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 ìm, 250–53 ìm and b53 ìm) in contrasting soils (clay, sandy and peaty) fertilized with manure or mineral fertilizers. We analyzed the organic (OC) and humic (HC) carbon of each aggregate fraction, also investigating the molecular weight of the humic substances (HSs) extracted. The aims were to evaluate the effect of the different fertilization types on the aggregates and organic matter distribution and investigate the composition and role of HC as binding agent. The results demonstrated that the addition of manure significantly increased the proportion of macroaggregates with respect to mineral fertilization (605 and 545 g aggr kg.1 of soil respectively), but only in the clay soil. However, manure application increased the OC and HC concentration in the aggregate fractions with respect to the mineral and control treatment, in all the three soils. Organic carbon concentration generally increased with increasing aggregate size as well as HC that followed the same pattern, showing a very high correlation with OC (rN0.95). 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