Abstract

A study was conducted to estimate different phosphorus fractions in some alkaline calcareous subtropical soils and evaluate their relationship with microbial and biochemical properties of the soils. Soil samples (0 to 15 cm) belonging to 15 soil series were collected from the agricultural fields of Potohar, northern Punjab, Pakistan in September, 2008 and analysed for P fractions and microbial parameters including microbial biomass C, microbial biomass N, microbial biomass P, and activities of dehydrogenase and alkaline phosphatase enzymes. The average size of different P fractions (% of total P) in the soils varied in the order: _dHCl-P (63.7%) > _cHCl-P_i(14.8%) > residual-P (9.4%) > _cHCl-P_o (3.8%) > NaHCO₃-P_i (2.4%) > NaOH-P_i(2.0%) > NaHCO₃-P_o (1.4%) > resin-P (1.3%) > NaOH-P_o (1.2%). The organic P fractions collectively formed 6.4% of the total P, while the inorganic P fractions were 93.6%. The bio-available P fractions that is, resin-P, NaHCO₃-P_i and NaHCO₃-P_oshowed significant positive relationship with Olsen-P, microbial biomass P, and total organic C, but also with microbial biomass C, microbial biomass N, dehydrogenase activity and alkaline phosphatase activity in the soils. Among other P fractions, NaOH-P_i had strong positive correlation with total organic C, alkaline phosphatase and dehydrogenase activities, while the NaOH-P_o had strong positive correlation with total organic C, dehydrogenase activity, microbial biomass N, alkaline phosphatase activity and microbial biomass C. The _dHCl-P fraction did not show any strong correlation with microbial parameters, whereas the _cHCl-P_o had strong positive correlation with microbial biomass N and microbial biomass C. The results demonstrate positive contribution of soil microbial biomass, especially the microbial biomass P to P availability in the soils under present study.

Key words: Phosphorus fractions, microbial biomass, enzyme activities, alkaline calcareous soils.