Effects of foliar application of humic acid on Cd²⁺ uptake and phyllosphere bacterial community in rice

Heavy metal pollution is one of the major constraints affecting the safe production and sustainable utilization of cultivated land. Developing efficient and convenient remediation technologies for heavy metal contamination is of great importance in ensuring national food security. As a plant regulator, humic acid (HA) demonstrates beneficial growth-promoting effects on rice. However, there remains a lack of systematic understanding regarding the impact and mechanisms of foliar HA application on heavy metal uptake in rice. This study investigated the effects of different HA spray dosages on rice growth and cadmium (Cd) absorption through pot experiments, while also analyzing the response of phyllosphere bacterial communities to HA application and their relationship with Cd uptake using high-throughput sequencing technology. The results showed that all HA treatments enhanced rice growth and Cd resistance in Cd-contaminated soil. Compared with the control, HA application significantly increased plant height, root length, fresh weight, and grain yield, with the promoting effects intensifying as HA dosage increased. HA also significantly reduced Cd content in rice roots, stems, leaves, and grains, but the efficacy followed a bell-shaped curve relative to HA dosage. HA application significantly altered the structure of phyllosphere bacterial communities and increased their diversity, but high dosages significantly reduced the network complexity of phyllosphere bacterial communities. Plant Cd content was significantly with the complexity of bacterial community interactions rather than the community structure or diversity of phyllosphere bacteria. These findings indicate that foliar HA application effectively improves Cd resistance in rice, modifies leaf microbial communities, and enhances both yield and safety of rice production. Furthermore, HA exhibits a distinct nonlinear dose-response effect. Under the experimental conditions, an optimal foliar HA dosage of 13.20 kg·hm⁻² was identified as most conducive to rice production in Cd-contaminated soil. This study provides scientific evidence for HA application in Cd-contaminated soil and offers new insights for ensuring agricultural product safety and sustainable soil utilization.