Cementing agents involved in the macro- and microaggregation of a Mediterranean shrubland soil under laboratory heating

The heating effect on aggregation and cementing agents of a Mediterranean soil was studied in two fractions (macro- and microaggregates) in different environments: under Quercus coccifera soil (UQ), under Rosmarinus officinalis soil (UR) and between plants or bare soil (BS). Samples were heated under laboratory conditions at different temperatures (220, 380, 500, 750 and 1000°C) to establish their effects on the studied soil properties by comparison with unheated control samples (25°C). Increasing temperature treatments caused significant changes in most soil variables. In both soil fractions, the presence of vegetation contributed to increase of the soil organic matter (SOM) content, which, in general, tended to decrease as temperature increased in all soils, reducing also their soil macroaggregate stability (SMS). Reduction in the stability of microaggregates (MSA) was not significant, possibly because the organic matter only controlled the stability of some of their fractions.Up to 500°C, the mean weight diameter (MWD), calcium carbonate (CaCO3) and extractable iron contents of macroaggregates presented different tendencies in all soils, but at 750°C relative maxima were observed in these three parameters, indicating a possible relationship between the potential re-aggregation of medium size macroaggregates into larger ones, and the increasing CaCO3 and extractable Fe contents. However, these newly formed “aggregates” would show low resistance to erosion as the decrease in SMS values suggested. The CaCO3 and extractable Fe contents of microaggregates in all soils tended to increase up to 750°C while their mean diameter (MMD) and stability hardly changed or tended to decrease. At 1000°C, the cementing agents showed their lowest values in each soil, while the macroaggregate stability tended to increase, and the mean diameter and stability of microaggregates reached their maximum values. This fact could be due to the possible re-aggregation of small fractions into large ones and some “hardening” processes that could have occurred. In general, the macro- and microaggregate stability appeared to depend on the organic matter content in all studied soils but, particularly, in those under canopy. The mean size of both fractions seemed to be controlled mainly by the calcium carbonate and iron contents.