Nitrogen Fertilizer Formulations Modulate the Yield–Cadmium Trade-Off in Rice Through Rhizosphere Processes and Translocation Nodes

Cadmium (Cd) contamination in paddy soils represents a major risk to global food security and human health because Cd can readily enter the food chain through rice consumption. Therefore, clarification of the key processes and mechanisms by which agronomic practices regulate Cd accumulation in rice is essential. Based on integrated two-year pot and field experiments, we showed that different nitrogen fertilizer formulations, including nitrate-N (N), ammonium-N (A), and urea (U), had distinct effects on Cd accumulation and grain yield in rice. The N treatment reduced Cd concentrations in brown rice by 25.29% to 70% in the pot experiment and by 4.25% to 89.97% in the field experiment but decreased grain yield by 5% to 25%. By contrast, the A treatment increased Cd concentrations in brown rice by 17.86% to 58.62%, while maintaining or slightly increasing grain yield (−3% to +5%), and the U treatment showed intermediate responses. These responses were mainly associated with nitrogen-induced shifts in rhizosphere chemistry, especially changes in soil Cd availability linked to pH and exchangeable H+, although unmeasured redox-related processes may have also contributed under flooded conditions. Further analyses of internal Cd distribution and translocation, together with exploratory random forest modeling, suggested that Cd transport efficiency at key stem internodes and Cd redistribution from the panicle to the grain were important regulatory nodes associated with Cd concentrations in brown rice. These regulatory nodes were markedly affected by fertilizer formulation. Overall, our results describe a continuous pathway from rhizosphere Cd availability to internal transport and partitioning, through which nitrogen fertilizer formulations regulate Cd accumulation in rice. This study aimed to clarify how different nitrogen fertilizer formulations regulate the trade-off between grain yield and Cd accumulation in rice, based on the hypothesis that these formulations differentially modify rhizosphere chemistry and Cd bioavailability, that specific stem nodes contribute to control of grain Cd accumulation, and that the main regulatory processes differ between pot and field systems. This study provides s a scientific basis for developing practical nitrogen-management strategies to support safe rice production in Cd-contaminated paddy fields.