Organic Matter Characteristics of Soil Aggregate Fractions Under Different Ecosystems.

ABSTRACT  The interaction of organic matter with mineral constituents influences how soil organic carbon is stabilized physio-chemically, and it may be influenced by specific ecosystems. Organic matter characteristics in different soil aggregate fractions likely reflect these effects. To elucidate these influences, surface soil samples (0-15cm) were collected from two sites which were under an agroforestry study in the Southeast U.S.: Colt (AR) and Archibald (LA). The Colt site was used in row crop production, and Archibald was used in pasture for up to 5 years. Three agroforestry treatments were established at each site: rotation of sorghum and soybean, switchgrass, and cottonwood. The collected soil samples were fractionated in 3 aggregate sizes: <53µm, 53-250µm, and 250-2000µm. Each fraction was characterized for soil organic matter characteristics. ATR-FTIR analysis showed that the <53µm and 250-2000µm fractions had stronger vibrational signal of phenolic compound (wavenumber 1360~1420 cm-1) than the 53-250µm fraction for soils of all ecosystems. Among different ecosystems, crop rotation generally accumulated more phenolic compounds than the other two ecosystems. However, switchgrass and cottonwood soils at the Archibald site exhibited relatively higher peak intensity of aromatic C=C band at wavenumber 1560~1600cm^-1 than that of phenolic OH band, whereas the opposite was shown at the Colt site. The latter could be due to a combined effect of igneous soil difference and land use history. Overall, the results indicated both ecosystems and aggregate fractions influence the chemistry of soil organic matter characteristics.