Possible solubilization of various mineral elements in the rhizosphere of Lupinus albus L

Abstract

ABSTRACT Lupinus albus L. (lupin) has a high tolerance for phosphorus deficient conditions as its roots can solubilize the unavailable phosphorus in the rhizosphere soil. The roots may also be able to solubilize other elements, but this requires further investigation. In this study, therefore, we conducted two experiments to comprehensively investigate the effects of lupin roots on the mineral dynamics of the rhizosphere soil. First, a mixed cropping experiment was conducted, in which lupin shared a rhizosphere with soybean (Glycine max (L.) Merr.) in a long-term experimental field with four fertilizer treatments: complete fertilization (+NPK), without nitrogen (−N), without phosphorus (−P), and without potassium (−K). The results of shoot dry weight of plants cultivated alone indicated that lupin is highly tolerant to all N, P, and K deficiencies, while soybean can adapt to N deficiency with the help of rhizobia but is less tolerant to P and K deficiencies than lupin. When mixed-cropped with lupin, the concentrations of many elements in the soybean leaf increased, particularly with the −N and −P treatments. Furthermore, soybean growth was significantly improved when cropped with lupin in the −N and −P treatments. Second, a comparison of the elemental profiles of hydroponically and field-soil-grown plants was conducted. Under hydroponic conditions, the rhizosphere effect is negligible when the culture medium is well circulated. The lupin/soybean ratios for leaf mineral concentrations were considerably larger in the field cultivated plants when compared with the hydroponic cultivations for elements such as sodium, potassium, cesium, phosphorus, iron, copper, and molybdenum, as there were lower concentrations of these elements in the soybean leaves in the field. These results indicate that lupin roots can solubilize a variety of insoluble elements in the soil, which may be the reason why lupin can adapt to various nutrient deficient soils. In the lupin rhizosphere, the solubilization of cesium, which is generally strongly fixed by soil minerals and not easily leached, was particularly pronounced. This implies that the surface structure of clay minerals might be altered in the lupin rhizosphere, resulting in the fixed forms of various elements becoming available.