EXTRACTABILITY OF MICROBIAL C-14 AND N-15 FOLLOWING ADDITION OF VARIABLE RATES OF LABELED GLUCOSE AND (NH4)2SO4 TO SOIL

The objective of this study was to investigate the effect of variable rates of substrate addition and length of incubation period on in situ estimates of the proportion of microbial C and N released by a chloroform-fumigation-extraction method. A Bradwell silty loam soil containing ca 340 μg microbial C g-1 soil was amended with [14C(U)]glucose and [15N](NH4):SO4 at 30 and 2. 300 and 2, and 300 and 20μg of C and N. respectively, g-1 soil. The following measurements were taken at 0.25, 0.5. 1. 3 and 7 days after the addition of substrates: respired 14C, mineral 15N, total 15N and increase in extractable 14C and 15N upon fumigation. 14C-efficiency, defined as the proportion of 14C in the microbial biomass relative to the total amount of 14C removed from the soil solution, was >70% in all treatments at 0.25 and 0.5 days and in the treatment receiving the lowest rate of glucose at all sampling times. The proportion of microbial 14 released by fumigation-extraction was >40% when the efficiency was >70%. but was 34% when the efficiency was lower. In the treatment receiving the low rate of glucose microbial N accounted for ca 50% of added 15N, of which >40% was released by fumigation-extraction; in the treatments receiving the high rate of glucose microbial N accounted for an average of 81% of added 15N, of with only 24% was released by fumigation-extraction at 3 and 7 days after the addition of substrates. Based on these observations, it was concluded that at low rates of glucose addition microbial 14C and 15N remained present among cytoplasmic constituents and little if any was utilized in biosynthesis; sufficient glucose to stimulate microbiul growth and a sufficient period of incubation would be required to obtain reliable estimates of the proportion of microbial C and N released upon fumigation-extraction. © 1990.