P-poor Soil Research Articles

Rock phosphate solubilization and colonization of maize rhizosphere by wild and genetically modified strains of Penicillium rugulosum.

Maize root colonization and phosphate solubilizing activity of the fungus Penicillium rugulosum were assessed in a greenhouse trial using soil-plant microcosms. The bacterial gene hph conferring resistance to hygromicin B was introduced by electroporation in the wild-type strain IR-94MF1 of P. rugulosum and one transformant, w-T3, was selected. Maize plants were grown for 5 weeks in a P-poor soil and fertilized with a Florida apatite mineral, with Navay, an apatite rock deposit from Venezuela, or with simple superphosphate. Inoculation treatments included strain IR-94MF1, transformant w-T3 and two IR-94MF1 UV-induced mutants with enhanced (Mps++) or reduced (Mps-) in vitro mineral phosphate solubilizing activity. In the absence of P fertilization, inoculation with any P. rugulosum isolate significantly reduced the size of the total and P-solubilizing bacterial community present in maize rhizosphere. The bacterial community significantly increased in maize inoculated with IR-94MF1 and w-T3 when P was added as apatites Navay or Florida. All P. rugulosum strains were able to stimulate the growth of maize plants as indicated by 3.6 to 28.6% increases in dry matter yields. In the presence of rock phosphate, P uptake by maize plants inoculated with the two mutants Mps++ and Mps- was not always in agreement with their P-solubilizing phenotypes. Strain IR-94MF1 and transformant w-T3 increased P assimilation by the plants fertilized with Navay rock phosphate by 26 and 38%, respectively. In this treatment, w-T3 showed its highest significant maize rhizosphere colonization. With the simple superphosphate treatment, w-T3 increased P uptake in plants by 8% over the uninoculated control and also decreased significantly the community size of total bacteria, total fungi, and P-solubilizing fungi in the rhizosphere.

Effects of the addition of Fe(0) on the reduction of Fe(III) in submerged rice soil

The present study aimed at determining firstly whether Fe(0) is oxidized to Fe(II) in submerged rice soil, what is the acceptor of electrons released from Fe(0), and finally how the Eh values changed in the presence of Fe(0). Under laboratory conditions, flooded soil with the addition of Fe(0) was incubated anaerobically. Although the addition of Fe(0) may increase the amount of Fe(II), it is difficult to determine whether Fe(II) is derived from Fe(III) or Fe(0). For the current experiments, therefore, P-poor soil with the addition of ferric phosphate (Fe(III)PO4) and P-rich soil were used. If the electrons from Fe(0) flows to Fe(III)PO4 and/or the complex of Fe(III) oxides and phosphate in soil, Fe(III) should be reduced to soluble Fe(II). This reduction increases the Bray-P content because of the production of Fe(II)3(PO4)2 that is more soluble than Fe(III)PO4. When P-poor soil with the addition of Fe(III)PO4 and Fe(0) was incubated, the amounts of both Fe(II) and P in soil increased. This finding indicates that Fe(0) was oxidized to Fe(II) and, subsequently, that the electrons released from Fe(0) flowed to Fe(III)PO4. When the P-rich soil with Fe(0) was incubated, the amount of Fe(II) increased, unlike the amount of P. These findings show that Fe(0) was oxidized to Fe(II). However, the electrons released from Fe(0) did not flow to the complex of Fe(III) oxides and phosphate in soil. On the basis of these results, it can be concluded that the electrons from Fe(0) flows more easily to Fe(III)PO4 than to the complex of Fe(III) oxides and phosphate in soil. In the presence of Fe(0) and Fe(III)PO4 , the changes in the Eh values were investigated. If Fe(0) alone contributes to the maintenance of the redox equilibria, then the Eh values should increase with the change of Fe(0) to Fe(II) because Fe(0) and Fe(II) are the reduced and oxidized forms, respectively. In the current study, however, the addition of Fe(0) did not increase the Eh values. The presence of Fe(0) decreased the Eh values and increased the pH and Fe(II) content during the incubation. These findings suggest that Fe(0) and Fe(III) oxides in soil contribute to the maintenance of the redox equilibria in soil.