What do you mean by soil P content? Total P content or available P content? This can make a huge difference!

Dear Francesca

Have you checked the pH of your soil? There is an optimum level of pH for P availability to plants irrespective to what is in the soil.

Mohsen

Dear Francesca,

I believe that your question is relevant to the P form which plant only uses available P-form decided by soil pH as mentioned by Mohsen. On soil with pH> 6.5-7, available P is very low and most P are binded by various ions such as Ca 2+.

Weikai

Ya, the optimum pH of soil available P is around 7.2. Either higher or lower phosphate will precipitate as Ca-phosphate or Al-, Fe-phosphate respectively. You can find this out by doing the soil P fractionation experitment.

Dear Francesca,

Even if the quantity of available P in the soil is the same among the succession, you could have functional change in the plant community that implies changes in roots P content. Another possibility is that in late stage of the succession the P is located in organic matter and as a result unavailable for plants.

Florian

Dear Francesca, first of all, excuse my english. All the answers cited above are possible, moreover You must check the physiological state of the plant. If it is in the reproductive stage the plant mobilize the P reserves to produce the flowers or seeds.

Daniel.

Avalability. What is the status of Ca, high levels will reduce the avialability.

P absorbtion is different in different types of soil. You need to see what type of soil you have. For example in acidic soils P is absorbed by different oxides and hydroxides forming coloids. Increasing ionic strength of soil, you may increase P absorbtion. Root architecture of the plant is also important. I think that the major problem is that generally P have low availability due to slow diffusion and high fixation in soils.

pH is the major factor on which the availability of the nutrient to plant depends upon. please see the attached photograph. the pH at which P is available is either 6.5-7.5 or 8.7 to 10

The Pi content of the roots depends of several factors such as the quantity orthophosphate in the soil, pH and the physiological status of the plant. You can obtain more informmation in this paper:

J Exp Bot. 2004 Feb;55(396):285-93. Epub 2004 Jan 12.

The transcriptional control of plant responses to phosphate limitation.

Franco-Zorrilla JM, González E, Bustos R, Linhares F, Leyva A, Paz-Ares J.

Thanks all,

as Laurent Cournac suggeted, the P I'm talking about is the available P, the one that plants can actually uptake. The total P has a very low value, the available P has a very high value and P in roots shows the same low value of the total P. If there is P available for plant uptake, I should find a P value in plant roots as high as the one of the available P in soil. It is just the opposite in my results. What might be the reason why plants are not using this available P?

1-it may the P not solve in the root environment.

2-may not be suitable soil pH for P uptake.

3-P uptake is an active process and needs energy of root respiration (as a form of ATP I mean), so waterlogged conditions led to hypoxy or anoxy or if the soil around the roots is cold then root respiration stopped and P will not absorb well.

4-P uptake in some plant species depends on mycorrhizal-root symbiosis. In the absence of the root symbiont, P will not absorb well.

Different species may have different P concentration in roots enve if they growth in soil with the same availability of P it's depend on their tissue characteristics...

Hi Florian, so you are suggesting as possible explanation for these results the tissue characteristic influence on P uptake. Do you have any interesting papers to advise reading? Many thanks and thanks to all of you, I'm really appreciating your collaboration

Hi Francesca, you could read "Plant and microbial strategies to improve the phosphorus efficiency of agriculture" Richardson et al. (2011). In this paper authors describe a large range of root strategies wich can have a strong influence on plant P uptake capacity. I think you could found some interesting grounds for thought in this paper. Florian

Ok Thanks, I will have a look at it...

why The soil P content, as available P (the one that plants can uptake) at the same stage is high but the root P value is very low, is that although the soil P content is high, but most of them are insoluble, and plant roots only uptake soluble phospharus, so the content of Pi in plant roots is low.

The term "available P" can mean different things. Different methods for determining available P will yield different concentrations. As Manish mentioned, pH is very important, especially with regard to P bound to calcium and aluminum. As Hamid said, flooding can also influence plant P concentrations. The decreased redox potential resulting from flooding, however, can actually increase root P concentrations.

Let's try to put relativism of P concentration in numbers; from tables if I feed 120 kg ha-1 of P2O5 in the soil the 'normal' durum weat concentration would be 0.7 to 0.9 g per 100 dry weight. But the soil volume and its weight for 50cm of dept in 1 ha is so high that weat is concentrating P if I'm right...

