Limitations in the application of the fumigation technique for biomass estimations in amended soils

The fumigation technique for the estimation of microbial biomass-C was applied at different periods after amendment of three agricultural soils with ¹⁴C-labelled glucose, cellulose and wheat roots. By daily monitoring of evolved CO₂ and ¹⁴CO₂ it was recognized that the CO₂ from the degradation of the amendment had an interfering effect on biomass calculations. Biomass estimations were valid only when CO₂ from the degradation of the amendment had slowed, 3 days after glucose amendment, 14 days after addition of cellulose, and 28 days after amendment with wheat roots.Fumigated, reinoculated soils degraded glucose faster than did the corresponding control samples, causing an overestimation of biomass-C. By contrast biomass-C was underestimated in soils amended with cellulose or wheat roots due to lower rates of degradation of the added C-sources in fumigated samples. The reduced capacity for degradation of complex organic materials may be due to smaller decomposer populations in inoculated fumigated soils; populations recovered within 20 days to only 10–20% of their original biomass-C content. Re-establishment of biomass in fumigated samples was tested with inocula in amounts increasing to 10, 50 and 100% of corresponding control samples. The K-factor was not influenced by these treatments. Estimates of biomass in soil during the rapid phase of degradation of wheat roots were influenced by the amount of inoculum.