Optimizing nitrate topdressing timing in sweet sorghum for integrated phytoremediation of cadmium-polluted soil and enhanced bioethanol yield

Phytoremediation of cadmium (Cd)-polluted soil is an environmentally-friendly, cost-effective, and sustainable technology. Sweet sorghum is a promising candidate for phytoremediation, as it combines high Cd accumulation with the production of bioethanol. Nitrate plays a superior role in improving efficiency of Cd phytoremediation using sweet sorghum. Nevertheless, the specific effects of nitrate topdressing timing on Cd accumulation, bioethanol yield, and the associated rhizosphere microbiome in sweet sorghum remain unclear. This study demonstrated that applying nitrate at the seedling stage was significantly more effective than application at the jointing, flowering, or milk stages regarding phytoremediation of Cd-polluted soil. This early topdressing increased both fresh weight of internodes and total Cd accumulation in the shoots. Consequently, it also yielded the highest theoretical bioethanol production, reinforcing the benefit of early nitrate application for integrated phytoremediation and bioenergy. Further analysis revealed that soil nitrate content and shoot Cd accumulation were key factors in reshaping the rhizosphere microbial community and its functions. Nitrate topdressing at seedling stage depressed rhizosphere nitrogen fixation (indicated by downregulated nifK/H ) and denitrification (downregulated narG/H/I ), while enhancing ammonia cycle (upregulated nirA ). This shift ultimately increased soil nitrate content, which was positively correlated with higher shoot Cd accumulation. Additionally, metal resistance genes znuC s were also positively correlated with shoot Cd accumulation in sweet sorghum. Altogether, nitrate topdressing at seedling stage reshapes rhizosphere soil microbial functions, thereby improving sweet sorghum-assisted phytoremediation of Cd-polluted soil. • N topdressing at seedling stage promotes total internodes fresh weight of sweet sorghum grown in the Cd-polluted soil. • N topdressing at seedling stage remarkably increases shoot Cd accumulation as well as theoretical bioethanol yield. • Rhizosphere soil microbial divergence is driven by shoot Cd accumulation and soil nitrate content. • Soil N cycle genes nifK / H and narG / H / I are negatively correlated with shoot Cd accumulation. • znuC expression in rhizosphere soil is positively correlated with shoot Cd accumulation.