Interactions of soil quality, ages of alluvial fans, and mechanisms controlling total soil organic carbon dynamics

Abstract

So far, no studies have tested how alluvial fan aging and associated geomorphological differences can affect the mechanisms that control soil organic carbon (SOC) dynamics. In order to understand the SOC dynamics, we quantified the relationship between soil quality (SQ) indicators and alluvial fans aging over area of about 10,000 km2 in north-eastern Iran, using a minimum data set (MDS) and regression analyses. Twenty seven soil attributes and weathering indices were measured at 90 old and young fans. Total SOC and its fractions were found as the key indicators of the MDS. Despite the higher content of SOC in younger fans (6.25 – 7.11 g kg−1), they exhibited a lower value of SQ (0.35 – 0.42). Younger fans had a low content of fine-sized pedogenic oxides, and, thus, a lower sorption potential, giving rise to a lower ability for stabilising SOC. The higher content of SOC in younger fans was due to an accumulation of coarse particulate organic carbon. Long-term exposure of the older surfaces to weathering caused a statistically significant increase in the chemical index of alteration (57.94 – 72.32 %), chemical index of weathering (64.3 – 77.2 %), the carbon size fraction 53 – 250 µm (2.66 – 5.56 g kg−1), the dithionite-extractable Al (48.4 – 59.2 g kg−1) and Fe (21.5 – 25.7 g kg−1) oxalate-extractable Al (25.6 – 32.7 g kg−1) and Fe (9.5 – 13.0 g kg−1). These characteristics promote physical protection of SOC against microbial decomposition. Alluvial fan aging leads to geomorphological differences in terms of the weathering degree, processes of flooding, inundation, and erosion, contributing to high variability of the SOC dynamics. A proper understanding of the relationships between the age evolution of landforms and associated variations in mechanism regulating the preservation and stability of soil C can help to develop new perspectives for geo-conservation and restoration efforts.