LAND-USE EFFECTS ON THE COMPOSITION OF ORGANIC-MATTER IN PARTICLE-SIZE SEPARATES OF SOILS .2. CPMAS AND SOLUTION C-13 NMR ANALYSIS

Soils from A horizons of Eutrochrepts under spruce forest (Sf), mixed deciduous forest (Df), permanent grassland (Gp), and arable rotation (Ar) were fractionated into clay- (<2 mu m), silt- (2-20 mu m) and sand- (20-2000 mu m) sized separates. C-13 NMR spectroscopy was used to compare SOM composition across size separates and between land-use regimes. CPMAS C-13 NMR spectroscopy showed that the intensity of signals assigned to carbohydrates (representing most O-alkyl C) and lignin (phenolic and methoxyl C) declined with decreasing particle size. Concurrently, alkyl C and C-substitution of aromatic C increased in the order sand, silt, clay. The amount of alkyl C correlated well with microbial resynthesis of carbohydrates. Solution C-13 NMR spectra suggested that humic acids (HA) extracted from the size separates were richer in carboxyl C and aromatic C than the bulk size separates. Also HA reflected increasing percentage of alkyl C with decreasing particle size. O-alkyl C were lower in silt HA than in clay HA whereas aromatic C tended to peak in silt HA. These results suggested that sand-sized separates were enriched in plant residues (primary resources) whereas clay-sized separates were dominated by products of microbial resynthesis (secondary resources). Silt was rich in selectively preserved and microbially transformed primary resources. C-13 NMR spectroscopy showed only small differences in SOM composition between land-use regimes, except that silt and silt HA from Ar were richer in aromatic C than those from the other plots. But enrichment factors (E = content in fraction/content in whole soil) revealed differences in the distribution of C species across the size separates. Relatively high E(aromatic) (0.9) and E(O-alkyl) (1.0) for sand from Gp indicated high amounts of plant residues, probably due to intense rhizodeposition and to occlusion of plant debris within aggregates. Low E(aromatic) (0.3) and E(O-alkyl) (0.3) for sand from Ar suggested depletion of primary resources, which could be attributed to disintegration of soil aggregates upon cultivation. A pronounced enrichment of alkyl C in Ar clay-sized separates (E(alkyl) = 3.1) suggested large amounts of microbial carbon. Microbial products attached to clay surfaces by a variety of physico-chemical bondings appeared more stable against mineralization induced by cultivation than plant residues sequestered in aggregates.