Saturday, 15 July 2006

Response of Arbuscular Mycorrhizal Fungi to Phosphorus Fertilization of Soils as Affected by Soil Conditions.

N. Mathimaran, R Ruh, J Jansa, and E. Frossard. ETH, Research station Eschikon, Lindau, 8315, Switzerland

Arbuscular Mycorrhizal Fungi (AMF) are widespread symbionts of plants. They colonize roots of a great majority of plant species and contribute to their uptake of mineral nutrients such as Phosphorus (P), nitrogen, and zinc from the soil. This is of particular importance for plants growing under nutrient limiting conditions and likely less important when the availability of nutrients in soil increases. The fungi are, on the other hand, fully dependent on supply of reduced carbon from their host plant under all conditions. Thus, an increase in soil P availability due to P fertilization is suggested to reduce AMF contribution to P uptake by plants (mycorrhizal benefits), whereas the carbon drain from the plants towards the fungi (mycorrhizal costs) remain the same. Here we studied whether P fertilization of two different soils (tropical ferralsol and temperate cambisol) had any influence on P acquisition and maize growth promotion capacities of indigenous AMF species. Five strains of Glomus intraradices were isolated from each P fertilized and unfertilized temperate soil and five strains of Gigaspora gigantea were obtained from each P fertilized and unfertilized tropical soil. These AMF species were the dominant components of indigenous AMF communities in their respective soils. Those twenty fungal strains were used for inoculation of maize in a fully factorial glasshouse pot experiment with two levels of P addition to the pots and four replicates per treatment. For testing the efficiency of P acquisition by AMF from the soil, we buried little tubes with radioactively (33P) labeled soil into each pot. The tubes were capped with a mesh that could be penetrated only by AMF hyphae, but not by plant roots. The maize was grown for six weeks in the glasshouse and then harvested. Nested ANOVA design was used to separate the effects of P fertilization history of the AMF on their contribution to plant P uptake and biomass production from the effects of P addition to the pots. Percentage of maize root length colonization by the AMF was used as the weight to remove the possible bias in the biomass and P uptake due to differences in colonization. The maize inoculated with any of the AMF species had higher biomass production and P uptake compared to nonmycorrhizal maize plants when no P was added to the pots. The P fertilization history of the AMF strains did affect neither the biomass production nor P uptake of the plants. P addition to pots inoculated either with Gigaspora gigantea or Glomus intraradices resulted in higher P uptake by maize plants but only led to increase in plant biomass production when inoculated with Gigaspora gigantea. The strains of Gigaspora gigantea from unfertilized soil took up and transferred much more 33P from labeled tubes to plants than the strains from fertilized soil. The strains from unfertilized soil also reduced their 33P uptake from the tubes significantly when P was added to the pots, whereas such response was not observed for strains from fertilized soil. The strains of Glomus intraradices from fertilized soil reduced their 33P uptake significantly when P was added to the pots, whereas the 33P uptake by strains from unfertilized soil was unaffected by P addition to the pots. Our results indicate that AMF adapt to their environment. Since Gigaspora gigantea from P deficient tropical soil is likely adapted to P poor soil, P fertilization seems to suppress its symbiotic benefits already long before the P availability is sufficient for plant growth. On the other hand, Glomus intraradices from P sufficient temperate soil is likely adapted to higher P availability in soil and particularly the strains from regularly P fertilized soil developed a strategy to save their investments into P acquisition upon condition when P is not limiting for plant growth.

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