LAND-USE EFFECTS ON THE COMPOSITION OF ORGANIC-MATTER IN PARTICLE-SIZE SEPARATES OF SOIL .1. LIGNIN AND CARBOHYDRATE SIGNATURE

Soil from Eutrochrept A horizons under long-term spruce forest (Sf), m ixed deciduous forest (Df), permanent grassland (Gp) and arable rotati on (Ar) was fractionated according to particle size and analysed for c ontents of C, N, lignin-derived phenols and carbohydrates. Whole soil from Sf, Df, Gp and Ar contained 84, 59, 73 and 25 g C kg(-1) soil, re spectively. For all sites, the C content declined and C/N ratio increa sed in the order: clay (<2 mu m), silt (2-20 mu m), sand (20-2000 mu m ). Clay and silt were significantly lower in C in Ar than in Sf, Df an d Gp, C associated with sand being substantially lower under arable ro tation. The yield of lignin-derived phenols decreased and carboxyl fun ctionality and methoxyl demethylation of lignin derivatives increased with decreasing particle size, indicating a progressive lignin alterat ion. Whole soil from Sf and Gp was substantially higher in vanillyl (V ), syringyl (S) and cinnamyl (C) units (VSC) than soil from Df and Ar. Compared to whole soil, clay was depleted and sand enriched in VSC. O nly sand appeared to be affected significantly by land use. Sand from Ar and Df was more enriched in VSC than sand from Gp and Sf. Whole soi l carbohydrates decreased in the order: Gp>Ar>Df>Sf. Sand- and clay-si zed separates were enriched in carbohydrates compared to silt. Carbohy drates in sand were mainly of plant origin whereas microbially-derived sugars accounted for a larger proportion in the clay. Compared to Sf, Df and Gp, clay from Ar was enriched and sand depleted in microbial s ugars. Lignin and carbohydrate distribution patterns indicate that org anic matter was in a more advanced stage of decomposition in the sand separates from forest than from agricultural A horizons. The forest so ils also show a higher degree of oxidative changes in lignin associate d with clay. In contrast, differences between silt from the four A hor izons were small.