It is a fairy ring of Marasmius oreades - an edible species. The ring grows every year to get new energy and affect the grass as it grows.

Agree with Mattias - this fungus is Marasmius oreades. For further details you can see Toohey J.I., 1983: Fungus fairy rings in soil etiology and chemical ecology. Canadian Field Naturalist: 9-15.

Bsets regards,

Elena

The comments are correct - Marasmius oreades ! The fungus ist spreading more and more in forming a ring that gets larger and larger. Can be stopped by aeration of soil. Also careful watering of the soil can stimulate lawn growth so that this is no further loosing nutrients to the fungus. Also a thin layer of sand can be helpful: in spring the grass will growth through easily and overgrow the fungus.

I think its genus Marasmius ...good luck.