Determination of extracellular neutral phosphomonoesterase activity in soil

A new approach was carried out to determine the extracellular neutral phosphomonoesterase activity of soil. Surface soils under continuous winter wheat, and characterized by different amounts of organic matter, were sampled, air-dried and stored for 2 years or kept moist and investigated immediately. Glucose and nitrate were added to each soil and phosphomonoesterase and ATP content were monitored for up to 96 h. In moist soils the enzyme activity was significantly (P < 0.05) correlated with the ATP content, an indicator of microbial biomass. At zero ATP, a positive intercept of neutral phosphomonoesterase activity on the ordinate was observed and this value was assumed to be equal to the extracellular component. The correlations were less significant in dry than fresh soil. Probably the negative intercepts observed in two air-dried soils were due to underestimation of the intracellular activity present in non-proliferating microorganisms whose number was higher than, in moist samples as the result of the prolonged storage under dry conditions.