The Growth of Fairy Rings of Agaricus Arvensis and Their Effect Upon Grassland Vegetation and Soil

SUMMARY (1) The effect of fairy rings of Agaricus arvensis upon grassland vegetation is described. The outermost zone of affected vegetation usually had a high proportion of dead material and bare ground. Leaves of Agrostis capillaris were bluish with characteristic reddening at the tips, and showed evidence of desiccation. Within the scorched zone, growth, particularly of grasses, was stimulated. Occasionally fairy rings produced two or three concentric regions of scorched or reddened foliage alternating with zones of more vigorous vegetation. It is suggested that the inner, reddened zones were areas of potassium deficiency, and represented positions of intense fungal activity in previous years. (2) The mean rate of advance of the fairy rings was 49 cm year-' in 1979-80, and 42 cm year-' in 1980-8 1. (3) Total respiration rate of the grassland sward was usually greatest in the outermost scorched zone, and lowest towards the inside of the ring. (4) Soil organic matter declined from 9 1% of the dry weight outside the ring to a minimum of 6.6% at 80 cm to the inside of the leading edge. Associated with this decline were reductions in the concentrations of soil nitrogen and phosphorus. Concentrations of acetic acid-extractable potassium in the soil at the leading edge were three times as high as outside the ring, but declined sharply further inside to less than half the concentration outside. The fungus may accumulate potassium as it grows through the soil. The need to concentrate relatively large amounts of potassium from nutrient-poor soils provides an explanation for certain features of fairy ring growth, in particular the closely packed mycelium which impedes drainage, and the apparent toxic effects on plants.