Recovery of fresh debris of different sizes in density fractions of two contrasting soils

Fresh plant residues are often identified with the light organic matter obt ained by fractionation in a dense liquid, but the extent to which residues can be extracted efficiently by densimetric methods has not been widely stu died. This paper presents the results of two experiments in which C-14-labe lled straw was mixed with two soils of differing texture, and the mixture w as subjected to densimetric fractionation. In the first experiment, soil wa s mixed with bulk milled straw, and in the second, with straw of different sizes (> 200 mu m, 200-50 mu m, 50-20 mu m and < 20 mu m). The recovery of straw in the light fraction was low, and decreased with decreasing straw si ze, reaching a minimum (less than 20 %) for straw < 20 mu m. Except for str aw < 20 mu m, the recovery in the light fraction was lower in soil richest in silt and clay. In the absence of soil (blanks), the recovery of fine str aw in the light fraction was lower than in the presence of soil, suggesting that the recovery of fine straw in the light fraction is partly due to its association with light coarse debris. Extractable and polytungstate-solubl e fractions accounted for a small proportion of the C-14-activity. These re sults suggest that densimetric methods are not efficient for recovering fre sh plant residues, except in the case of large residues in coarse-textured soils. (C) Elsevier, Paris.