IMPACT OF PHOSPHORUS FERTILIZATION AND LIMING ON THE PRESENCE OF ARBUSCULAR MYCORRHIZAL SPORES IN A SWEDISH LONG-TERM FIELD EXPERIMENT

Occurrence of arbuscular mycorrhizal fungal (AMF) spores was investigated in a long-term field experiment located at two separate sites. In addition to treatments with no liming or phosphorus application, the original experimental design included liming to 75% of base saturation, a basal dressing with 540 kg P/ha, and annual addditions of 0, 15 and 45 kg P/ha. The number of AMF spores decreased in the 540 kg P/ha treatment as well as in those plots given annual additions of 45 kg P/ha over a 9-year period. Indices suggest a negative relationship between a decrease of AMF spores and elevated levels of readily available soil phosphorus. Liming to 75% degree of base saturation at one of the field sites, i.e. increasing soil pH from 6.3 to 6.7, did not affect the presence of AMF spores. On the other hand, in the second soil, liming increased pH from 5.1 to 6.3, which significantly decreased the number of AMF spores. This may be explained by an increasing level of readily available soil phosphorus becoming soluble when pH was raised more than one unit by liming, which also has a negative effect on the occurrence of AMF spores. Presence of AMF spores did not affect harvest yields. Yield differences are mainly explained as being due to changes in chemical soil parameters, such as pH and amounts of readily available soil phosphorus.