Methodological bias associated with soluble protein recovery from soil

Lucy M Greenfield,Davey L Jones,Eric Paterson,Paul W Hill,Elizabeth M Baggs

doi:10.1038/s41598-018-29559-4

Lucy M Greenfield, Davey L Jones + Show 3 more

Open Access

https://doi.org/10.1038/s41598-018-29559-4

Copy DOI

Abstract

Proteins play a crucial role in many soil processes, however, standardised methods to extract soluble protein from soil are lacking. The aim of this study was to compare the ability of different extractants to quantify the recovery of soluble proteins from three soil types (Cambisol, Ferralsol and Histosol) with contrasting clay and organic matter contents. Known amounts of plant-derived 14C-labelled soluble proteins were incubated with soil and then extracted with solutions of contrasting pH, concentration and polarity. Protein recovery proved highly solvent and soil dependent (Histosol > Cambisol > Ferralsol) and no single extractant was capable of complete protein recovery. In comparison to deionised water (10–60% of the total protein recovered), maximal recovery was observed with NaOH (0.1 M; 61–80%) and Na-pyrophosphate (0.05 M, pH 7.0; 45–75% recovery). We conclude that the dependence of protein recovery on both extractant and soil type prevents direct comparison of studies using different recovery methods, particularly if no extraction controls are used. We present recommendations for a standard protein extraction protocol.

Highlights

Protein represents the dominant form of organic nitrogen (N) entering soil ecosystems and frequently the bottleneck in soil N cycling[1]
Previous studies have examined a range of protein extraction methods, these have been largely restricted to single unrepresentative proteins (e.g. BSA), single soils or have used quantification methods known to suffer from severe interference by the co-extraction of humic substances[4,5,6]
Protein recovery by deionised water varied from 10–60% between soil types

Summary

Introduction

Protein represents the dominant form of organic nitrogen (N) entering soil ecosystems and frequently the bottleneck in soil N cycling[1]. Extractants that have commonly been used for soil protein recovery include simple salts (e.g. K2SO4, Na-pyrophosphate, Na-phosphate), bases (e.g. NaOH), organic acids (e.g. Na-citrate) and surfactants (e.g. Tris-SDS) (Supplementary Table S1). Soil type has a large influence on protein recovery. Some studies suggest that organic matter and clay content are the key soil properties which affect protein recovery[5,7,8] whilst other studies suggest soil pH is important[9,10,11]. Clay content and pH influence the adsorption of protein in soil and, affects the ease to which it can be extracted. Our aim was to focus on soluble proteins and to compare the recovery of a mixture of 14C-labelled plant proteins from soil using 39 different extractants. Our secondary aim was to evaluate the influence of soil type on protein recovery

Objectives

Methods

Results

Conclusion