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

F. Watteau, G. Villemin, F. Bartoli, C. Schwartz, J.L. Morel

Watteau & al., 2011
Watteau & al., Soil Biology & Biochemistry 46 (2012) 103-114

Soil structure describes the arrangement of the solid parts of the soil and of the pore space located between them [1] but also describes the manner in which soil particles are aggregated. This multi-scale geometry is among others governed by parent material-dependent soil texture and aggregation processes due to the combined physical, chemical and biological interactions occurring between soil constituents [2]. These interactions are themselves functions of the environmental conditions under which the soil was formed and continues to evolve and vary according to recent management history. Water/nutrient storage and habitat for soil organisms are vital soil characteristics related to soil structure and leading to sustained biological productivity. Thus, soil structure appears to be a key characteristic of soil biofunctioning. Here, transmission electronic microscopy (TEM) was used to test the sensitivity of micro-aggregates to land use and cultivation practices. The aims of this study were therefore to use TEM to characterize the water-stable 0-20 µm micro-aggregates of a cultivated topsoil and to highlight any impact of cultivation practices and of inter-annual variation on soil structure dynamics.

Main results

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 fertilization 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-1 for 4 years. Granulometric soil fractionations involving water or sodic resin, of samples from plots with or without sewage sludge addition were performed (figure 1).

Fig.1 Watteau 2011

Figure 1: Differential size distributions in 2002 and 2003 of the topsoils of the control (Fig. 1a) and the amended soil (Fig. 1b). dW (%) = substraction of the weight percentages of water-stable fractions from sodic resin-stable fractions

Then morphological and analytical characterization by TEM of the water-stable <20 µm micro-aggregates were carried out to specify their aggregative organic matter (figure 2).

Fig.2 Watteau 2011

Figure 2: Ultrastructural examination of the (2–20 µm) fractions of the amended topsoil. 4.1 – Global view of the fraction in 2002. 4.2 – Global view of the fraction in 2003. 4.3 – Specific micro-aggregate. Spectrum 1 : EDX analysis of the organic matter of the micro-aggregate. Spectrum 2 : EDX analysis of the mineral of the micro-aggregate Caption – agI, agII, agIII: micro-aggregate of type I, II or III; b: bacteria; c: clay; cw: cell wall residue; ex: exopolymers; h: hole in the resin due to the micro-quartz; m: mineral; om: organic matter; q: quartz grains;  *1 and *2: localization of EDX analysis 1 and 2.

0-20 µm water-stable fraction contained more than 60% of the soil carbon. A typology of <20 µm micro-aggregates was established according the nature of aggregative organic matter. It appeared that among 0-2 µm and 2-20 µm 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 µm water-dispersibility and a decrease in the mean size of the 2-20 µm water-stable aggregates. Sewage sludge addition slightly increased the proportion of the 50-200 µm 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 µm 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. 

References

1. Marshall, T.J., Holmes, J.W., 1979. Soil Physics. Cambridge University Press.

2. Gobat, J.-M., Aragno, M., Matthey, W., 1998. Le sol vivant, bases de pédologie, Biologie des sols. Presses Polytechniques et Universitaires Romandes, Lausanne.

Affiliations

Laboratoire Sols et Environnement, Nancy Université, INRA, 2 avenue de la Forêt de Haye, BP 172, F-54505 Vandoeuvre-lès-Nancy Cedex, France