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

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. Soil samples were taken from three localities (Sobice, Bab, Vieska nad A 1/2 itavou). Each study locality included both a forest and an agricultural soil-sampling area. We found that in forest soils, the proportion of water-stable macroaggregates (WSA(ma)) relative to water-stable microaggregates (WSA(mi)) was greater than in agricultural soils. When all soils were assessed together, positive statistically significant correlations were observed between the size classes WSA(ma) > 1 mm and organic carbon (C-org) content; however, the WSA(mi) content was negatively correlated with C-org content. Favorable humus quality positively influenced the stabilization of WSA(ma) > 5 mm; however, we found it had a negative statistically significant effect on stabilization of WSA(ma) 1-0.25 mm. In agricultural soils, the stabilization of WSA(ma) was associated with humified, i.e., stable SOM. The WSA(ma) 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 WSA(ma) 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 WSA(mi) 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 WSA(mi). 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.