Water satability of soil aggregates in a 50-year-old soil formation experiment on calcareous glacial till
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Soil formation on the human time scale is immensely time consuming, although it can be significantly accelerated through the effects of vegetation. The content of water-stable aggregates (WSAs) is a useful indicator for determining both the soil development level and the soil quality. However, in severely degraded soils, especially in the Baltic pedoclimatic region, the effects of vegetation on the aggregate stability have been poorly studied. Therefore, to obtain more knowledge about the impact of vegetation on WSA, and thereby knowing how to improve it, this study was conducted on a long-term soil formation experiment in Estonia near Tartu. In 1964, the initial soil from an area of 20 × 8 m down to 100 cm depth was replaced with a sandy loam calcareous glacial till. The experiment started on April 26, 1965, when plants were sown on the plot. The topsoil (0–20 cm) samples were analyzed in 1966, 2000, 2007, and 2014. The study indicated that perennial grasses (meadow fescue and common meadow-grass) fertilized with P40K75, compared to N150P40K75, decreased the WSA content, as well at the accumulation rate of soil organic carbon (SOC) and the total nitrogen content (Ntot). The hybrid alfalfa treatment resulted in the significantly highest SOC and Ntot accumulation, but not in the overall highest WSA content. Under barley, manure positively affected the WSA and SOC, though many other physical properties were not improved. Compared to the initial till under bare fallow, the SOC and Ntot contents were significantly higher under grown crops, but the WSA content remained the same. In addition, regardless of the grown crops, the WSA of larger (0.25–2 mm) aggregates was substantially higher than that of smaller (0.25–1 mm) aggregates. Also, as the relationship between WSA and SOC in the study was linear, the soil was far from C saturation and still in development. Overall, it can be concluded that the cultivation of perennial grasses and hybrid alfalfa on the severely eroded soil is the most rational option to improve the water stability of aggregates and increase the SOC and Ntot contents. However, because of the complexity of the aggregation process, further research is still needed.