Origin and fate of organic matter in sandy soils along a primary vegetation succession

Until now little is known about the role vegetation plays in the organic matter formation, particularly at the molecular level. Most ecosystems have a long history, which is unknown or too complex to find distinct relations between vegetation and the chemical composition of soil organic matter. To gain a better insight in such relationships, the relative simple soil-vegetation system of the Hulshorster Sands and Leuvenum Forest is used. The complete vegetation succession has been investigated, making it possible to monitor the organic matter development. During the succession, thus with time, the pH decreases, the ectorganic layer becomes thicker and the soil develops towards a micro-podzol.

Chapter 2 describes the organic matter composition and relation to the first stages of vegetation development in the pre-podzol phase. In the Ah horizon under grass ( Corynephorus canescens ), the organic matter has virtually no contribution of the covering vegetation. Algae, from an earlier stage, and mosses ( Polytrichum piliferum and Bryum sp.), from adjacent spots, are the main suppliers. Under mosses, the organic matter is very similar to that of the original species. An aliphatic biopolymer, derived from mosses, that produces a series of branched alkene/alkane doublets (C ₂₀ -C ₃₂ ) together with a homologous series of n -alkenes and n -alkanes (C ₁₀ up to C ₃₄ ) upon pyrolysis, appears to be very recalcitrant in the first stages of succession.

In the vegetation stages following mosses, these aliphatic macromolecules are still an important source of the aliphatic part of organic matter. Apart from the aliphatic moss contribution, the organic matter under Callunavulgaris and Pinus sylvestris has a close relationship with the contributing plant parts: flowers, leaves and branches dominate the organic layer under Calluna , needles predominate the ectorganic horizon under pines, and the soil organic matter in the Ah horizons in both cases is almost completely determined by roots.

In Chapter 3 the water soluble organic matter (WSOM) of the Ah horizons is compared with the bulk horizons. The WSOM contains little lignin, also when this compound was present in significant amounts in the Ah horizons. In contrast, the moss-derived aliphatics are prominently present in the WSOM. Their presence in water is probably due to the formation of micelle-like arrangements in which the hydrophobic parts are inside a hydrophilic mantle resulting into 'soluble' aliphatics.

Chapter 4 deals with the fractionation of three H horizons under different vegetation in the forest with incipient podzolization: a water soluble fraction, a NaOH soluble fraction and the residue, the insoluble fraction (humin). The water soluble fractions are very similar to each other with regard to their chemical composition, and so are the NaOH soluble factions. The insoluble fractions differ in particular in their aliphatic part. During beech forest development, an increasing amount of suberin, mainly derived from beech roots, is observed. The bulk of the organic matter in the H horizons, however, appears to have a pine origin as concluded from the chemical data. Pollen analysis supports that beech litter did not contribute to the H horizons.

Chapter 5 addresses the analysis (composition of polysaccharides, NMR, pyrolysis and thermochemolysis) and the unraveling of the origin of the ectorganic layers in the succession from pine to beech forest. Under pine, the bulk organic matter shows only little effect of the ground vegetation Deschampsia and Empetrum respectively. The chemical composition of the L and F1 horizons are very similar to each other, but towards the H horizons an increase of aliphatics, a decrease of polysaccharides and a more or less constant fraction of aromatic compounds is noticed. The lignin composition is dominated by guaiacol building blocks, and hardly any angiosperm-lignin is present in the ectorganic layers implying no significant contributions of the undergrowth. This contrasts with macromorphological analyses, because abundant above- and below-ground parts of Deschampsia and Empetrum are present in the organic horizons suggesting an important input of the understorey. The composition of the aliphatic fraction also shows an effect of the undergrowth.

The distribution of cutin and suberin building blocks as measured upon thermochemolysis reveals the clear impact of Deschampsia and Empetrum in addition to pine, especially in the F1 and F2 horizons. The transition towards the beech forest is characterized by a more distinct influence of beech litter. From L to H horizons polysaccharide contents decrease clearly, aromatics remain constant and the amounts of aliphatic compounds increase relatively.

Furthermore, with depth a clear decrease of syringyl lignin compared with guaiacyl lignin is observed. The latter is mainly due to a decrease of beech litter towards the H horizon: the L consists only of beech litter, the F is also dominated by beech litter in addition to beech roots, and the H horizon is virtually only derived from pine litter. Nevertheless, the composition of the H horizons is affected by beech through their roots, which is clearly marked by the presence of beech-suberin, and the contents of that biopolymer increases from younger to older beech stands.

In Chapter 6 the WSOM fractions from L, F, and H horizons are compared on their chemical composition with those of the bulk of B horizons and fibers of incipient podzols. The WSOM shows hardly any resemblance with the B horizons, whereas it appears to be very similar to the fibers. This strongly suggests that in the initial phases of podzolization almost all WSOM moves through the B horizons and accumulates when the water movement stops resulting into the formation of humus fibers. In Chapter 7 it is elucidated that the organic matter in the B horizons is mainly composed of root-derived material, whereas hardly any organic matter is complexed with metals or clay. The organic matter in B horizons is highly aliphatic, consisting partly of suberan-like polymers and partly of suberin, whereas lignin and polysaccharides are virtually absent. In Chapter 8, the chemical implications of the negligible organic matter sources of both illuviation as mixing of above-ground litter is described. The clear chemical evidence of root-derived organic matter could be established.