The influence of soil organic matter fractions on aggregates stabilization in agricultural and forest soils of selected Slovak and Czech hilly lands

PURPOSE: Because the stability of soil aggregates is affected by many factors, we studied aggregates formed in forest and agricultural soils in different soil types (Cambisols, Luvisols, Chernozems). We evaluated: (1) the differences in water-stable aggregates (WSA) as related to soil type and land management and (2) the relationships between quantitative and qualitative parameters of soil organic matter (SOM), particle-size distribution and individual size classes of WSA. MATERIALS AND METHODS: Soil samples were taken from three localities (Soběšice, Báb, Vieska nad Žitavou). Each study locality included both a forest and an agricultural soil-sampling area. RESULTS AND DISCUSSION: We found that in forest soils, the proportion of water-stable macroaggregates (WSAₘₐ) relative to water-stable microaggregates (WSAₘᵢ) was greater than in agricultural soils. When all soils were assessed together, positive statistically significant correlations were observed between the size classes WSAₘₐ > 1 mm and organic carbon (Cₒᵣg) content; however, the WSAₘᵢ content was negatively correlated with Cₒᵣg content. Favorable humus quality positively influenced the stabilization of WSAₘₐ > 5 mm; however, we found it had a negative statistically significant effect on stabilization of WSAₘₐ 1–0.25 mm. In agricultural soils, the stabilization of WSAₘₐ was associated with humified, i.e., stable SOM. The WSAₘₐ content was highly positively influenced mainly by fulvic acids bound with clay and sesquioxides; therefore, we consider this humus fraction to be a key to macroaggregate stability in the studied agricultural soils. On the other side, all fractions of humic and fulvic acids participated on the formation of WSAₘₐ in forest soil, which is a major difference in organic stabilization agents of macroaggregates between studied forest and agricultural soils. Another considerable difference is that WSAₘᵢ in agricultural soils were stabilized primarily with humic acids and in forest soils by fulvic acids. Moreover, in forest soils, a higher content of labile carbon in WSA had a positive effect on formation of WSAₘᵢ. CONCLUSIONS: The observed changes in individual size classes of WSA and interactions between SOM, particle-size distribution, and WSA have a negative impact on soil fertility and thereby endanger agricultural sustainability.