Interpretive Summary: Interventions to maintain soil productivity and sustainability depend upon understanding contributions to soil organic matter. In 1996 we discovered a glycoprotein, glomalin, in soils. Glomalin is produced by a ubiquitous root-colonizing group of fungi ' the arbuscular mycorrhizal fungi. The purpose of the current work was to define carbon inputs to soils from extractable fractions of soil organic matter. Conventional methods were used to extract particulate organic matter (POM), glomalin, and humic acid (HA) and fulvic acid (FA). POM and glomalin contributed the greatest amounts to soil organic matter. We found that glomalin contributed almost seven times more carbon to soil than HA. This information is of great interest to soil scientists and ecologists because HA was previously thought to be a major contributor to soil carbon. These results will be used by scientists interested in soil carbon.

Technical Abstract:
Humic substances [humic acid (HA), fulvic acid (FA) and insoluble humin], particulate organic matter (POM) and glomalin comprise the majority of extractable soil organic matter (SOM). The purpose of this work was to define carbon (C) inputs to soils from each fraction. POM was extracted and examined separately to contrast amounts of C associated with POM and extractable fractions of POM vs. POM-free soil. Bulk soil samples (0 ' 10 cm depth) from six undisturbed soils ' two each from Colorado (CO), Maryland (MD) and Georgia (GA) ' were analyzed. The following extraction sequence was performed to maximize the purity of each individual fraction: (1) POM extraction with 12% NaCl , (2) glomalin extraction with 50 mM citrate at 121oC, and (3) co-extraction of HA and FA in 0.1N NaOH. Glomalin, HA and FA were similarly extracted from the POM fraction. POM was separated into glomalin and residual POM. POM-glomalin contributed only 3% to the C in the fractions examined, while residual POM contributed 20% of the C in fractions examined across all soils. Mean combined values for chemically extracted fractions from POM and from POM-free soil were: 8.14 g glomalin, 1.22 g HA, and 0.55 g FA kg-1 soil. Percentage carbon was greater in HA than in glomalin, but glomalin contributed almost seven times more carbon to soil than HA. Total protein and carbon, nitrogen and hydrogen concentrations showed that glomalin and HA were, for the most part, isolated fractions, but protein was detected in HA extracts.