BACTERIAL PRODUCTION DETERMINED BY [H-3] THYMIDINE INCORPORATION IN-FIELD RHIZOSPHERES AS EVALUATED BY COMPARISON TO RHIZODEPOSITION

In a sandy loam soil cropped to barley bacterial production in the rhi zosphere was compared to the results of a parallel investigation on rh izodeposition. Bacterial production was stimulated in the rhizosphere as revealed by an increased biomass of bacteria (643-883 mug C g-1 soi l) and protozoa (7.2-15 x 10(4) cells g-1 soil) as well as elevated th ymidine incorporation (9.7-12 pmol g-1 soil) in rhizosphere soil compa red to bulk soil. Rhizodeposition, as determined by several pulse labe llings with CO-2(14), was estimated to be 412 mug C g-1 dry wt soil in the 0-15 cm layer. Bacterial production, as determined by incorporati on of H-3-labelled thymidine converted to bacterial C, revealed a plan t-induced formation of 1348 mug bacterial C g-1 soil in the 0-15 cm la yer. This is probably the first estimate for bacterial production base d on thymidine incorporation which has been compared to an estimate of C availability in a terrestrial ecosystem. The discrepancy between th e rhizodeposition data and the estimate of bacterial production sugges ts that conversion factors between [H-3]thymidine incorporation and ba cterial production obtained in laboratory incubations are not generall y applicable to the field situation. It is proposed that estimates of bacterial production should be based on measurements of actual convers ion factors to improve the accuracy of the method.