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To examine the influence of vesicular-arbuscular (VA) mycorrhizal fungi on phosphorus (P) depletion in the rhizosphere, mycorrhizal and non-mycorrhizal white clover (Trifolium repens L.) were grown for seven weeks in a sterilized calcareous soil in pots with three compartments, a central one for root growth and two outer ones for hyphae growth. Compartmentation was accomplished by a 30-/zm nylon net. The root compartment received a uniform level of P (50 mg kg -1 soil) in combination with low or high levels of P (50 or 150 mg kg -1 soil) in the hyphal compartments. Plants were inoculated with Glomus mosseae (Nicol. & Gerd.) Gerd. & Trappe or remained uninfected.Mycorrhizal inoculation doubled P concentration in shoot and root, and increased dry weight, especially of the shoot, irrespective of P levels. Mycorrhizal contribution accounted for 76% of total P uptake at the low P level and 79% at the high P level, and almost all of this P was delivered by the hyphae from the outer compartment. In the non-mycorrhizal plants, the depletion of NaHCO 3-extractable P (Olsen-P) extended about 1 cm into the outer compartment, but in the mycorrhizal plants a uniform P depletion zone extended up to 11.7 cm (the length of the hyphal compartment) from the -3 root surface. In the outer compartment, the mycorrhizal hyphae length density was high (2.5-7 m cm soil) at the various distances (0-11.7 cm) from the root surface. Uptake rate of P by mycorrhizal hyphae was in the range of 3.3 -4.3 • 10 -15 mol s -1 cm -1

SUMMARYMaize {Zea mays L.) was grown in fertilized calcareous soil in pots which were separated by 30 fim nylon nets into three compartments, the central one for root growth and the two outer ones for hyphal growth. The size of each compartment was 40 x 25 x 3 cm. The treatments comprised of sterilized soil, either inoculated witb rhizosphere microorganisms (other than VA mycorrhizal fungi), with rhizosphere microorganisms together witb a VA mycorrbizal fungus [Glomus mosseae (Nicol. & Gerd.) Gerdemann & Trappe] or remained non-inoculated (sterile control). As inoculum for rbizospbere microorganisms the roots with adhering rhizospbere soil of nonmycorrhizal maize plants was used.Compared to the non-inoculated (sterile) control, inoculation with rhizosphere microorganisms did not affect shoot dry weight and morphology, but increased total root length (17 %) and root length per unit root dry weight (35%). The additional inoculation with VA mycorrhizal fungi had no influence on tbe shoot dry weight but increased area and dry weight of the leaf blades by about 30% and the ratio leaf blade:leaf sheath + stem (w/w) by 41 %. Tbe most profound effect of VA mycorrhizal fungi inoculation was on root growth and morphology. Compared to the non-inoculated control, root dry weight was decreased by 16%, root length by 31 % and root hair density and length by 41 and 43 %, respectively.In mycorrhizal plants the transpiration rates per plant were about 30 % higher than in the other treatments and this is attributed to tbe larger leaf area. Water uptake rate per unit root length and per unit time was about twice as high in mycorrhizal plants. For several reasons a substantial hyphal water transport seems unlikely. The results stress the necessity of detailed studies on root morphology for interpretation of effects of mycorrhizal fungi on mineral nutrient uptake and water relations in plants.

Phosphorus is one of the major limiting factors of primary productivity in terrestrial ecosystems and, thus, the P demand of plants might be among the most important drivers of soil and ecosystem development. The P cycling in forest ecosystems seems an ideal example to illustrate the concept of ecosystem nutrition. Ecosystem nutrition combines and extents the traditional concepts of nutrient cycling and ecosystem ecology. The major extension is to consider also the loading and unloading of nutrient cycles and the impact of nutrient acquiring and recycling processes on overall ecosystem properties. Ecosystem nutrition aims to integrate nutrient related aspects at different scales and in different ecosystem compartments including all processes, interactions and feedbacks associated with the nutrition of an ecosystem. We review numerous previous studies dealing with P nutrition from this ecosystem nutrition perspective. The available information contributes to the description of basic ecosystem characteristics such as emergence, hierarchy, and robustness. In result, we were able to refine Odum's hypothesis on P nutrition strategies along ecosystem succession to substrate related ecosystem nutrition and development. We hypothesize that at sites rich in mineral-bound P, plant and microbial communities tend to introduce P from primary minerals into the biogeochemical P cycle (acquiring systems), and hence the tightness of the P cycle is of minor relevance for ecosystem functioning. In contrast, tight P recycling is a crucial emergent property of forest ecosystems established at sites poor in mineral bound P (recycling systems). We conclude that the integration of knowledge on nutrient cycling, soil science, and ecosystem ecology into holistic ecosystem nutrition will provide an entirely new view on soil-plant-microbe interactions.

To test the ability of vesicular–arbuscular mycorrhizal (VAM) hyphae to take up water, phosphorus, nitrogen, and potassium, mycorrhizal and nonmycorrhizal couchgrass (Agropyron repens) or white clover (Trifolium repens) plants were grown in pots with separated compartments for roots and hyphae growth. Soil solution transfer between compartments was blocked by a 2-mm air gap. Total shoot contents of phosphate and nitrogen, but not of potassium, were higher in mycorrhizal plants with access to the hyphal compartment. Hyphal uptake from the outer compartment accounted for 49% of the total phosphate and 35% of the total nitrogen taken up by mycorrhizal plants. This was associated with depletion of extractable phosphate, [Formula: see text]-nitrogen, and also [Formula: see text]-nitrogen in the soil of the hyphal compartments. In contrast, no difference in water loss from the hyphal compartments was measured by tensiometers under well-watered and water-stress conditions whether hyphae were present or not. Severance of the external hyphae did not affect water loss from the outer compartments. The results show the ability of VAM hyphae to transport considerable quantities of phosphate and nitrogen to the plant from soil zones several centimetres from the root. However, there was no evidence for a significant direct water transport by VAM hyphae to plants. Key words: Agropyron repens (couchgrass), Glomus mosseae, nitrogen, phosphorus, vesicular–arbuscular mycorrhiza, water.

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Fine-root (<2 mm) biomass and biomass distribution were determined in different age-classes of three European forest chronosequences dominated by Fagus sylvatica L., Picea abies (L.) Karst., and Quercus cerris L., respectively. Root samples were taken with the auger method. There was a clear effect of stand age on standing fine-root biomass, with the highest fine-root biomass in adult but not mature stands. The vertical fine-root biomass distribution showed, at all sites, high densities of roots in the top soil layers and with depth a gradual decrease of fine-root biomass density. The difference in total fine-root biomass between the different age-classes appeared to be due to differences in the top soil layers. Fine-root biomass in the lower soil layers was less variable along the life cycle of the forests. Only in very young stands, specific root length of fine roots was higher than in the other age-classes. The present data together with other published values suggest that fine-root biomass in tree stands develops in three phases: rapid increase after a clear-cut harvest up to a maximum of fine-root biomass; a decrease during maturation of the stand; and a steady-state in mature stands.