Effects of phosphate fertilizer applications on microorganisms in pastoral soil

Abstract

Microbial and earthworm populations and some soil chemical characteristics were measured in soil samples collected from an existing fertilizer trial that had been in progress for 3 years. The samples were collected two weeks before and two weeks after the annual applications of single superphosphate (SSP) or North Carolina phosphate rock (NCPR). There were no significant long-term fertilizer effects on any of the characteristics measured but there were some significant changes between the two samplings. Total bacterial count (69 x 106 to 6 x 106 g-1 soil) and the fungal count (l3 x 105to 5.2 x 105 g-1 of soil) declined significantly in all treatments between the two samplings. This decline was significantly smaller for fungi in fertilized plots. There were significant increases in populations of gram negative bacteria (28 x 105 to 52 x 105 bacteria g-1 of soil) and phosphate rock (PR) dissolving bacteria (7 6 x 105 to l3 x 105 bacteria g-1 of soil) two weeks after fertilizer application. Populations of these bacteria did not change significantly in unfertilized plots. The water-extractable carbon declined significantly in unfertilized and SSP-treated plots but not in NCPR-treated plots. The levels of ammonium oxidizing bacteria, PR-dissolving fungi, fluorescent Pseudomonas spp. and microbial P were unaffected by fertilizer. Two groups of earthworms were examined once before the fertilizer application. There were no significant fertilizer effects on adult or juvenile populations of Lumbricus rubellus or Aporrectodea caliginosa.