DNA-based determination of soil microbial biomass in alkaline and carbonaceous soils of semi-arid climate

The applicability of DNA-based analysis of soil microbial biomass was proven under conditions when the common approaches – chloroform fumigation-extraction (CFE) and substrate-induced respiration (SIR) – are restricted. These restrictions include certain soil properties typical for arid areas (alkaline or carbonaceous soils) or limitation by sample preparation (frozen samples). To prove the suitability and correspondence of the methods, microbial biomass was determined by CFE, SIR and by DNA quantification in slightly alkaline Chernozem and strongly alkaline Calcisol of semi-arid climate under contrasting land use. Quantification of double-stranded DNA (dsDNA) revealed an excellent agreement (R2 = 0.96) with SIR-based microbial biomass (SIR-Cmic) for soils with pH lower than 8. DNA- and CFE-based microbial biomass (CFE-Cmic) correlated well (R2 = 0.97) for all studied soils. The conversion factors from dsDNA to SIR-Cmic of 5.1 and to CFE-Cmic of 4.4 were obtained. In alkaline soils (pH > 8), microbial biomass measured by SIR was strongly underestimated because of CO2 retention in soil solution due to high pH and CO3 2− exchange with carbonates. Thus, the dsDNA quantification provides a simple and durable approach for microbial biomass analysis as alternative for SIR and CFE and can be successfully used in alkaline or calcareous semi-arid soils.