EVALUATION OF THE USE OF A MODEL RHIZODEPOSITION TECHNIQUE TO SEPARATE ROOT AND MICROBIAL RESPIRATION IN SOIL

A model rhizodeposition technique to estimate the foot and microbial components of C-14 soil/root respiration in pulse-labelling experiments is described. The method involves the injection of model rhizodeposits, consisting of C-14-labelled glucose, root extract or root cell wall material, into the rooted soil of an unlabelled plant, simultaneously with the pulse-labelling of a separate but similar plant with (CO2)-C-14. In a growth chamber experiment with 30 day old wheat and barley the contribution of direct root respiration to C-14 soil/root respiration over a 26 day period after labelling was estimated 89-95%. Estimates of direct root respiration in field-grown wheat and barley at different development stages in most cases accounted for at least 75% of C-14 soil/root respiration over a 21 day period after labelling. The mineralization rate of injected C-14-glucose was positively correlated with the concentration of glucose-C established in soil. The use of the method in rhizosphere carbon budget estimations is evaluated.