EFFECT OF ARBUSCULAR MYCORRHIZAS ON UPTAKE OF NITROGEN BY BRACHIARIA-ARRECTA AND SORGHUM-VULGARE FROM SOILS LABELED FOR SEVERAL YEARS WITH N-15

A pot experiment was conducted under glasshouse conditions to investig ate the effect of the inoculation of arbuscular mycorrhizal (AM) fungi on the growth and N uptake of two graminaceous plants: Brachiaria arr ecta (Hack. ex. Th. Dur & Schinz) Stent and Sorghum vulgare (Pers.) gr owing in four fumigated soils which had been labelled for several year s with N-15. The four soils had different chemical characteristics, es pecially in their P availability. The levels of root colonization by A M fungi were inversely related to the P concentration in these soils. Positive responses to AM fungal inoculation were observed for all the yield parameters except in the soil with the highest P availability (4 0 mu g P g(-1) soil, extractant Mehlich-1). In the other three soils, AM fungal inoculation of both crops increased N accumulation by betwee n 17 and 46%, but the N-15 enrichment of the N absorbed was significan tly lower in the mycorrhizal plants than in those not colonized by myc orrhizal fungi. This suggests that some of the N absorbed from the soi l by the mycorrhizal roots was derived from N sources other than the m ineral N.