Density fractionation of soil macroorganic matter using silica suspensions

Abstract

A new density fractiorlation method for soil macroorganic matter is described which uses a stable silica suspension (LudoxTM) as heavy liquid. The method has several advantages over other density methods: it is fast and cheap, the silica suspension is not toxic and does not alter the characteristics of the organic matter fractions and large soil samples (500 g) can be fractionated. One method used to investigate soil macroorganic matter in more detail involves densiometric techniques. The concept behind density fractionation assumes that soil organic matter can be divided into pools differing in structure and function. The so-called light fraction consists of mineral-free, partly decomposed plant debris. It has a relatively high C-to-N ratio, a rapid turnover and a specific density considerably lower than that of soil minerals (Greelnland and Ford, 1964; Sollins e/ cl., 1984). The heavy fraction includes the organomineral complexed soil organic matter. It has a narrow C-to-N ratio, a slower turnover rate and a higher specific density than the light fraction due to its intimate association with soil minerals (Greenland and Ford, 1964). Originally, organic liquids such as tetrabromoethane, bromoform and tetrachloroethane were used for density fractionation (Monnier ef AI.. 1962; Greenland and Ford, 1964; Turchenek and Oades, 1979). One of the drawbacks of these liquids is the high potential toxicity of halogenated hydrocarbons. During the last decade, aqueous solutions of inorganic salts such as sodium iodide (Sollins et c/l.. 1984) and sodium-polytungstate (Cambardella and Elliott, 1992) have become more popular. Because inorganic salts are expensive and toxic to the environment, only small amounts (about IO g) of dried soil samples are commonly used in density fractionation studies (e.e. Cambardelta and Elliott. 1992: Janzen el (I/.. 1992). The p&longed period needed to equilibrate the suspended material with the salt (‘Cambardella and Elliott, 1992; Janzen VI (I/., 1992) and the decrease in mineralization after introducing organic rmaterial into the salt solution (Sollins et cl., 1984) are other drawbacks