Effect of Nitrogen Forms on Aluminum Toxicity to Soybean (Glycine max ) and Aluminum Speciation on Root Surface

Al toxicity, being a major factor limiting plant growth in acid soils, has been investigated extensively, but little has been reported on relationship between Al toxicity and Al speciation on plant root surface. A hydroponic experiment, designed to have two treatments, alternation of N and Al, or both N and Al simultaneously, was carried out to explore effects of forms of nitrogen on Al toxicity to soybean(Glycine max) and joint effect of N and Al on Al speciation on soybean root surface. Results show that regardless of treatment, the amount of Al ions adsorbed on soybean root surface was higher in the nitrate system than in the ammonium system when N and Al were added simultaneously. In the treatment of alternation, Al toxicity to soybean was observed, significantly inhibiting extension of soybean roots and reducing biomass of the plant, and did not differed much in extent between the ammonium system and the nitrate system. However, the growth of soybean was better in the nitrate system than in the ammonium system no matter whether in treatments with or without aluminum. Nitrate also enhanced the absorption of Mg, K, and Mn by soybean roots. Ammonium alleviated Al toxicity to soybean by competing for adsorption sites with Al when both N and Al were present in the solution.