STUDY OF FREE AND OCCLUDED PARTICULATE ORGANIC-MATTER IN SOILS BY SOLID-STATE C-13 CP MAS NMR-SPECTROSCOPY AND SCANNING ELECTRON-MICROSCOPY/

A simple densimetric method for the separation of free and occluded pa rticulate organic materials was developed and applied to five virgin s oils. The free organic matter was isolated by suspending the soil in s odium polytungstate solution (d = 1.6 Mg m-3) and decanting the light material. The remaining soil was disaggregated by sonification for lib eration of occluded organic materials. The free light fraction consist ed of large, undecomposed or partly decomposed root and plant fragment s. This fraction comprised 0.59-4.34% of soil dry weight and accounted for 6.9-31.3% and 5.9-22.1% of total soil carbon and nitrogen respect ively. Identifiable components of the occluded fraction were small par ticles of incompletely decomposed organic residues, pollen grains, par ticles of plant tissue such as lignin coils and phytoliths. This fract ion comprised 0.69-1.81% of soil dry weight and represented 9.2-17.5% and 6.2-14.1% of the total soil carbon and nitrogen. The proportion of soil organic carbon recovered as the occluded fraction was high in so ils with high clay contents. The chemical composition of occluded and free organic materials was investigated by solid-state C-13 CP/MAS NMR spectroscopy. Despite the differences in soils, environmental conditi ons and vegetation, the organic structure of the free light fraction w as similar in four of the five soils. This fraction consisted of 55-63 % O-alkyl C, 18-25% alkyl C, 14-18% aromatic C, and 5-7% carbonyl C. I n the other soil, this fraction showed a higher proportion of alkyl C (31%) and lower O-alkyl C (46%). Most of the differences between soils were associated with organic materials contained in the occluded ligh t fraction. The differences in chemical structure between the occluded light fraction and free light fractions were similar in all examined soils. The NMR data showed that the proportion of O-alkyl C was lower and alkyl C higher in the occluded light fractions than in the free li ght fractions. The proportion of aromatic and carbonyl carbon was high er in the occluded fractions of three soils while the percentage of th ese two types of carbon remained unchanged in the two other soils. It is considered that the occluded organic matter is an old pool of carbo n that has been accreted within aggregates during decades of root grow th and it is that pool which is lost due to cultivation.