Role of allophanes in the accumulation of glomalin-related soil protein in tropical soils (Martinique, French West Indies)

Thermo-stable, operationally defined soil protein, known as glomalin, may make an important contribution to carbon storage in soils. The term glomalin is used because this putative protein, or group of proteins, was originally thought to be produced only by Glomus fungi. There is currently little information on the glomalin-related soil protein (GRSP) content of tropical soils, particularly allophanic soils that are known to have different carbon dynamics to temperate climate soils. We have measured the Bradford-reactive GRSP content of soils sampled from forests and grasslands on the tropical island of Martinique and compared the observations with soil composition. Two operationally defined fractions of GRSP were measured, namely easily-extractable and total GRSP. The contents of GRSP in moist soils were in the range of 2-36 g kg(-1), accounting for about 8% of soil organic carbon, and were greater in topsoils than in corresponding subsoils. Both the GRSP contents and the fraction of soil organic carbon attributed to GRSP were greater than those reported for temperate climate soils. Both total and easily extractable GRSP contents were positively correlated to soil organic carbon content. The fraction of soil organic carbon that could be attributed to soil protein decreased with increasing allophane content for allophanic soils. No other trends of GRSP content with soil properties or land use were found. GRSP extraction was decreased about seven-fold by air-drying of soils, confirming the irreversible change in the soil microstructure of allophanic soils. Total and easily extractable GRSP were correlated and we conclude that both are good probes of thermo-stable soil protein content for these soils. No attempt was made to verify the fungal origin of the protein detected.