Vanakkam Murali Ayya,

Both, mycorrhizal fungi and their host plants need phosphates, and work hard to accumulate P.  One of the main functions of mycorrhiza is to extract P from the soil. But when soluble P are abundant, the host plants tend to increase their restrictions to mycorrhizal fungi, resulting in lower levels of root colonisation. Therefore, under conditions of high available P, the restrictions are increased. This has been proved by Shukla et al (2012) who studied different P levels on AM colonisation on different plants. This is highly depends on soil conditions eg. - Higher P availability result in large amount of N loss in P-poor soils (Shi et al. 2013). 

Anbudan,

Raja

High soil P levels have been found to be detrimental for the symbiotic association of arbuscular mycorrhizal fungi with the crop plants.There are several reports from different  agro climatic and cropping situations to show the reduction in AM colonization in roots due to the application of phosphatic fertilizers.AM infection intensity was found to be inversely correlated to soil P levels and at very high P fertility levels the root system of normally mycorrhizal plants was sparsely infected.This was found to be true with the acidic soils of Kerala which are having high available phosphorus content as revealed by a study conducted earlier. However, the spore counts were found to be affected to a lesser extent than root colonization when phosphorus  levels were high in soils.In many farmers plots where farmers apply less or negligible chemical fertilizers and where organic manure application is a practice along with the cropping system approach in perennial plantations,AMF spore counts,species diversity and richness were found to be very high. In such situations the crops depend more on mycorrhizae for the uptake of phosphorus and other immobile elements.

Dear Murali Gopal,

High soil P levels have been found to be detrimental for the symbiotic association of arbuscular mycorrhizal fungi with plants. These fungi need the root exudates to their vegetative growth. When the content of P is high in soil, P results in decreasing exudates of root by affecting phospholipid membrane. Therefore, due to decreasing in root exudates, which is important to vegetative growth of these fungi, the symbiotic association of arbuscular mycorrhizal fungi with plants is reduced.

Best regards,

My understanding of the literature on AMF is that while they were first discovered as being very important for phosphorous acquisition, they are generally important in mediating plant stress and have important roles in the acquisition of many nutrient resources, and in the biosequestration of potentially toxic elements (especially in heavy-metal contaminated soils). This is to say that while many studies do show that increasing phosphorous can suppress AMF colonization, it is important to understand that AMF colonization will be greatest when they offer the greatest reduction in plant stress, so it is also important to understand what are the limiting factors to plant growth in your soils. If there is an evolutionary history of phosphorus NOT being a limiting factor in plant growth, it is possible the AMF have adapted to providing other resources.

Thank You Rajasekaran Murugan avargale, George V. Thomas sir, Hassan Etesami and Dietrich Epp Schmidt for your critical inputs on AM sporulation and root colonization in acid soils of Kerala paradoxically having high available P. 

I understand from your replies:

i) High available P influences AM colonization of the roots negatively but not the sporulation.

ii)  AM association with plants is energy-driven trade-off wherein the mutual benefits for the host and the symbiont may surpass the energy lost by them during their co-existence. So, it could be possible that even if P is available in high concentrations in soil, plants may yet allow AM to colonize their roots in good measure for mobilizing nutrients other than P and reduce the plants from various biotic and abiotic stresses.

Our studies on AM association with coconut and arecanut and their cropping systems in Kerala somewhat reflect both the above observations. However,  we have not proved the second one directly. 

My intent to raise the query was to know if AM bioinoculant will be effective for crops grown under situation where soils, though acidic, have high available P in them?

Thanks,

Murali

Where there is an abundant supply of P, plants are not dependent upon AM fungi.

Thank you Lynda Hanlon for the response.  Plants are not dependent on AM fungi for what? P perhaps??

Will be grateful for experiences and  inputs from researchers who would have tried applying arbuscular mycorrhizal fungi to  plants that grow in acid soils having surplus available phosphorus. 

Thanks,

murali

We have developed a soil based AM inoculant containing Claroideoglomus etunicatum . This AM was found to be predominant species in the rhizosphere of coconut gardens. It was tested against coconut seedlings grown in soils having poor available P and found to produce good growth promotion. We want to popularize the use of this AM inoculant in Kerala. However, as I mentioned earlier, the soils here are having surplus available P. Would it be successful if used for coconut cultivation in Kerala?

Thanks

Interesting discussion . Can we have some indicator AMF  species describing crop  specific   rhizosphere . Probably then only , AMF-based inoculant  will be effectively  possible, and probably then only , inoculation with rhizo-competent AMF  will make any difference with respect to  both soil health as well as plant health .

Thank you Dr. Anoop Srivastava. We have not been able to find any indicator AMF specific for coconut or the inter crop rhizosphere. For example along with the main crop coconut there are black pepper, banana, nutmeg etc that are grown as intercrops in coconut based cropping system. Our studies found that no particular AM species is specific for any of these crops. The C. etunicatum was found to be present in 100% of the soils of coconut rhizosphere. I do not know if this data qualifies for it to be rhizo-competent. But its application definitely improve the growth parameters of coconut seedlings.

Thanks.

Did you find any   microbial compositional change  in rhizosphere of black pepper, banana, nutmeg  in relation to available pool of nutrients . Such attempts will give you an aided advantage to develop AM-based microbial consortium , which will be like a hot cake to provide the required resilience against both diseases as well as possible nutrient mining .

A good suggestion Dr. Srivastava. We have analyzed the soil nutrient and microbial populations of these crops. What kind of specific relationship should we look for to aid our understanding of the paradoxical situation: high available P as well as AM association.

Thanks

Normally, the application of soluble phosphorus decreased root colonization like has been seen by Abbott and Robson (1984) Improvement of phosphorus nutrition following AM colonization of phosphorus-deficient roots results in a decrease in membrane permeability and reduction in root exudation (Graham et al., 1981). The reduction of that activity affects the establishment of mycorrhizal association. However remember also that sporulation rates of AM fungi have been found to be host-dependent and depend also from other multiple abiotic factors, and then you need to take into account such other factors before to arrive to a definitively conclusion.  

Thank you EF Gomez for your valuable inputs.

Dear Dietrich Epp Schmidt,

I had been perusing literatures available on the P status in Kerala soils. It appears that there has been abundant P available for last 10 years or more making P as not a limiting nutrient. Could our observation of presence of am spores and root colonization in coconut based cropping systems  indicate a different function? And, do you have access to any publications that shows good am occurrence in high p soils ?

Thanks 

Is there any relation between total soil and available soil P?

As per my research, the site with a higher available P had very little spore density and root colonization compared to the site with a low available P.

Can someone highlight , how AMF differentiate between indigenous soil P and exogenously applied P in a given ecosystem...

AMF is an obligate symbiont, which assist plants in the uptake of water and nutrients the plant roots alone cannot access. In return, the plants give the symbiont photosynthates. In its simplest form, when fertilisers are added to the soil, there is less need for the three-way exchange between plant, soil and fungus.

Generally soils with high P content reduce root colonization and AM fungi efficiency to mobilize nutrients especially P