ASSESSMENT OF THE INFLUENCE OF PHOSPHATE FERTILIZERS ON THE MICROBIALACTIVITY OF PASTURE SOILS

The objective of the present work was to examine the effects of phosph ate fertilizers on the microbial activity of pasture soils. Various mi crobial characteristics were measured using soils from an existing lon g-term phosphate fertilizer field trial and a short-term incubation ex periment. The measurements included basal respiration, substrate induc ed respiration, inhibition of substrate-induced respiration by strepto mycin sulphate (fungal activity) and actidione (bacterial activity) an d microbial biomass C. The long-term field trials was initiated during 1985 to examine the effectiveness of different sources of phosphate f ertilizers (single superphosphate, North Carolina phosphate rock, part ially acidulated North Carolina phosphate rock, and diammonium phospha te) on pasture yield. The incubation experiment was conducted for 8 we eks using the same soil and the sources of phosphate fertilizers used in the field trial. In the incubation experiment the fertilizer additi on caused an initial decrease in basal and substrate-induced respirati on but had no effect on total microbial biomass. The initial decline i n basal and substrate-induced respiration with the fertilizer addition was restored within 8 weeks after incubation. In the field experiment the fertilizer addition had no significant effect on basal respiratio n but increased substrate-induced respiration and microbial biomass C. The short-term and the long-term effects of phosphate fertilizer addi tion on the microbial characteristics of the soils are discussed in re lation to its effects on pH, salt concentration, and the nutrient stat us of the soils.