Bioavailability index for quantitative evaluation of plant availability of extractable soil trace elements

A new index, Bioavailability Index (BI) and the corresponding experimental method were developed for quantitative evaluation of bioavailability of the extractable soil trace elements. Soils were first treated with various extractants (DTPA, HCl, NH2OH . HCl + HCl) separately to remove the extractable elements. The soils after extraction were washed with deionised water to eliminate the extractant and its pH was adjusted with CaO and finally restored to its original pH level. Wheat (Triticum aestivum L.) and rape (Brassica chinensis) were planted in the untreated and treated soils for 8 weeks. The concentrations of the trace elements in plants were determined after harvest. Nutrient accumulation by plants is significantly reduced due to removal of extractable trace elements from the soil. BI of the extractable fraction was proportional to the ratio of plant accumulation reduction to trace element extractability. In the present study, BI value of the total content of soil trace elements was designated as 1. Though only a minor fraction of the total soil nutrient, generally less than 5%, was removed by DTPA, the nutrient accumulation by plants, especially for wheat, was reduced greatly, leading to relatively large BI values. For wheat, the average BI values of the eight nutrients Cu, Mn, Zn, Ni, Co, Pb, Cr, and V were found to be 22.7, 17.6 and 17.4 for the three testing soils, and for rape, the corresponding values of 8.9, 10.0 and 11.0 were obtained, indicating that the DTPA-extractable elements represent the highly available fraction of the total content. The BI values for HCl-extractable elements were much lower compared with those for DTPA. For wheat, the average BI values for the three soils are 2.0, 1.9 and 2.4, and for rape, the corresponding values are 4.8, 4.1 and 3.7. The high availability of DTPA-extractable trace elements and relatively low availability of HCl-extractable trace elements highlight the significant role that chelation action might play in plant nutrient acquisition. The different responses of wheat and rape to the soils previously subjected to the same extraction procedure could be explained by their genotypical differences in sensitivity to nutrient deficiencies. The quantitative nature of BI makes it valuable in the study of nutrient bioavailability and plant accumulation mechanisms.