Sorry for trunkated answer; would then I suggest to check reference units in root and soil figures. If the units are for P in solution like ppm in water, the water root reference should be at least equal to environment or if they are in dry matter the reference units and the space sampled of soil is critical, as well the presence of an equilibrium like it happens in stirred solutions. P. see http://www.plantbio.uga.edu/~haines/Phosphorus_poster_3.pdf. GB

From what I understand from my study, the way in which labs determine "bioavailable" elements is by performing a acetic acid leach on the soils, and analysis the elemental content of the leachate. However, this is not always going to give you a true indication of what is available to every plant. Organic P is not available to plants, and the P needs to be mineralised to an inorganic phosphate before it can be taken up by plants (this is what is put into fertilisers).

Phosphorus is quite immobile. After absorption from soil, it is incorporated in nucleic acids (DNA and RNA). Some plants can store it as inositol phosphate.

The results you report is not unusual. Uptake (accumulation) of nutrients depends on several factors. Low P status in root despite high soil P status is because of the capability of the plant for P accumulation. This phenomenon can be defended by active nutrients absorption. You need a book on plant physiology and go through the mineral nutrient absorption instead of getting confused from so many answers may be relevant to your work or not.

Hi Nafees, thanks for your help and advise. Actually I'm getting into plant nutrient physiology and nutrient allocation and I would recommend some interesting reports on New Physiologist, in particular : "N:P ratios in terrestrial plants: variation and functional significance", Sabine Gusewell (2004).

P is mostly fixed generally in alkaline and spedifically in sodic soil having high level of Na. Sufficient quantity of P is present in soil but due to its fixation with Na, it is not avaiable for plant. Soil having less than 7ppm of P are considered deficient and 14 ppm is optimum level for soil.

Available soil P content!

Yah, I agree with R. maheswari. As the absorbed P might be utilized in metabolic processes. all metabolic products are made up of P compunds with major proportion.

Low P contents in roots may be due to one or combination of followings;

Soil pH: At high soil (above pH 7.0), soil P is fixed into less soluble/available forms by excess calcium. At low soil pH (below pH 6.0), soil P is fixed into less soluble/available forms by excess aluminum.

Soil Moisture: Under moisture stress conditions, P availability and uptake generally decreased, though at higher levels of soil P, this can change.

Soil Compaction: As P moves slowly in the soil, anything that inhibits root growth is likely to reduce P uptake.

Soil Aeration: Lack of soil O2 can reduce P uptake.

Soil Temperature: At low Tem. P uptake is reduced.

Soil Texture: Some clay types generally exhibit high P fixation.

Soil Organic Matter: Generally, high organic matter contents result in greater fixation of Fe and Al, resulting in less P fixation.

Plant Root Systems: As soil P is vastly immobile, well developed root systems could only ensure better uptake of P.

Availability of PSBs: With the availability of PSBs, insoluble phosphates too can be solubilized.

Mycorrhizae: The thread-like hyphae of Mycorrhizae could absorb and transport P to the plant roots.

Other Elements: P uptake is enhanced when in combination with ammonium N (NH4-N). Potassium has been shown to co-precipitate with P when soluble phosphoric fertilizers are applied to soils. Calcium may combine with P to make insoluble compounds that are unavailable to plants in the short term. Mg fertilization can enhance P uptake by plants.

Due to the possible interactions between P and Zn, high levels of either element can depress the uptake of the other. Other elements like Cu, B, Mo, Si etc., have also effects on P uptake.

If water is limited, we found that the genes for P uptake were downregulated (including several acid phosphates, P transporters, the phosphate inducible gene, and the P membrane transporter (antigen peptide transporter 2 [APT2]) even though soil P was high. See The Plant Genome paper (Hansen et al. 2013) for more information

http://doi: 10.3835/plantgenome2012.11.0029

What is the soil pH? is an agricultural land?

That such functional content in soil microorganisms in particular (Arbuscular mycorrhiza and phosphate solubilizing bacteria?

P dynamics in soil solutions is entirely differ than those of in plant systems. In soil solutions p movement is by and large governed by ph and temperature, however moisture regim also play a crucial role, due to its slugging movement and limited movements from it place.Under plant systems p movement ismore vibrant due to adequate energy balance and water potential. Both were is not upto the marks in soil solutions in general.

hi francesca. which type is your soil? in some tropical soils with high content of Fe or Al oxides for instance (which exhibit AEC), PO4 can be retained towards plants while displaced by HCO3- as from the olsen procedure... in line with nina: what is your extraction procedure and what is the soil type?

Is possible in soils with high acidity and low cation exchange