Distribution of organic and humic carbon in wet-sieved aggregates of different soils under long-term fertilization experiment

Aggregate fractionation has become a very common approach to study the link between aggregate formation and soil organic matter (SOM) turnover. According to the hierarchical theory, microaggregates are assumed to be stabilized by persisting binding agents, whereas macroaggregates by transient or temporary organic materials. Humic substances, considered to be recalcitrant, could probably act as persistent binding agents and hence relatively more would be found at microaggregate level. In a long-term experiment established in the early 1960s in north-eastern Italy, we wet-sieved large macroaggregates into three aggregate-size classes (2000–250 μm, 250–53 μm and  0.95). The HC/OC ratio ranged narrowly among aggregate classes and was not higher at microaggregate level, suggesting no hierarchical role of HC as persisting binding agents. Apparent molecular weight distribution of HC extracted, suggested a likely different origin and chemical composition depending on aggregate fractions and soils.