Humus substances and soil aggregates in the soils with different texture

Humus substances (HS) influence the incorporation of carbon into soil aggregates in many ways. In this study the influence of HS and their fractions in the soil on the proportions of carbon (total organic, labile, non-labile) in water-resistant macro-aggregates (WSA) and differences between the amount of carbon in WSA in coarse-grained (CGS) and fine-grained (FGS) soils with dependence on the proportions of HS in the soil were determined. The experiment included three soils (Haplic Chernozem, Haplic Luvisol, Eutric Cambisol), each of them with two different soil textures (CGS, FGS) from four ecosystems (forest, meadow, urban, and agro-ecosystem). In CGS, higher proportions (52 and 50%) of smaller (< 1 mm) dry-sieved macro-aggregates (DSA) and also WSA were determined, while in FGS, higher proportions (51 and 53%) of larger DSA (> 7 mm) and WSA (> 2 mm) were detected. A negative correlation was recorded between the content of organic carbon in the fractions of WSA and the amount of extracted humic acids (HA) in CGS, and fulvic acids (FA) in FGS. In CGS, the correlation between the carbon content in WSA and HA bound with Ca²⁺ and Mg²⁺, which forms humates (HA2), was negative. In FGS, a negative correlation was recorded between the carbon content in WSA and free aggressive FA (FA1a) and free FA and those, which are bound with monovalent cations and mobile R₂O₃ (FA1) in the soil. In the case of FA1a, a negative correlation was recorded in FGS and also in CGS, however this influence was more marked in CGS than in FGS (by about 21% higher correlation). In CGS, the influence of HA and FA in soil on the content of labile carbon in aggregates was stronger than in FGS. In CGS, a higher proportion of carbon in aggregates was detected in the case of lower stability of HS and HA and, on the contrary, in FGS, a higher content of carbon in aggregates was detected in the case of their higher stability.