Persistent enhancement of panicle number underlies long-term yield responses to a single Fe amendment in rice paddies

Iron (Fe) is an essential micronutrient for rice and a key modifier of soil chemical processes in paddy fields. Although Fe-based amendments have been used to improve soil fertility and crop performance, most studies have focused on short-term responses or repeated applications, leaving the long-term agronomic effect of a single Fe input under field conditions poorly understood. Here, with a seven-year field experiment at two paddy sites in Tokamachi and Nagaoka, Niigata, Japan, we examined whether a one-time application of zero-valent Fe powder could produce persistent agronomic benefits over multiple cropping seasons. Linear mixed-effects models showed that, at both sites, soil free iron oxide remained persistently higher through the seventh year after application, while rice panicle number exhibited a significant positive overall response to Fe amendment across the seven-year period (+33.0 m⁻² at Tokamachi and +11.4 m⁻² at Nagaoka). However, its conversion into grain yield differed between sites. At Tokamachi, increased panicle number was maintained as higher total grain number (+3084 m⁻²) and grain yield (+0.482 t ha⁻¹), whereas at Nagaoka, it was offset by fewer grains per panicle (−5.1 grains panicle⁻¹), resulting in no significant yield increase. To further explore the nutrient-related clues underlying the common panicle-number enhancement, we analyzed plant N and Fe uptake. Interestingly, structural equation models showed that the panicle-number response was more closely associated with enhanced N acquisition and its coordination with Fe uptake than with Fe uptake alone. Our findings indicate that Fe-based management can have long-lasting effects on rice yield formation, while highlighting the need to understand site-specific constraints on yield realization. Given that yield-component compensation was substantially weaker at Tokamachi, where background soil Fe was lower, future work should examine whether the yield benefit of Fe amendment depends on background soil Fe status.