Municipal solid waste compost dose effects on soil microbial biomass determined by chloroform fumigation-extraction and DNA methods

We evaluated the relationship between microbial biomass C and N (B-C and B-N) as estimated by the chloroform fumigation-extraction (CFE) method and microbial biomass DNA concentration in a loam-clayey wheat cultivated soil. The soil received municipal solid waste compost at rates of 40 or 80 t ha(-1) and farmyard manure at 40 t ha(-1). Microbial biomasses C and N and DNA concentration showed the highest values for microorganisms counts with compost and farmyard manure at 40 t ha(-1). Compost applications at 40 t ha-1 improve the micro-organisms growth than that of 80 t ha(-1). Moreover a significant decrease of soil microbial biomass was noted after fertilisation for three years. The presence of humic acid and proteins impurities in DNA extracts; even in important level as in F-treated soil; did not affect the microbial biomass. The decrease of microbial biomass was due to heavy metals content elevation in compost at 80 t ha(-1) treated soil. Thus the highest rate of municipal solid waste compost induced the lowest ratio of biomass C to soil organic carbon and the lowest ratio of biomass N to soil organic nitrogen. There was a positive relationship between B-C, B-N and D-NA concentration. DNA concentration was significantly and positively correlated with B-C and with B-N. However there was a negative correlation between either micro-organisms numbers and DNA concentration, or B-C and BN. The comparison of the two used methods DNA extraction and CFE showed the lowest coefficient of variation (cv %) with DNA extraction method. This last method can be used as an alternative method to measure the microbial biomass in amended soils.