Evaluation of effect of different sterilization methods on soil biomass phosphorus extracted with NaHCO3
Sinegani, A.A.S.,Bu-Ali Sina Univ., Hamedan (Iran Islamic Republic) | Hosseinpur, A.,Shaher-e-Kord Univ. (Iran Islamic Republic)
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 deg 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 deg C for 0.5 h, oven drying at 70 deg C for 48 h, oven drying at 180 deg 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. The method of soil sterilization and soil type had significant effects on biomass P. There were no significant differences between extracted biomass P at chloroform fumigation, autoclaving at 121 deg C for 0.5 h, and oven drying at 70 deg C for 48 h. However biomass P was significantly more extracted from soil by oven drying at 180 deg 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 deg 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.
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