Factors controlling water-stable macroaggregation of Kastanozems exposed to water erosion

The longterm agricultural use of Kastanozems in Bulgaria caused deterioration of soil structure, depletion of soil organic matter content and intensive water erosion on sloping lands. One of the indicators of these processes is the significant reduction of the water stability of soil aggregates. The aim of this study was to assess the influence of chemical, biochemical, biological and physical soil properties on the aggregate stability of Kastanozems on sloping lands. The topsoil layers of seven different in position, land cover and tillage variants (V) were investigated: noncultivated site under forest (V1); newly ploughed area (V2); high part of the slope with inclination 7.5(o) (V3); middle part of the slope with inclination of 50 with sowing of milk thistle (Silybum marianum)(V4); low part of the slope with deposition of eroded material (V5); maize sowed across the slope under erosion control technology combined with surface mulching with manure (V6); maize sowed along the slope under conventional technology (V7). The highest content of water stable aggregates (> 0.25 mm) was 41.9% observed at V1. The water stability of aggregates in V2 and V3 decreased by 40%, which corresponded to the same degree of decrease in total organic carbon. In the other variants the aggregate fraction 1-3 mm contained almost no water stable aggregates, although the content of organic carbon was higher due to overlay of the eroded material (V4, V5, V7) or to the applied manure (V6). Formation of water stable macro aggregates of eroded particles is very difficult when sloping lands are cultivated. The strongest positive relationship was found between water stability of macro aggregates and the content of microbial carbon, which in turn depended on the amount of actinomycetes and fungi. The instability of soil aggregate led to the reduction of the available water capacity of all variants, which was most evident in the site with deposition of eroded material (V4) and less pronounced in the variant with mulching with manure (V6).