By investigating both semi-natural and experimentally implemented pH variation in nutrient-poor sandy grasslands, we examined how soil decalcification may influence the arbuscular mycorrhizal (AM) fungal biomass in plant roots and in the soil, as well as the allocation of carbon (C) to AM fungal biomass. We estimated the amount of AM fungal mycelium, by using the fatty acid 16:1 omega 5, in the roots and in the soil surrounding the roots of two grass species in a pH gradient, and in one grass in an experimental area. We investigated the allocation of C to the AM fungi by using in situ C-13 labelling. Contrary to our expectations, we found a decrease in AM fungal mycelium in the soil with increasing soil pH, and no significant relationship between soil pH and the amount of AM fungi in the roots. We also found that the allocation of C below ground was very small in the grasses investigated: the excess of C-13 in the roots after five days was only between 0.06 and 1.2% of the excess of C-13 in the shoots directly after labelling. The allocation of C to the extraradical AM mycelium also decreased with increasing pH, ranging from not detectable to 1.2 mu g C-13 g(-1) dry soil in the range of soil pH from 5.5 to 8.5. Experimental manipulation of decalcification through soil perturbation further supported the finding that AM fungal density is higher in decalcified areas, although it was not statistically significant in this case. We conclude that the amount of AM fungi is very low in this type of nutrient and drought stressed habitat, and that a high pH in the topsoil does not leads to higher AM fungal biomass. (C) 2012 Elsevier B.V. All rights reserved.