Topsoil chemical differentiation in mapped brown forest soil units of Southern Bulgaria

hemical differentiation within the uppermost centimetres of mineral soil is rarely resolved in regional surveys, because sampling depths of 0–20 cm average across the layer in which surface accumulation actually occurs. This study compares two successive topsoil intervals, 0–5 and 5–10 cm, at fifteen sampling points distributed across the Western Rhodopes in Southern Bulgaria, all located within areas mapped as brown forest soils. Twenty-nine samples were analysed for soil reaction, the components of the sorption complex, organic carbon, humus and total nitrogen. The upper interval contained approximately 53% more organic carbon (3.48 versus 2.28%) and 70% more total nitrogen (2.17 versus 1.28 g kg−1) than the interval immediately below it, and both contrasts remained significant after false discovery rate adjustment under parametric and distribution-free tests alike. Soil reaction was slightly lower at the surface (6.50 versus 6.66), a difference that was statistically supported but quantitatively small, with a 95% confidence interval reaching only -0.29 pH units. The components of the sorption complex–total exchange capacity, exchangeable calcium and magnesium, hydrolytic acidity and base saturation–showed no significant differentiation between the two intervals. Expressed as stratification ratios, organic carbon and nitrogen gave mean values of 1.67 and 1.87, whereas exchange capacity, base saturation and pH all gave ratios close to unity. Surface accumulation of organic matter and nitrogen therefore proceeds without a corresponding reorganisation of the exchange complex, and a 0–10 cm composite would have concealed the greater part of it.