycle. Grain yield exhibited negative correlation with the difference in NBI dynamics between SCRNFs and CK, suggesting that synchronizing N supply between one-time application of SCRNFs and conventional high-yield fertilization is crucial for high yield. These findings demonstrate the potential of N status diagnosed by leaf NBI to evaluate N supply characteristics of SCRNFs and highlight the importance of synchronized N supply for a one-time SCRNF application.