Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 23, 2006
Publication Date: September 6, 2006
Citation: Halvorson, J.J., Gonzalez, J.M. 2006. Bradford reactive soil protein in Appalachian soils: distribution and response to incubation, extraction reagent and tannins. Plant and Soil Journal, 286:339-356. Interpretive Summary: Glomalin, thought to be an important soil protein that acts as glue in soils and represents an important portion of stable soil organic matter, is determined by the Bradford method. However, more information is needed about its importance in Appalachian soils and its relationships with other soil properties. We measured glomalin in soil from pastures, hayfields, cultivated fields and forest areas in southern West Virginia. Highest amounts of glomalin were found near the soil surface, decreasing with depth for all land uses except cultivated sites. Forest and pasture sites contained more glomalin than hayfields, or cultivated fields but differences were seen only in the 0-5 cm depth. Overall averages of carbon (C) and nitrogen (N) in glomalin represented about 4.0 and 6.5% of the total soil C and N, respectively. After an incubation we found only small changes in glomalin (less than 10%). Extraction of glomalin appears to be affected by the electrical charge of the chemical solution used as extractant and buffering capacity of both the extractant and soil. Extractants with low electrical charge have little buffering capacity and extract little glomalin. Also, apparent recovery of glomalin increased after treatment with tannins, common plant compounds, but less soluble-N was recovered from tannin-treated samples than from untreated controls. Formation of dark-colored substances during extraction was observed, suggesting that the Bradford method may overestimate soil protein when tannins are present. Recovery of less soluble -N from soil extracts suggests tannins may combine with soil constituents themselves or form non-extractable N-containing compounds. Our data indicate that glomalin is a resistant fraction of soil organic matter in Appalachian soils that will be found in relative abundance at the soil surface and thus likely affected by silvopastoral management. More information is now needed to determine the interactions between plant-derived compounds like tannin and glomalin in soil and its role as a reservoir of nutrients and soil organic matter.
Technical Abstract: Total protein, determined with the Bradford assay, is thought to correlate to glomalin, an important soil glycoprotein that promotes soil aggregate formation and may represent a significant pool of stable soil organic matter. However, more information is needed about its importance in Appalachian soils and its relationships with other soil properties. We measured Bradford reactive protein (BRP) in 0-20 cm soil from pastures, hayfields, cultivated fields or forest areas in southern West Virginia. Highest amounts of BRP were found near the soil surface, and decreased significantly with depth for all land uses excerpt cultivated sites. Forest and pasture sites contained more BRP than hayfields, or cultivated fields but these differences occurred only in the 0-5 cm depth. Overall averages of C and N in glomalin represented about 4.0 and 6.5% of the total soil C and N respectively. After a 395-day incubation, we found CO2-C evolution rates comparable to other studies but only small changes in BRP (<10%) including some evidence for increases in BRP during the incubation. Sodium citrate, sodium pyrophosphate, and sodium oxalate recovered significantly more BRP from soil than the other extractants we tested with highest extraction efficiencies observed for sodium citrate and pyrophosphate. Recovery of BRP appears related to negative charge and buffering capacity of both the soil and extractant. Extractants with low negative charge have little buffering capacity and yield little BRP. Apparent recovery of BRP increased after treatment with tannic acid, but less soluble-N was recovered from tannin-treated samples than from untreated controls and E4/E6, the ratio of absorbance at 465 and 665 nm, decreased, evidence for the formation of larger or heavier molecules. Formation of dark-colored substances during extraction suggests the colorimetric Bradford assay may overestimate soil protein when tannins are present. Recovery of less soluble -N from soil extracts and lower E4/E6 ratios suggests tannins may bind with soil constituents themselves or form non-extractable N-containing complexes.