Soil profile improvement as a by-product of gully stabilization measures
Abstract
In this paper, we characterize the long-term effect of reforestation on the stabilization of soils affected by strong gully erosion in Prietrž village in South-Western Slovakia. Permanent gullies there were reforested a century ago with the black locust (Robinia pseudoacacia). Reforestation measures induced soil profile development on the gully slopes with distinct humus horizons. Soil-forming processes caused organic matter accumulation, carbonate leaching and a decrease in pH in the surface horizons. One century after reforestation, the key properties of the gully soils closely resemble those of the surrounding agricultural soils. Soil aggregate stability was used as a useful indicator which reflected the impact of reforestation on soil stabilization over the last hundred years. The overall effect of the observed changes is an increase in the aggregate stability of these reforested gully soils. Soil aggregate stability measured by the rainfall simulator method positively correlated with the organic carbon content, and it had negative correlations with carbonate content and soil pH. The values determined for these gully soils do not differ significantly from those in the surrounding non-eroded agricultural soils. The extreme difference in aggregate stability between the topsoils and the parent marl material is responsible for this landscape's susceptibility to severe gully erosion.
Conclusions
Field research of reforested soils on steep walls of gullies showed that prevailing climatic conditions, and the beneficial reforestation effects which accrued during the century following reforestation, proved sufficient for the development of a clear soil profile with distinct soil horizons. Detailed analysis showed that soil properties are now gradually approaching the values determined in the surrounding agricultural soils. A remarkable decrease in soil pH, carbonate leaching and soil organic matter accumulation were observed in the newly developed soils of these gullies. Statistical evaluation has revealed that the A horizon of gully soils has properties significantly different from the parent marl soil material, but very closely approximating the A horizon of surrounding agricultural soils. Irrespective of some controversial information in the literature concerning relationships between the aggregate stability and individual soil properties, the overall effect of the evolution of soil properties has resulted in an increase in aggregate stability in the A horizon of gully soils. The aggregate stability is even so high that its difference from the surrounding agricultural soils is not statistically significant. The main reason for the region's susceptibility to gully erosion can be derived from the analysis of the aggregate stability in the studied soils. This showed that landscape instability here can be attributed to the very low aggregate stability of the parent marl material below a relatively stable soil cover. The obtained results also showed that reforestation at the end of the 19th century with black locust was an effective measure in changing unsuitable properties in the marl material exposed in these gullies. This reforestation was also beneficial in the development of soil profiles and for stabilization of the walls of the permanent gullies in this region.