Effects of toposequences on soil physicochemical properties and enzyme activities in semi-arid landscapes

This study investigated the influence of landscape position and parent material on soil physicochemical properties and biological functioning along the Çankırı–Acıçay River toposequence. Our objectives were to (i) characterize six representative soil profiles, (ii) quantify the range and spatial variation of key hydrolytic enzyme activities (urease, phosphatase, arylsulfatase, and β-glucosidase), and (iii) evaluate the relationships between soil properties and enzymatic activity. The transect encompasses distinct lithologies: right-bank soils developed on Quaternary alluvial deposits (terraces/floodplains), while left-bank soils evolved from a complex mix including Oligo-Miocene gypsum and rock-salt strata (steep slopes). These differences resulted in classifications as Entisols, Inceptisols, and Mollisols, and distinct variations in particle-size distribution and organic matter (OM) accumulation across the profiles. Results showed that topography-driven erosion and deposition strongly controlled soil OM dynamics, which was the primary regulator of enzymatic activity. OM correlated with all enzymes, r = 0.81** to 0.89**. The old river terrace soils (depositional areas) exhibited the highest enzyme activities due to greater OM and finer texture, confirming their function as biological "hotspots". Conversely, upper-slope and gypsiferous soils displayed minimal enzymatic potential. In conclusion, variations in landscape position and parent material fundamentally alter both soil development and enzyme-mediated nutrient cycling within the semi-arid landscape of the Çankırı–Acıçay River basin.