IMPACT OF PHOSPHORUS FERTILIZATION ON VAM DIASPORES IN 2 SWEDISH LONG-TERM FIELD EXPERIMENT

The presence of vesicular-arbuscular mycorrhizae diaspores in two long -term Swedish field experiments at Ultuna and Offer was studied. The n umber of diaspores decreased rapidly over time with increasing annual additions of an easily soluble phosphorus fertilizer, 45 kg P ha-1 yea r-1. Five years after the start of the experiment, at this level of fe rtilization, the spore frequencies were reduced by 50% and 7% at Ultun a and Offer, respectively. The spore frequencies later stabilized and fluctuated between 0.08 and 0.56 diaspores g-1 of soil. Moderate addit ions of easily soluble phosphorus fertilizers, 5 and 15 kg P ha-1 year -1, did not affect spore frequencies, which fluctuated between 0.48 an d 1.96 diaspores g-1 of soil. When excluding phosphorus fertilization, spore frequencies doubled within 5-14 years. Twenty-eight years after the start of the experiment, the spore frequencies of the zero-phosph orus fertilized soils had doubled and trebled, respectively at the two locations. The phosphorus additions increased the amounts of easily s oluble phosphorus in the soils, which adversely affected the diaspores . No relationships existed between the number of VAM diaspores and oth er soil parameters such as the amount of difficultly soluble phosphoru s, pH, organic matter, or total nitrogen content. A plant-production p arameter, estimated as dry matter production, was independent of the p resence of VAM diaspores but another plant production related paramete r, uptake of phosphorus by the plants, was dependent on the presence o f VAM diaspores.