Medium-term transformation of Chernozems under broadleaf forests in the temperate climate of south-east Poland

Water erosion, accelerated in sloped landscapes by intense cultivation (ploughing), can rapidly degrade humus-rich topsoil horizons of chernozemic soils, leading to an irreversible loss of the most valuable soil resources, in particular in areas with relict Chernozems/Phaeozems. An afforestation can effectively control erosion rates, but dense canopy forests are commonly listed among factors that support the transformation of Chernozems into Luvisols/Retisols. A comparison of the morphology and physico-chemical properties of adjacent arable and forest soils was carried out in four catenas located in the patches of relict chernozemic soils in the loess belt of south-east Poland. An analysis has evidenced that mixed broadleaf forests have generated during the 200–300-year-long period (an approximate time since afforestation of the sites under study) topsoil acidification (by approximately 1–1.5 grade compared to adjacent arable soils), base cations leaching (below 10 cmol(+) kg−1), decrease of soil organic carbon pools (by 15–35% in the soil column of 0–50 cm) and clay depletion (by ca. 4%). The subsurface Bt (argic) horizons have developed in all afforested and some of arable soil profiles. However, all studied forest soils preserved the mollic or chernic horizons and in most cases retained the same reference soil group as adjacent arable soils (Chernozem or Phaeozem). It means that even in temperate humid climate as in south-east Poland, mixed broadleaf forests, consisting of tree species known with little acidifying impact on soil, may not rapidly degrade Chernozems. The observed evidence of physicochemical and morphological transformation is apparently weaker than the reported soil destruction caused by water erosion and justifies the controlled afforestation of the relict Chernozems and Phaeozems as a means of their preservation against rapid loss.