Effect of arbuscular mycorrhizas on uptake of nitrogen byBrachiaria arrectaandSorghum vulgarefrom soils labelled for several years with15N

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Abstract
Summary: A pot experiment was conducted under glasshouse conditions to investigate the effect of the inoculation of arbuscular mycorrhizal (AM) fungi on the growth and N uptake of two graminaceous plants:Brachiaria arrecta(Hack. ex. Th. Dur & Schinz) Stent andSorghum vulgare(Pers.) growing in four fumigated soils which had been labelled for several years with15N. The four soils had different chemical characteristics, especially in their P availability. The levels of root colonization by AM 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 (40 μg P g−1soil, extractant Mehlich‐1). In the other three soils, AM fungal inoculation of both crops increased N accumulation by between 17 and 46%, but the15N enrichment of the N absorbed was significantly lower in the mycorrhizal plants than in those not colonized by mycorrhizal fungi. This suggests that some of the N absorbed from the soil by the mycorrhizal roots was derived from N sources other than the mineral N.