Mycorrhizal effectiveness on physic nut as influenced by phosphate fertilization levels

In recent years, physic nut (Jatropha curcas L.) has attracted attention because of its potential for biofuel production. Although it is adapted to low-fertility soils, physic nut requires soil acidity corrections and addition of a considerable amount of fertilizer for high productivity. The objective of this research was to evaluate the effectiveness of arbuscular mycorrhizal fungi (AMF) (control without AMF inoculation, Gigaspora margarita inoculation or Glomus clarum inoculation) on increasing growth and yield of physic nut seedlings under different rates of P fertilization (0, 25, 50, 100, 200, and 400 mg kg-1 P soil) in greenhouse. The experiment was arranged in a completely randomized, block in a factorial scheme design with four replications. The physic nut plants were harvested 180 days after the beginning of the experiment. Mycorrhizal inoculation increased physic nut growth, plant P concentration and root P uptake efficiency at low soil P concentrations. The P use quotient of the plants decreased as the amount of P applied increased, and the P use efficiency index increased at low P levels and decreased at high P levels. Mycorrhizal root colonization and AMF sporulation were negatively affected by P addition. The highest mycorrhizal efficiency was observed when the soil contained between 7.8 and 25 mgkg-1 of P. The physic nut plants responded strongly to P application, independent of mycorrhizal inoculation.

Phosphorus availability changes the internal and external endomycorrhizal colonization and affects symbiotic effectivenes

Despite the awareness that high phosphorus (P) availability decreases the mycorrhizal root colonization in higher plants, the effects on the external mycelium are contradicting. The aim of this work was to assess the external mycelium of the arbuscular mycorrhizal fungi (AMF) Glomus intraradices and Gigaspora rosea, associated to soybean under increasing P levels (0, 25, 50, 100 and 200 mg kg-1). The tested hypotheses were: a) the amount of external mycelium depends on the fungal species; b) the P levels affect diferentially each mycorrhizal species; c) the mycorrhizal effectiveness depends on the amount of external mycelium. Mycorrhizal stimulus on plant biomass occurred at P levels of 0, 25 and 200 mg kg-1. Mycorrhizal root colonization and total external mycelium (TEM) decreased with increasing P levels. At zero P level, G. rosea produced less TEM than G. intraradices, which may be related to the lower mycorrhizal effectiveness for G. rosea. Both AMF increased the active external mycelium along P levels, but this increase was not related with symbiotic effectiveness. TEM was found to be closely related to root colonization and mycorrhizal effectiveness. The root colonization and TEM are mutually afected by P availability. Althoug G. intraradices produced more TEM than G. rosea along the P levels, they seemed to affect similarly the TEM for both AMF. More AMF species should be assessed in order to attest whether P levels affect the external mycelium differentially among them.

Mycorrhizal effectiveness and manganese toxicity in soybean as affected by soil type and endophyte

Mycorrhizal plants may present Mn toxicity alleviation and this depends on the plant-endophyte-environment interaction. The effectiveness of three arbuscular mycorrhizal fungi (AMF) (Glomus macrocarpum, G. etunicatum, G. intraradices) and a control without AMF in two soils: Typic Rhodudalf with high Mn availability and a Typic Quartzipsamment, with low Mn availability, was evaluated in a time-course experiment at 3, 6, 9 and 12 weeks after soybean (Glycine max L.) seedling emergence. The objective was to select the most effective AMF species to enhance plant growth and to assess its effects upon Mn uptake by plants and Mn availability in the soil. For the sandy soil, AMF inoculation resulted in increased plant biomass, especially with G. intraradices and G. etunicatum. Lower Mn concentrations were observed in shoot and root of mycorrhizal plants. For the clayey soil, there was also an increase in plant biomass, but only for plants inoculated with G. intraradices and G. etunicatum. Mycorrhizal plants presented higher Mn concentrations in shoot and root and there was an increase of available Mn in the soil, in relation to the control, especially in the treatment with G. macrocarpum. When inoculated with G. macrocarpum, plants presented Mn toxicity symptoms and reduced biomass in comparison to control plants. The effects of mycorrhizal inoculation, either positive or negative, were most intense at 9 and 12 weeks.

The significance of grazing on fungi in nutrient cycling

Excretion of N by fungal grazers is not the dominant process by which N is released in nutrient cycling: it accounts for one eighth or less of total net N mineralization. Fungivores comprise between 21 and 76% of the fauna biomass. Other fauna, as well as fungi and bacteria, all participate in the mineralization process. Microcosm studies have shown fungal grazing can promote release of N, but immobilization by concomitant microbe production can occur in tandem with that release. Studies using field applications of biocides have had inconsistent outcomes. Fungivores contribute to nutrient cycling by the combined action of comminution, mixing, and dispersal of inoculum, which promote microbial activity. Passage through the Collembola gut has been estimated to have the capacity to bring about a 42-fold increase in nitrate concentration from food to faeces, which on an ecosystem scale could conceivably translate into a doubling of levels of nitrate. Recent laboratory work has shown that fungivores may prefer the thinner mycorrhizal hyphae that occur some distance away from the more coarse mycorrhizal hyphae in the rhizoplane. Where this occurs, grazing can be expected to have only a small impact on the effectiveness of mycorrhizal fungi for the promotion of plant nutrient absorption in the field. Key words: fungivores, fauna, soil, litter, mineralization, mycorrhizal effectiveness.

Determining vesicular–arbuscular mycorrhizal effectiveness by monitoring P status of leaf disks

A greenhouse experiment was conducted to monitor the development of symbiotic interaction between the vesicular–arbuscular mycorrhizal (VAM) fungus Glomus aggregatum and Vigna unguiculata grown in a typical Oxisol before and after the soil was subjected to simulated erosion and at various levels of phosphorus in the soil solution. VAM development monitored in terms of P status of cowpea leaf disks revealed that VAM activity was not detected in the eroded soil unless the soil was amended with P. When P was not limiting, VAM activity (effectiveness) was detected as early as 17 days from planting, the activity peaking 5–10 days thereafter. Peak VAM activity was observed at a soil solution P level of 0.026 μg/mL and the peak values were similar in the eroded and uneroded soil samples. Maximum mycorrhizal inoculation effect was also observed at this level of soil solution P. Our results illustrate the usefulness of the leaf-disk technique for monitoring the development of the VAM symbiosis and the significance of soil solution P in regulating host response to VAM inoculation.