I am studying the effect of native AMF communities on P nutrition of soybean. I am designing a pot experiment with soil from long term experiments with monoculture of soybean under no-tillage not fertilized. Soils are Mollisol (USDA classif), and they maintain high levels of P at 0-10 cm deep (more than 30 ppm P Bray I). I wonder if there is some method to extract or remove partially soil P without negative and significant effects on AMF native communities. I am thinking about extracting with 0.1 N Cl2Ca and washing with distilled water, then I will dilute soil with sand. I will appreciate any comments and suggestions!
A very interesting question Valera. Salts such as CaCl2 are very inefficient in removing the P that is sorbed to the surfaces of hydrous oxides, or present as Ca phases, e.g. associated with/precipitated on CaCO3 (if CaCO3 is present in your soils). Chemicals that are efficient tend to be harsh, e.g. acids or alkalis, and to have associated effects that are difficult or impossible to reverse, Consequently, even as a chemist I would not use chemical extraction. You might exhaust the P in the potted soil by growing soybeans, then do the plus/minus AMF treatment. But if the soil is rich in organic matter, that will continue to degrade and release P, so exhaustion may take several crops and factors such as nutrients, salinity and perhaps pH will need to be managed. A comparison of soil P propertties at different stages of the run-down, using chemical techniques and synchrotron X-ray spectroscopy, would be very informative.
Hi Valeria, For your experimental trial with or without P, purging the P from the soyabean pot may be difficult. Can you not use vermiculite or washed- sand for planting soyabean and AMF inoculation.
Dear Valeria
I would suggest taking your soil for soybean which is high in Phosphorus and planting a rotation cover crop of Sunflower. Take all the material top and bottom out of the soil to reduce the soil P content.
Sunflower has a high content of Phosphorus the elimination of the seed tops and roots will lower the Phosphorus which is removed from the system. Sunflower is a good host for AM fungi and the soil will not lose its AM potential or be altered by the fumigation giving an unnatural simulation.
The reduction of Phosphorus in the soil will lead to greater signal by the AM to establish favorable relationship with soybean plants. The AM fungi should be pretty typical of your origin soil.
Some Thoughts....I am seeing some good results from your planning and upcoming experiment.
Sincerely, Paul Reed Hepperly, PhD
Dear researchers
Thank you very much for your interesting answers! Every day since I published this question I read each answer while I was preparing the experiment.
The soil is a silty loam Mollisol with 6.4 pH, 3% organic matter and 26-36 ppm P Bray I. I made some tests in order to reduce the P content. Washing with Cl2Ca 0.1 N reduced P Bray from 36 ppm to 27. Unfortunately, in my laboratory it was impossible to do it with quantities of soil higher than 20 g because of the low percolation capacity of clay and silt and therefore the impossibility of removing the salt. Even with the use of vacuum...
For this reason, after reading your suggestions I decided to "dilute" the soil with sand (sterilized, P free). I am testing two dilutions: 10% and 30% soil:sand. I don't want to be short in AMF inoculum but I need low P content. I planted soybean seedlings after applying P in certain treatments (5 mM). Additionally, I fertilized with N (5 mM) in order to supress nodulation. I don't like vermiculite due to the possible immobilization P.
To eliminate the native community of AMF I used folmaldehyde because according to some publications is the method with lower impact on soil properties, specially available P.
Thank you very much for your suggestions and comments. I really appreciate it! They are so useful for me!!!
Kind regards
Valeria
I’m agree with you, in considering the appropriate solution the dilution with sand, without chemical artifacts. I would like to add the following information, if it may be helpful to you. In discussing about the topic P and AMF with other researchers, there is an increasing body of evidence, and confirmed in some experiments that I have performed in the past, that AMF activity and contribute is lower in low P soil, becoming higher in soil with a moderate soil P content.
Good luck for your experiments
Kind regards
Vincenza
When I worked with soybean the root rot was horrible. When the planting medium is soil based mixing with 50% sand greatly reduces problems with drainage with facilitates the rot. As you ably point out sand is extremely low in Phosporus and using a 1:1 mix with you native clay soil and with sand will lower P values probably by one half. Allophane is also very well known for Phosphorus fixation. It is the soluble Phosphate which signals the change of plants from susceptible to mycorrhizae to resistant and immobilizing and diluting or some combination should work.
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