THE USE OF PHOSPHOLIPID FATTY-ACID ANALYSIS TO ESTIMATE BACTERIAL ANDFUNGAL BIOMASS IN SOIL

The cell content of 12 bacterial phospholipid fatty acids (PLFA) was d etermined in bacteria extracted from soil by homogenization/centrifuga tion. The bacteria were enumerated using acridine orange direct counts . An average of 1.40x10(-17) mol bacterial PLFA cell(-1) was found in bacteria extracted from 15 soils covering a wide range of pH and organ ic matter contents. With this factor, the bacterial biomass based on P LFA analyses of whole soil samples was calculated as 1.0-4.8 mg bacter ial C g(-1) soil C. The corresponding range based on microscopical cou nts was 0.3-3.0 mg bacterial C g(-1) soil C. The recovery of bacteria from the soils using homogenization/centrifugation was 2.6-16% (mean 8 .7%) measured by PLFA analysis, and 12-61% (mean 26%) measured as micr oscopical counts. The soil content of the PLFA 18:2 omega 6 was correl ated with the ergosterol content (r=0.92), which supports the use of t his PLFA as an indicator of fungal biomass. The ratio 18:2 omega 6 to bacterial PLFA is therefore suggested as an index of the fungal:bacter ial biomass ratio in soil. An advantage with the method based on PLFA analyses is that the same technique and even the same sample is used t o determine both fungi and bacteria. The fungal:bacterial biomass rati o calculated in this way was positively correlated with the organic ma tter content of the soils (r=0.94).