Polypeptide urea inhibits cadmium uptake by enhancing Ammonium Nitrogen absorption in rice roots

Cadmium (Cd) contamination in paddy soils poses a significant global environmental management challenge, threatening food safety and agricultural sustainability. In the soil contaminated with Cd, the absorption of Cd by rice roots is significantly affected by the type of nitrogen (N) fertilizer. However, the mechanism by which polypeptide urea (PASP-urea) regulates root Cd uptake remains insufficiently understood. This study utilized rice varieties Zhonghua 11 and Shuhui 498 as test materials in three-year natural field pot experiments and systematically compared the effects of conventional urea and PASP-urea on soil nitrogen dynamics and Cd bioavailability. In particular, differences in NH₄⁺ and Cd²⁺ uptake by distinct root zones of rice under different fertilization treatments were investigated. Experiments in 2022-2023 showed that PASP-urea significantly increased the soil available N content, promoted rice biomass accumulation, and reduced Cd²⁺ concentrations in rice roots during the early growth stage. The optimized fertilization trial in 2024 further demonstrated that PASP-urea significantly decreased the total Cd accumulation and grain Cd concentrations at maturity in both cultivars, with reductions of 34.39% and 32.70% in Zhonghua 11 and Shuhui 498, respectively. Random forest analysis further identified soil NH₄⁺-N as the dominant predictor of root Cd concentration. Using non invasive micro test technology, we identified the meristematic zone as the key site for NH₄⁺-Cd interaction. In this region, PASP-urea enhanced NH₄⁺ uptake by 118.83% and 96.72% in the two cultivars, while decreasing Cd²⁺ uptake by 41.97% and 56.57%. Our study revealed a root zone specific mechanism in which enhanced ammonium uptake competitively inhibits the influx of Cd²⁺ at its primary uptake sites. This finding provides key physiological evidence for the development of next generation fertilizers that can ensure both high yields and safe rice production in Cd contaminated paddy fields.