0-20 mu m aggregate typology based on the nature of aggregative organic materials in a cultivated silty topsoil

Soil structure is a key characteristic of soil functioning. More, silt and clay-sized organo-mineral fractions are usually considered as the most reactive soil fractions. As transmission electronic microscopy (TEM) appeared to be well-suited to study soil microstructures, it was used to test the sensitivity of micro-aggregates to land use and cultivation practices. The aims of this study were therefore (1) to use TEM to characterize the organic materials involved in the stable 0-20 mu m micro-aggregates of a cultivated topsoil and (2) to highlight any impact of cultivation practices and of inter-annual variation on micro-aggregation. For this, the study was conducted on a Calcic Cambisol (pH of 7.1, organic matter content of 33 g kg(-1), C-to-N ratio around 10), developed in Eastern France and sampled two consecutive years before and after the exceptional dry period occurring in 2003. It was cropped under maize following moderate tillage and fertilisation within an experimental station. Digested sewage sludge has been applied since 1996 according to regulations for environmental controls at the rate of 10 Mg ha(-1) year for 4 years. Granulometric soil fractionations of samples from plots with or without sewage sludge addition were performed. Then, morphological and analytical characterization by TEM of the water-stable <20 mu m micro-aggregates were carried out to specify their aggregative organic matter. 0-20 mu m water-stable fraction contained more than 60% of the soil carbon. A typology of <20 mu m micro-aggregates was established according the nature of aggregative organic matter. It appeared that among 0-2 mu m and 2-20 mu m micro-aggregates, those containing young organic matter of plant and microbial origin were involved in the most stable associations. The impact of the dry period on soil microstructure between 2002 and 2003 corresponded to both an increase in the 0-2 mu m water-dispersibility and a decrease in the mean size of the 2-20 mu m water-stable aggregates. Sewage sludge addition slightly increased the proportion of the 50-200 mu m water-stable macro-aggregates and induced the formation of micro-aggregates containing sludge flocs, which could be considered as specific sewage sludge fingerprints. Descriptors of 0-20 mu m stable micro-aggregates, i.e. size, stability and typology, appeared as a promising tool to specify the dynamics of the organo-mineral associations in soils subjected to environmental changes or cultivation practices. (C) 2011 Elsevier Ltd. All rights reserved.