Ultrastructural study of the biogeochemical cycle of silicon in the soil and litter of a temperate forest

Abstract

SummaryThe biogeochemical cycle of silicon is closely linked to the plant cycle. We have studied the relations between silica and organic matter during humification in a temperate deciduous forest, an acid brown soil ecosystem, the characteristics of which suggest that plants play a major role in the recycling of soluble silica. Our ultrastructural approach consists in identifying the occurrence of biogenic silica in the beech roots, in young and old beech leaves in litter and in the underlying microaggregates of the topsoil. The microaggregates were either sampled directly in situ or isolated by size fractionation.Observations of ultrathin sections by transmission electron microscopy at a micrometric scale were combined with electron energy loss microanalysis, which enabled us to identify the silica phases and their associated mineral and organic environments. Some silica deposits were identified in the cell walls of both the beech roots and the beech leaves, whereas pure silica occurred in the root cortical cells as a layer which was closely associated with polyphenolic substances. The ultrastructural study of the microaggregates showed a close association between Si‐cell wall residues and Si‐polyphenolic residues. We also identified a specific bacterial activity that seemed to be related to the degradation of siliceous cell walls. The microquartz in the topsoil showed morphological and analytical transformations that might be considered as weathering. Both ultrastructural observations and the electron energy loss microanalysis showed the presence of biogenic silica and silica‐organic material in litter and soil, and these techniques should enable us to follow the biogeochemical cycle in ecosystems and the bioavailability of silicon.