Phosphorus controls the level of colonisation by arbuscular mycorrhizal fungi in conventional and biodynamic irrigated dairy pastures

The broad-scale factors determining the level of colonisation by arbuscular mycorrhizal fungi in irrigated permanent pasture were investigated using 10 pairs of dairy farms located in northern Victoria, Australia. Each pair consisted of adjacent farms, one under conventional management and the other under biodynamic management. Significant quantities of fertilisers were applied only to the conventional pastures and this was the major difference in inputs between the 2 management systems. Colonisation by arbuscular mycorrhizal fungi of white clover (Trifolium repens L.), perennial ryegrass (Lolium perenne L.) and paspalum (Paspalum dilatatum Poir.) was lower in the conventionally managed pastures than in the biodynamic pastures. On all farms, clover was more highly colonised than the grasses. The level of colonisation in clover exhibited a strong negative relationship with the concentration of phosphorus in the pasture shoots and a weak negative correlation with soil-extractable phosphorus. Similar relationships were evident for the grasses, but these were not as strong. Soil pH, soil organic carbon and the concentration of other soil nutrients (Kjeldahl nitrogen, sulfate, EDTA copper, EDTA zinc, chlorine, and exchangeable potassium, sodium, calcium and magnesium) and pasture shoot nutrients (chlorine, sodium, potassium, calcium, magnesium, manganese, copper, zinc, iron, nitrogen, sulfur and boron) could not account for differences in colonisation levels. On 3 farm pairs, colonisation levels by arbuscular mycorrhizal fungi were monitored over 3.5 years. Colonisation did not vary seasonally. It is concluded that phosphorus levels and host plant will be the major determinants of colonisation levels by arbuscular mycorrhizal fungi in perennial irrigated pasture-based agricultural systems.