In vivo Measurement of Root-induced pH Changes at the Soil-Root Interface: Effect of Plant Species and Nitrogen Source

In order to measure the pH at the soil-root interface (rhizosphere) of growing roots, 4 different plant species (maize, wheat, chickpea and white clover) were grown in plexiglass boxes in a soil of pH 6.0. Nitrogen was supplied at different rates and in different forms (nitrate, ammonium and N2 fixation, resp.). After a growth period between 4 and 10 days the soil and the roots were infiltrated with agar (35–38°C) containing bromocresol purple as pH indicator. The pH in the rhizosphere was also measured by microelectrodes inserted into the agar. Compared to the bulk soil (pH 6.0) in the rhizosphere of maize plants the pH increased to about 7.5 with NO3--N and dropped to about 4.0 with NH4+-N as the N supply. However, with NO3--N striking pH differences were seen both along the roots and between the various root zones of the maize plants. In contrast to maize, the pH of the rhizosphere of chickpea seedlings decreased to about 4.5 even when supplied with NO3--N. In white clover the rhizosphere pH depended on whether nitrogen was supplied as NO3--N or via N2 fixation (i.e. rhizobial infection): with N3--N the rhizosphere pH increased to about 6.5 and with N2 fixation decreased to about 4.5. In mixed culture of maize and chickpea there were striking differences in pH values in the rhizosphere varying from 4.5 to 7.0. The results demonstrate the extent to which the pH in the rhizosphere can differ from that further from the roots, depending on plant species and nitrogen source. The method described in this paper offers a simple way to obtain an overall view of this pH pattern in the rhizosphere of intact roots.