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NUTRIENT CYCLING

Direct Measurement of Arbuscular Mycorrhizal Phosphorus Uptake into Field-Grown Winter Wheat

^a Risø Natl. Lab., Roskilde), Dep. of Microbial Ecology, Ecology Bldg., Lund Univ., S-223 62 Lund, Sweden
^b Plant Microbe Symbioses, Plant Biology and Biogeochemistry Dep., Risø Natl. Lab., DK-4000 Roskilde, Denmark

iver.jakobsen{at}risoe.dk

The aim of the study was to measure hyphal P uptake by native arbuscular mycorrhizal fungi using a test system designed for field conditions. Phosphorus uptake by external hyphae of a native arbuscular mycorrhizal (AM) fungal community was studied in the winter wheat (Triticum aestivum L. cv. Stava) crop of an organic cropping rotation. Mesh containers containing a soil volume accessible only to external hyphae were buried in the ground. The soil inside these hyphal compartments (HCs) was labeled with ³²P to distinguish P taken up from inside the HCs from overall P uptake. As an additional control treatment, half of the HCs received the fungicide carbendazim [methyl benzimidazol 2-y carbamate] at a rate known to disrupt AM hyphal P uptake. Wheat tillers were harvested throughout the experimental period to monitor the time course of hyphal P uptake. Final harvest was at plant maturity, at which time the HCs were excavated and the soil inside was analyzed for hyphal length, NaHCO₃–extractable P and specific activity of the extracted P. External hyphae of the native AM fungi grew into the HCs without added carbendazim and took up substantial amounts of P. Addition of carbendazim at a rate known to inhibit hyphal P uptake resulted in no P uptake from inside those HCs. This is due to the design of the HCs used in this study, which successfully prevented P uptake by plant roots and root hairs from inside the HCs. The HCs are therefore well suited for field measurements of plant P uptake via external hyphae of the native AM fungi. Estimates for length-specific hyphal P uptake per unit time (37.8 ± 5.2 fmol P m^-1 s^-1) were similar to published estimates obtained in growth chamber studies with cultured isolates of AM fungi (1–430 fmol P m^-1 s^-1). The results of this study demonstrate the considerable contribution of native AM fungi to overall P uptake of field-grown winter wheat, even at typical field soil fertility levels (28 µg NaHCO₃–extractable P g^-1 soil).

Abbreviations: AM, arbuscular mycorrhizal • HC, hyphal compartment

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