Fungicide application and phosphorus uptake by hyphae of arbuscular mycorrhizal fungi into field-grown peas

The effect of two commercial fungicide formulations on phosphorus (P) uptake into peas via hyphae of a native arbuscular mycorrhizal (AM) fungal community was examined in the field. The fungicides contained carbendazim or a mixture of propiconazole and fenpropimorph as their active ingredients and were applied at 1 x and 100 x the rate recommended for field use. Hyphal compartments (HCs; soil cylinders completely surrounded by nylon mesh of 25 mum mesh size) were buried between two rows of peas. The soil inside these HCs was labelled with P-32 to distinguish hyphal P uptake from inside the HCs from overall P uptake, Fungicides were added to the soil inside the HCs at concentrations assumed to reflect their concentration in the surrounding soil. At two harvests, plant growth, total P and P-32 uptake as well as root length density and AM root colonisation were measured. Length of hyphae inside the HCs was measured at the final harvest. Both carbendazim and the propiconazole/fenpropimorph-mixture stimulated hyphal P uptake from inside HCs when applied at rates recommended for normal field use, This increase in hyphal P uptake was significant only in the case of carbendazim. A negative effect of the fungicides on other components of the soil microbial community with which AM fungi interact is considered the most likely explanation for the observed stimulation in hyphal P uptake. Hyphal P uptake was completely inhibited by application of carbendazim at 100 x the recommended rate, Plant growth and overall P uptake were not affected by fungicide applications apart from application of the propiconazole/fenpropimorph mixture at 100 x the recommended rate. This rate completely inhibited plant growth. AM root colonisation was reduced by the high rate of carbendazim application only. This reduction had no effect on plant growth which may be due to the fertile soil conditions at the field site. It is concluded that the testing for side effects of pesticides on non-target organisms should include functional parameters such as P uptake by AM fungi. Such measures may be more sensitive to harmful effects than measures of plant growth or biomass. (C) 2001 Elsevier Science Ltd. All rights reserved.