Bradford quantification of Glomalin-Related Soil Protein in coloured extracts of forest soils

• Published in: Geoderma Vol. 372; p. 114394
• Main Authors: Cissé, G., Essi, M., Nicolas, M., Staunton, S.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.08.2020; Elsevier
• Subjects: absorbance; Agricultural sciences; color; colorimetry; Coloured extracts; Earth Sciences; forest soils; Forest soils, Autoclaved-citrate extractable protein; forests; Geochemistry; Humic substances; Life Sciences; Quantification of GRSP; Sciences of the Universe; Silviculture, forestry; soil organic matter; Soil study; topsoil; Forest soils, Autoclaved-citrate extractable protein; Coloured extracts; Quantification of GRSP; Humic substances; Autoclaved-citrate extractable protein; Forest soils
• ISSN: 0016-7061; 1872-6259
• DOI: 10.1016/j.geoderma.2020.114394

•Bradford assay to quantify soil proteins in autoclaved-citrate extracts is subject to interference.•Analysis of GRSP from organic-rich, highly coloured, forest soils is particularly challenging.•Sample dilution and subtraction of pH-dependent colour improve protein determination. Glomalin-related soil protein (GRSP) is thought to represent a fraction of recalcitrant organic matter in soil. But it is recognized that the autoclaved-citrate extraction procedure causes the co-extraction of humic substances which interfere (directly and indirectly) with the Bradford colorimetric assay. The aim of this work was to propose a reliable quantification method of GRSP from forest soil, very rich in organic matter and therefore in colour. We estimated the quantities of GRSP in the topsoil (0–10 cm) of 102 French forests using five methods: i) direct calibration, reasoned dilution with colour correction, ii) direct calibration, reasoned dilution but no colour correction, iii) direct calibration, 1:2 dilution, no colour correction, iv) indirect calibration and v) dilution method. Our results concur that the interference caused by the co-extracted compounds is not related simply to either the colour of the extracts or total soil organic matter content. These findings suggest that for improved accuracy of GRSP estimates using the Bradford method, extracts should be diluted, and the pH-specific absorbance of coloured extracts should be subtracted.