Co-occurring increased phosphatase activity and labile P depletion in the rhizosphere of Lupinus angustifolius assessed with a novel, combined 2D-imaging approach

Christina Hummel,Gustavo Boitt,Jakob Santner,Niklas J Lehto,Leo Condron,Walter W Wenzel

doi:10.1016/j.soilbio.2020.107963

Abstract

Innovative non-destructive 2D imaging methods such as zymography and diffusive gradients in thin films (DGT) have been developed recently to assess the distribution of phosphatase activity and labile solutes at the root-soil interface. We report on the first combination of these techniques for the spatial distribution and potential interaction of labile phosphorus (P) and associated acid phosphatase activity in the rhizosphere of blue lupin (Lupinus angustifolius) in two contrasting soils (‘Sand’ and ‘Loam’). Zymography, based on 4-methylumbelliferylphosphate (MU-P) for acid phosphatase activity mapping, and DGT gels capable of binding labile P were deployed to individual root axes to visualise P mobilisation and depletion in the rhizosphere of blue lupin grown in rhizotrons in glasshouse conditions for 45 days. Acid phosphatase activity was evidently higher in the rhizosphere and co-occurred with P-depletion zones around the roots in both soils. Lateral root profiles showed that elevated acid phosphatase activity as well as P-depletion extended up to 2 mm from the root centre into the rhizosphere. Despite larger total and organic P pools in the Loam, P was less plant available (DGT labile P) than in the Sand. However, phosphatase activities in the rhizosphere and plant P contents were similar in both soils. These results indicate that enzyme-catalysed hydrolysis of organic P was limited in the Loam, due to low P availability, hence explaining similar P contents in lupins from both soils. P did not accumulate in labile pools in the rhizosphere indicating that P supply via phosphatase could not compensate the plants demand for P resulting in P depletion zones. While our work demonstrates the applicability of combined zymography and DGT deployments to study rhizosphere processes at microscale, we also discuss limitations and perspectives of this approach.

Highlights

Worldwide, inorganic forms of phosphorus represent only about 57% of total soil P, while the rest is mainly present in organic P forms, especially as phosphate monoesters which account for 33% of total soil P (Menezes-Blackburn et al, 2018)
The sandy soil, was collected from a coastal dune near Haast, New Zealand and was slightly more acidic than the other soil. It developed on unconsolidated sand, had no horizons and is clas sified as Eutric Arenosol according to the World Reference Base for Soil Resources (WRB)
Irrespective of P pools and organic matter content of the soils investigated, acid phosphatase ac tivity was similar in both rhizospheres, higher in the rhizo sphere as compared to bulk soil, and co-localised with P-depletion zones around the roots

Summary

Introduction

Inorganic forms of phosphorus represent only about 57% of total soil P, while the rest is mainly present in organic P forms, especially as phosphate monoesters which account for 33% of total soil P (Menezes-Blackburn et al, 2018). Mineralisation of soil organic P to plant-available phosphate is an important source of P for plants (George et al, 2018; Menezes-Blackburn et al, 2018; Nash et al, 2014; Shen et al, 2011). Understanding the func tionality of phosphatase enzymes and how they are linked to organic P dynamics and P bioavailability is crucial to improve our understanding of how plant-soil-microbial interactions affect plant nutrition (George et al, 2018). Zymography is a relatively new technique to visualise the spatial distribution of potentially active enzymes in soil with 2D images (Spohn et al, 2013). It is based on the visualisation of substrates that become

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