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 number of diaspores decreased rapidly over time with increasing annual additions of an easily soluble phosphorus fertilizer, 45 kg P ha-1 year-1. Five years after the start of the experiment, at this level of fertilization, the spore frequencies were reduced by 50% and 7% at Ultuna and Offer, respectively. The spore frequencies later stabilized and fluctuated between 0.08 and 0.56 diaspores g-1 of soil. Moderate additions of easily soluble phosphorus fertilizers, 5 and 15 kg P ha-1 year-1, did not affect spore frequencies, which fluctuated between 0.48 and 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-phosphorus fertilized soils had doubled and trebled, respectively at the two locations. The phosphorus additions increased the amounts of easily soluble phosphorus in the soils, which adversely affected the diaspores. No relationships existed between the number of VAM diaspores and other soil parameters such as the amount of difficultly soluble phosphorus, pH, organic matter, or total nitrogen content. A plant-production parameter, estimated as dry matter production, was independent of the presence of VAM diaspores but another plant production related parameter, uptake of phosphorus by the plants, was dependent on the presence of VAM diaspores.