Seasonal Shifts in the Biochemical Compoments of the Light Fraction, Particulate Organic Matter, and Two Humic Acid Fractions in An Iowa Soil.

Soil organic matter is conventionally extracted through either physical or chemical means.  The benefits of integrating both approaches into one fractionation procedure were evaluated on an Iowa corn-soybean soil used for a cover crop study.  The light fraction was first extracted from the 0-5 cm soil depth, followed by two fractions of particulate organic matter (>500 µm and >53 µm) and finally two humic fractions that were distinguished based on whether they were bound to soil Ca.  Whole soil organic matter was obtained separately by dissolution of the soil mineral phase through HF washes. Several of the fractions decreased in quantity between early season and late season samplings, and their quantities responded in opposite manners to the cover crop.  The chemical natures of the fractions and of whole soil organic matter will be compared at each sampling time to that of the original soil by chromatographic analysis for carbohydrates, amino acids, amino sugars, and phenols.  These chromatographic results will be compared to previous analyses of these samples by ¹³C nuclear magnetic resonance (NMR) spectroscopy, which found the light fraction and both particulate organic matter fractions had stronger signals for aliphatic compounds while the two humic fractions had stronger signals for aromatic rings and COOH.  The NMR signals of whole soil organic matter reflected contributions from both groups of fractions.