Mobilization of potassium from Soils by rhizobium phaseoli

Potassium (K) is one of the most important essential elements for plant growth and function. However, both the concentration and the availability of K are very low in most soils. The chemical K supplemented through fertilization is rapidly adsorbed by soil minerals resulting in a 20%--35% of K use efficiency. Studies have found that rhizobia, which have the capacity not only to fix nitrogen from the atmosphere to contribute plant nitrogen nutrition, but also to mobilize soil K for plant use. As a result, the improvement of plant K use efficiency through some specific rhizobia to mobilize soil K has attracted much attention around the whole world. However, less information is available to illustrate the mechanisms how rhizobia could directly mobilize K from the soil. Eight strains of rhizobium sp., which isolated from a gray brown purple soil in Chongqing, southern China and coded as R 01, R 02, R 03, R 04, R 05, R 06, R 07 and R 08, respectively, were grown a liquid medium to study their capacity to mobilize soil K. The medium contained 1L H sub(2)O, 10 g mannitol, 1.0 g yeast powder, 0.5 g NaH sub(2)PO sub(4), 0.2 g MgSO sub(4) times 7H sub(2)O, 0.1 g NaCl, 0.1 g CaCl sub(2) times 6H sub(2)O and 4.0 mL microelement solution (each 1 L contained 5.0 g H sub(3)BO sub(3) and 5.0 g Na sub(2)MoO sub(4) dH sub(2)O). Compared to the non-rhizobium control, concentrations of proton in the liquid media under all eight rhizobium treatments were increased by 22 times at least leading a significant pH decrease after 7 days of incubation. All rhizobium strains exuded oxalic acid and malic acid and most of them exuded acetic acid, but varied their capacity to exude succinic acid, formic acid, citric acid and lactic acid. Soluble K in the liquid medium was significantly higher whilst mineral structure K was significantly lower in the soil in the rhizobium treatments than in the non-rhizobium control. Mineral structure K in soil negatively correlated with total organic acids (r=-0.878**, n=9), but positively correlated with pH in the liquid medium (r=0.863*, n=9). So, rhizobia could mobilize soil K through the exudation of organic acids and protons. Oxalic acid has a strong capacity to complex calcium, magnesium, iron and aluminum and positively correlated with total organic acids (r=0.870**, n=9), which suggests that oxalic acid exuded by rhizobia might contribute directly in the mobilization of soil K. In addition, protons ionized from organic acids were from 4.15% to 27