MICROBIAL UTILIZATION OF C-14[U]GLUCOSE IN SOIL IS AFFECTED BY THE AMOUNT AND TIMING OF GLUCOSE ADDITIONS

Microbial utilization of glucose-C-14 by soil microbes was investigated in two laboratory experiments. In the first experiment, C-14-labelled glucose was added to two soils at seven rates ranging from 36 to 2304 mug C g-1 soil. An average of 42% of added C-14 was mineralized by day 3 at glucose rates greater-than-or-equal-to 288 mug C g-1 soil in both soils, but this proportion declined at lower rates. Only 30% of added C-14 was mineralized at the lowest rate of glucose addition. The fraction of soil C-14 released by fumigation-extraction (FE-C-14) ranged from 11 to 36%, and decreased linearly with the proportion of C-14 mineralized in both soils. The effects of addition rate persisted until the end of the experiment, at 35 days. In the second experiment, addition of unlabelled glucose at 6, 24 or 72 h after addition of glucose-C-14 at 30 mug C g-1 soil did not appreciably affect the proportion of C-14 mineralized, while addition 72 h before or immediately after C-14 addition increased C-14 mineralization by about 50%. Fumigation-labile C-14 was reduced in all cases by addition of unlabelled glucose, with the greatest reduction when unlabelled glucose was added immediately after glucose-C-14. We conclude that assimilated glucose-C-14 was incompletely metabolized at low rates of glucose addition unless soil microorganisms were 'activated' by a prior addition of glucose. The proportion of C-14 mineralized at low rates of glucose addition to soils may be useful as an indicator of C availability: a small proportion mineralized may indicate low C availability whereas a large proportion mineralized may indicate high C availability.