Submitted to: Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 18, 2005
Publication Date: December 1, 2006
Citation: Nichols, K.A., Wright, S.E. 2006. Comparision of glomalin and humic acid in eight native u.s. soils. Soil Science. 170(12):985-997. Interpretive Summary: Interventions to maintain soil productivity and sustainability depend upon understanding organic matter. In 1996 we discovered an abundant glycoprotein, glomalin, that contributes a substantial amount to organic matter and carbon in soils. Glomalin is produced by arbuscular mycorrhizal fungi which are ubiquitous, beneficial root-colonizing fungi. It was important to determine why glomalin had not been found during 50 years of research on soil organic matter fractions. Chemical co-extraction of humic acid (HA) and fulvic acid (FA) was performed, and then glomalin was extracted from the residual soil. Also, glomalin was extracted first, and then HA and FA were co-extracted from the residual soil. This study showed that the methods used to extract HA and FA are generally specific for these components, but small amounts of glomalin or a glomalin-like compound are in the HA fraction. Glomalin is a different fraction from HA and is concentrated in the soil that was discarded after co-extraction of HA and FA. These results will be used by soil scientists to refine methods to study soil organic matter, and provide further evidence that glomalin is an important in soil health.
Technical Abstract: Two important extractable fractions of soil organic matter (SOM) are humic acid (HA) and glomalin. Optimizing the purity of each fraction is necessary to correlate fraction quantity and molecular characteristics with soil quality. Manipulation of the extraction sequence and controlled precipitation of HA were tested to evaluate co-extraction of HA and glomalin. Eight bulk soil samples (0 ' 10 cm depth) from four U.S. states (Colorado, Nebraska, Maryland and Georgia) were used. In Experiment 1 glomalin extraction (50 mM citrate, pH 8.0, at 121 oC) was followed by HA extraction (0.1 N NaOH), and Experiment 2 employed the reciprocal sequence. Experiment 2 HA was precipitated stepwise at pH 2.5, 2.0 and 1.0. Gravimetric, total protein (Bradford assay) and immunoreactive protein along with weights and concentrations of carbon, hydrogen, nitrogen and iron were used to compare glomalin and HA. The HA fraction from Experiment 2 contained two-fold greater amounts of protein than HA from Experiment 1 and showed that pre-extraction of glomalin improved the purity of HA, but protein-free HA was not obtained. Protein was more concentrated in HA precipitated at pH 2.5 or 2.0, depending on the soil, but HA precipitated at pH 1.0 also contained protein. Iron concentration in the soil was significantly and positively correlated with gravimetric weight, immunoreactive protein and percentage Fe in glomalin from Experiment 1. Glomalin weight also was correlated with soil organic C and clay content. In Experiment 1 a recalcitrant pool of glomalin was released by treating soil with NaOH, suggesting that glomalin extraction efficiency could be improved.