Shen, Q., Hedley, M., Camps Arbestain, M., Kirschbaum, M.U.F. (2017). Tephra is an effective P diffusion barrier in root exclusion experiments. Plant and Soil 410: 51–61.


Background and Aims

To understand the bioavailability of phosphorus (P) under different conditions, it is important to consider the P uptake by plant roots and arbuscular mycorrhizal fungi (AMF) hyphae separately. This can be done with root study containers that separate soil into root– and hyphal–zone by a layer of nylon 30 µm mesh, which impedes root access to parts of the soil while enabling exploration by AMF (termed the hyphal zone). However, diffusion of P from the hyphal zone to the root zone can complicate the analysis of such experiments when a concentration gradient is created by a P–rich patch in the hyphal zone. This study tested whether a layer of tephra with high P sorption characteristics can be arranged to prevent P diffusion between the root zone and the hyphal zone.


A study of P sorption on tephra was conducted to determine the thickness of the tephra needed. Once this was established, the effectiveness of the “P diffusion break” in the modified root study container was tested. For this, Lotus pedunculatus cv. barsille was grown in the container having a low–P soil (Olsen P 4.3 mg kg-1) in the root zone and either low– or high–P soil (Olsen P 33.3 mg kg-1) in the hyphal zone.


A 3–mm thick layer of tephra was shown to be adequate to prevent P diffusion for one year from the high–P soil to the low–P soil, as assessed by the absence of P accumulation in resin–P (greater than 5 mg kg-1) and total P in the tephra layer. Plant P uptake was increased by 1.9 mg pot-1 when AMF could access the high–P soil, resulting in an increase in plant yield by 56 %.


Given that (i) P diffusion across the tephra layer was prevented; (ii) roots were confined to the root zone; and (iii) AMF hyphae were detected in the tephra layer, the additional P uptake must have been through AMF acquisition. This experimental design can then be applied to study the influence of soil amendments (e.g. fertilizers, lime, biochar) either containing P or affecting P availability in soil to cause an increasing P transfer by AMF hyphae.

Keywords: Arbuscular mycorrhizal fungi; Langmuir adsorption – isotherm; Lotus pedunculatus cv barsille; P diffusion; Root study container; Tephra

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