ASSIMILATE TRANSLOCATION TO THE RHIZOSPHERE OF 2 WHEAT LINES AND SUBSEQUENT UTILIZATION BY RHIZOSPHERE MICROORGANISMS AT 2 SOIL-NITROGEN CONCENTRATIONS

The transport of photosynthates from the leaves to the rhizosphere of wheat and incorporation in the soil microbial biomass was investigated by growing plants in a continuously 14C-labelled atmosphere. Two different varieties of wheat (C-R5B and C-R5D) were used and two nitrogen concentrations in soil were applied. At the high N relatively more 14C was released by the roots as indicated by higher percentages of the translocated 14C found in root-soil respiration and soil residue. The amount of 14C-labelled microbial biomass in the soil, as welt as rhizosphere bacterial numbers, was significantly higher at the high N. It would appear that most of the increase in microbial biomass 14C resulted from a higher exudation rate at the higher N. However, in response to a higher N concentration the relative increase in 14C-labelled microbial biomass was larger than the relative increase in release of 14C-labelled C from the roots. This indicated that the root exudates were more efficiently used at the high N. No differences in soil microbial biomass or rhizosphere bacterial numbers were found between the two wheat lines. While the higher N treatment stimulated the decomposition and microbial utilization of root-released materials it appeared to have a negative effect on the decomposition of native soil organic matter since the rate of respiration of unlabelled C from the soil decreased. © 1990.