Microbial utilization of 14C[U]glucose in soil is affected by the amount and timing of glucose additions

Abstract

Microbial utilization of glucose- 14C by soil microbes was investigated in two laboratory experiments. In the first experiment, 14C-labelled glucose was added to two soils at seven rates ranging from 36 to 2304 μg C g −1 soil. An average of 42% of added 14C was mineralized by day 3 at glucose rates ⩾288 μgCg −1 soil in both soils, but this proportion declined at lower rates. Only 30% of added 14was mineralized at the lowest rate of glucose addition. The fraction of soil 14C released by fumigation-extraction (FE- 14C) ranged from 11 to 36%, and decreased linearly with the proportion of 14C 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- 14C at 30 μg C g −1 soil did not appreciably affect the proportion of 14C mineralized, while addition 72 h before or immediately after 14C addition increased 14C mineralization by about 50%. Fumigation-labile 14C was reduced in all cases by addition of unlabelled glucose, with the greatest reduction when unlabelled glucose was added immediately after glucose- 14C. We conclude that assimilated glucose- 14C was incompletely metabolized at low rates of glucose addition unless soil microorganisms were ‘activated’ by a prior addition of glucose. The proportion of 14C 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.