Evaluation of effect of different sterilization methods on soil biomass phosphorus extracted with NaHCO3

Soil microbial biomass is a dynamic force driving soil phosphorus cycling in soils. The temperature, time and method for killing soil organisms in soil biomass P determination are so important factors that affect the results. The aim of this study was to compare some methods of soil sterilization and soil microbial P (Pm) release in extraction with NaHCO3. Five samples of calcareous soils in three replicates were incubated in field capacity and 28°C. The texture of soils differed from loamy sand to sandy loam. They had 10-15% equivalent calcium carbonate, 1-3% organic carbon and 40-90 mg/kg available P content. After 1, 10, 30, and 50 days of incubation a portion of each soil was sterilized by five methods (chloroform fumigation, autoclaving at 121°C for 0.5 h, oven drying at 70°C for 48 h, oven drying at 180°C for 2 h, and ultrasonification). Phosphorus of the sterilized and control soils were extracted with 0.5M NaHCO3 (pH 8.5) and determined spectrophotometrically as blue molybdate-phosphate complexes under partial reduction with ascorbic acid. Results showed that the method of soil sterilization and soil type had significant effects on biomass P estimated in soils (P < 0.01). There were no significant differences between extracted biomass P at chloroform fumigation, autoclaving at 121°C for 0.5 h, and oven drying at 70°C for 48 h. However biomass P was significantly more extracted from soil by oven drying at 180°C for 2 h and by ultrasonification. Fixation of Pm in soil during the extraction period can reduce the precision of biomass P estimates. This effect was more obvious in biomass P determination by fumigation and oven drying at 70°C for 48 h. Biomass P determined by these methods and autoclaving had positive and significant correlations with soil sand contents, respectively due to long sterilization period and temperature. Ultrasonification released more P from these calcareous soils especially at the start of soil incubation. It released higher microbial P and reduced P sorption in soils; biomass P determined by this method compared to other methods had relatively higher correlations with fungi, glumales spore and P solubilizing bacteria numbers in soil.