Functional limitations of glomalin-related soil protein as an indicator of arbuscular mycorrhizal fungi while remaining relevant to soil health

This review critically examines the chemical nature of Glomalin-related soil proteins (GRSP), the biases introduced by non-mycorrhizal sources, and emphasizes the need to avoid misinterpreting GRSP as an arbuscular mycorrhizal fungi (AMF) marker. GRSP have been widely used in agricultural science as indicators of soil health and as a quantitative indicator of the accumulation of decomposed biomass of AMF in soil. GRSP is typically extracted by repeated autoclaving of soil in a citrate buffer, followed by protein quantification using the Bradford assay. However, recent discussions on the composition, structure, and interpretation of GRSP raise concerns about its specificity as an AMF marker, and we have serious doubts about the protein dominance of this mixture. A key limitation is that conventionally measured GRSP concentrations also correlate with plant litter decomposition and organic matter inputs, indicating that GRSP reflects general organic accumulation rather than AMF-specific processes. This underscores the need for more direct and representative measurements of mycorrhizal interactions. To address this issue, we propose renaming GRSP as Bradford-reactive soil compounds (BRSC), a term that more accurately reflects the chemical heterogeneity of the fraction and avoids implying any specific association with AMF. • Glomalin is widely used as a soil health and mycorrhizal fungi indicator. • The extraction method raises concerns about its specificity to mycorrhizal fungi. • Glomalin content correlates with plant litter decomposition and added organic matter. • High-temperature citrate extracts include diverse compounds, not just glomalin. • Glomalin is unsuitable for standardized measurements of mycorrhizal fungi.