Geomorphological controls on soil fertility of semi-arid alluvial fans: A case study of the Joghatay Mountains, Northeast Iran

Abstract

Alluvial fans are ideal locations to examine the effect of geomorphic processes and landforms on the soil characteristics. The rate of soil development and fertility differs between active surfaces, exposed to aggradation, and old or relict surfaces, subject to degradation. The aim of this research is to evaluate the effect of landforms and processes on soil fertility based on the studies on three alluvial fans of different ages (relict, old and young). A total of 36 soil samples were collected from these three alluvial fan surfaces. The samples were gathered from different locations of fans (apex and toe) and landforms (gully beds, interfluves, swales, and bars) and for each sample, nine variables including soil texture, acidity (pH), electrical conductivity (EC), total neutralizing value (TNV), available phosphorus (AP), available potassium (AK), total nitrogen (TN), total organic carbon (TOC), and hydraulic conductivity (K) were measured. The results show that the values of TNV, AP, AK, TN and TOC increase as clay% increases. In the relict and old fans, the values of TNV, TOC, TN, AP, AK, clay%, and silt% are higher in the interfluves than in the gully beds. Also, in the relict and old fans, the rates of clay%, silt%, TOC, TN, and AP are higher in the apexes than in the toes. In the young fan, the values of most variables are higher in the toe compared to the apex. The wide and relatively flat interfluves of the apex of old and relict fans have soils that are more fertile compared to the extensively entrenched surfaces of relict and old fan toes with relatively steep sloped interfluves. The less fertile soils in the apex of young fan can be attributed to its coarse-textured soils and also to repeated flooding and depositional processes. Overall, this study reveals that geomorphological landforms and processes such as aggradation and degradation play an important role in the spatial variation of soil fertility.