Abstract
This study compared the validity of using the isotopically exchangeable phosphorus (P) as an accurate measurement of plant available P by comparing the specific activity of P, i.e. the 32P/31P ratio, in soil solution (Ss) against the specific activity of P in plants (Sp) growing in a loamy soil after applying a 32P-labelled fertilizer (NaH2PO42H2O) at different rates (F) and specific activities (Sf). Non-mycorrhizal and mycorrhizal (Glomus intraradices) plants of two species (soybean and barley) were grown in greenhouse experiments. Ss values were determined on 1:10 soil suspension after periods of incubation ranged from 1 min to 35 d. At a given rate of P application, the Sp values of both non-mycorrhizal and mycorrhizal soybean and barley did not show significant difference although the plant P uptake varied 18 fold for all the (crop species × mycorrhizal infection) treatments over soil solution P values ranging from 0.02 to 5.46 mg P L-1 (0.6–176 μM). Ss values decreased with time and reached a steady state after 35 d of equilibration period. Both Sp/Sf and Ss/Sf increased with applied P and there is a 1:1 correspondence between Sp/Sf and Ss/Sf values. The identity between the isotopic composition of both P in soil solution and in plant indicates that the isotopically exchangeable P (E=F(Sf/Ss−1)) is the only source of phosphate in solid soil phase which replenishes P of the soil solution after P has been removed by the plant, i.e. the only source of P which participates in plant nutrition. The isotopically exchangeable P of a loamy soil is the P available to growing plants and mycorrhizal fungi increases the P uptake giving plants wider access to isotopically exchangeable P in soil, and not making previously non-exchangeable P available. An immediate application of the 1:1 correspondence between a soil parameter (Ss/Sf) and a plant parameter (Sp/Sf) concerns the agronomic evaluation of P fertilizers.
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Abbreviations
- R:
-
amount of 32P associated with the amount of 31P (F) in fertilizer
- Rp:
-
amount of 32P taken up by plant
- Rs:
-
amount of 32P in soil solution
- Qf:
-
amount of 31P taken up by plant from the fertilizer (Qf=FRp/R)
- Qs:
-
amount of 31P taken up by plant from the soil
- Qt:
-
total amount of 31P taken up by plant (Qt=Qs+Qf)
- Wf:
-
amount of 31P in soil solution derived from the fertilizer (Wf=FRs/R)
- Ws:
-
amount of 31P in soil solution originated from the soil
- Wt:
-
total amount of 31P in soil solution (Wt=Ws+Wf)
- Sf:
-
specific activity of P in the applied fertilizer (Sf=R/F)
- Ss:
-
specific activity of P in the soil solution (Ss=Rs/Wt)
- Sp:
-
specific activity of P taken up by the plant (Sp=Rp/Qt)
- A value:
-
FQs/Qf
- L value:
-
F(Sf/Sp−1)
- E value:
-
F(Sf/Ss-1)
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Morel, C., Plenchette, C. Is the isotopically exchangeable phosphate of a loamy soil the plant-available P?. Plant Soil 158, 287–297 (1994). https://doi.org/10.1007/BF00009502
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DOI: https://doi.org/10.1007/BF00009502