ESTIMATION OF SOIL MICROBIAL C BY A FUMIGATION EXTRACTION METHOD - USE ON SOILS OF HIGH ORGANIC-MATTER CONTENT, AND A REASSESSMENT OF THE KEC-FACTOR

The fumigation-extraction method to estimate microbial C was applied to a range of seven soils with organic-C contents up to 47%. The additional oxidizable organic-C released by chcl3 fumigation at -5 kPa water potential followed by extraction with 0.5 m K2SO4. was compared with the microbial C estimated by a modified substrate-induced-respiration (SIR) method, and by in situ labelling of cells with [14C]glucose. The extractable organic-C flush comprised 37-63% of the microbial C. The overall kecfactors, to convert from the organic-C flush to microbial C were 0.37 for the SIR method and 0.42 by 14C-labelling. The relationship between microbial C and the oxidizable C released by fumigation was similar for both organic and mineral soils. Accumulated data from 168 comparisons of the relationship between the organic-C flush and microbial C were re-examined. The kec-factors varied widely between soils and were strongly influenced by soil water content. It is recommended that soils should be rewetted. when required, to the -5kPa water potential, to ensure that the fumigation stage is fully effective. The reliability of the SIR and fumigation-incubation methods is discussed and a revised calibration for the SIR method: microbial C (/gmg g-1) = 50(/gmlCO2g-1h-1) is proposed, based on 14C-labelling and alternative methods of calcula the CO2-C flush when using the fumigation-incubation method. Taking account of possible underestimation of microbial C by these calibrations, a value of 0.35 is suggested as being representative of an overall kec-factor for New Zealand soils. Variability between soils means there is an error in applying an average factor to all soils, but a standard method should be adequate for most applications of microbial C measurements. © 1990.