A New Method to Isolate Silt-Sized Microaggregates, Presumed Sites of Organic Matter Stabilization in Soils.
I. Virto, Victor Lopez, and Claire Chenu. Insitut National Agronomique, Thiverval-Grignon, Batiment Eger, Thiverval-Grignon, 78850, France
It is now established that sites for physical protection of SOM are microaggregates (<200 µm), that are formed within macroaggregates. However, most studies so far have focused on sand sized microaggregates (50-250 µm), disregarding smaller units. We hypothesize that clay and silt-sized microaggregates are preferential sites for OM stabilization, due to their very strong physical stability. Here we present a new method to separate silt-sized microaggregates (2-50 µm) from free mineral (Min) and free organic (Free OM) particles of the same size. We developed a combination of density and size fractionation procedures that allowed us to quantify the 2-50 µm microaggregates in a cultivated silty soil, and to analyze them. In a sequential protocol, an innovative methodology was designed to recover clay and silt-sized microaggregates and particles as they are released from macroaggregates while dispersion in water. Sedimentation and sieving of the recovered material allowed us to separate the 2-20 and 20-50 mm fractions. The goodness of the method was tested through laser-analysis of the fractions. The parameters of the new method (time, intensity of dispersion, etc) were also adjusted by laser-analysis. Assuming the both organic and mineral composition of aggregates, we inferred a density between 1.8 and 2.5 g cm-3 for silt-sized microaggregates for density fractionation. The composition of the isolated fraction was verified through microscopic observations. The study allowed us to verify the existence of true aggregates in the silt-size fraction, and also to corroborate the theories presented before by other authors about the existence of an important amount of silt-sized particles within silt-sized microaggregates. The analysis of the isolated fraction proved that silt-sized aggregates are a site for C storage, and their ability to stock C in our soil was bigger as they decreased in size.