IN-VITRO WEATHERING OF PHLOGOPITE BY ECTOMYCORRHIZAL FUNGI .2. EFFECTOF K+ AND MG2+ DEFICIENCY AND N SOURCES ON ACCUMULATION OF OXALATE AND H+

Oxalate accumulation in external medium under hyphal mats of two ectom ycorrhizal species is strongly stimulated (1.7 to 35 fold) by a simult aneous depletion of available K+ and Mg2+. Pisolithus tinctorius strai n 441 accumulates oxalate both on NH4-N and on NO3-N whereas Paxillux involutus strain COU only accumulates oxalate on NO3-N. On NO3-N, unde r a simultaneous K+ and Mg2+ deficiency, P. involutus COU is a very ac tive oxalate producer compared to P. tinctorius 441. The present resul ts could explain the various mineralogical evolutions of a phlogopite mica previously recorded under P. involutus COU or P. tinctorius 441 a nd suggest a key role for fungal oxalic acid during mineral weathering in response to nutrient deficiency.