Combined Effect and Mechanism of Acidity and Lead Ion on Soybean Biomass
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Heavy metal pollution and soil acidification are serious global environmental issues. The combined pollution from acidification and heavy metal has become a new environmental issue in regions where the two issues simultaneously occur. However, studies on combined pollution are still limited. In the current study, we investigated the combined effect and mechanism of acidity and heavy metal [lead ion (Pb2+)] on soybean biomass as well as on growth, nitrogen nutrition, and antioxidant system in soybean roots. Results showed that the combined treatment with acidity and Pb2+ decreased the soybean biomass. At pH 4.5, the soybean biomass in the combined treatment with acidity and 0.9 mmol L−1 Pb2+ was lower than that in the combined treatment with acidity and Pb2+ at 0.3 or 1.5 mmol L−1. This result was also observed at pH 3.5 and 3.0. The combined treatment with acidity and Pb2+ also resulted in the following consequences: root growth inhibition; decrease in nitrate, ammonium, and malondialdehyde contents; increase in nitrite reductase activity; and decrease in peroxidase activity. The extent at which the test indexes decreased/increased in the combined treatment was higher than that in the single acidity treatment. The correlation analysis results indicated that the decrease in the soybean biomass in the combined treatment with acidity and Pb2+ resulted from the decrease in the root growth, nitrate–nitrogen assimilation, and peroxidase activity.