Composition and stability of soil aggregates in Fluvisols under forest, meadows, and 100 years of conventional tillage

Conversion of meadow and forest ecosystems to agricultural land generally leads to changes in soil structure. This comparative study presents the composition and stability of structural aggregates in humus horizons (0-30 cm) of noncarbonate silty-clay Fluvisols in the Kolubara River Valley, W Serbia. Aggregates collected from under a native forest were compared to aggregates from meadows and arable fields which underwent crop rotation for > 100 y. The results show that size distribution and stability of structural aggregates in the humus horizons of arable soil are significantly impaired due to long-term anthropogenization. In the humus horizons, the content of the agronomically most valuable aggregates (0.25-10 mm) decreased by a factor of [almost equal to] 2, from 68%-74% to 37%-39%, while the percentage of cloddy aggregates (>10 mm) increased by a factor of [almost equal to] 2, from 23%-31% to 48%-62%, compared to forest aggregates. The long-term-arable soil had significantly (p < 0.05) lower aggregate stability, determined by wet sieving, than meadow and forest soils. The lowest aggregate stability was found in aggregates > 3 mm. Their content is [almost equal to] 2.5-3 times lower in arable soil (13%-16%) than in forest soil (32%-42%) at a depth of 0-20 cm. The largest mean weight diameters of dry aggregates (dMWD) with a range between 12.6 and 14.7 mm were found in arable soil, vs. 9.5-9.9 mm in meadow and 6.5-8.3 mm in forest. The arable soil had significantly lower mean weight diameters of wet-stable aggregates (wMWD) and a lower structure coefficient (Ks) than forest and meadow soils. The dispersion ratio (DR) of arable soil was significantly higher than that of forest and meadow soils. Forest and meadow showed a significantly higher soil organic-matter content (SOM) by 74% and 39%, respectively, compared with arable soil, while meadow uses decreased the SOM content by 57% compared with forest at a depth of 0-10 cm. In conclusion, the results showed that long-term conventional tillage of soils from natural forest and meadow in the lowland ecosystems of W Serbia degraded soil aggregate-size distribution and stability and reduced SOM content, probably resulting in lower productivity and reduced crop yields.