Carbon and nitrogen in operationally defined soil organic matter pools

Humic substances [humic acid (HA), fulvic acid (FA), and insoluble humin], particulate organic matter (POM), and glomalin comprise the majority (ca 75%) of operationally defined extractable soil organic matter (SOM). The purpose of this work was to compare amounts of carbon (C) and nitrogen (N) in HA, FA, POM, and glomalin pools in six undisturbed soils. POM, glomalin, HA, and FA in POM, and glomalin, HA, and FA in POM-free soil were extracted in the following sequence: (1) POM fraction separation from the soil, (2) glomalin extraction from the POM fraction and POM-free soil, and (3) co-extraction of HA and FA from the POM fraction and POM-free soil. Only trace amounts of HA and FA were present in the POM fraction, while POM-associated glomalin (POM-glomalin) and POM alone contributed 2 and 12%, respectively, of the total C in the soil. Mean combined weights for chemically extracted pools from POM and from POM-free soil were 9.92 g glomalin, 1.12 g HA, and 0.88 g FA kg−1 soil. Total protein and C, N, and H concentrations showed that glomalin and HA were, for the most part, separate pools, although protein was detected in HA extracts. Even though percentage carbon was higher in HA than in glomalin, glomalin was a larger (almost nine times) operationally defined pool of soil organic C. Glomalin was also the largest pool of soil N of all the pools isolated, but all pools combined only contained 31% of the total N in the soil.