Water dispersible colloids associated organic carbon along an alluvial fan transect in a hyper-arid region of the Atacama Desert

Organic carbon (OC) in the hyper-arid Atacama Desert soils is known to be extremely low (0.1-0.01%). OC can accumulate on soil colloids (1-1000 nm) and nanoparticles (1-100 nm) due to its high specific surface area. Small-sized colloids may be transferred to deeper depth through the macropores in the soil. However, little is known about the colloidal-OC soil transfer under hyper-arid conditions. In this study, the Water Dispersible Colloids (WDCs, <300 nm) associated OC (WDC-OC) was analyzed using Asymmetric Field-Flow-Field Fractionation (AF⁴) coupled online to an Organic Carbon Detector (OCD). The experimental site is located at 1450 m altitude near Paposo (Antofagasta region, Chile) and receives <2 mm rain per annum. Samples were taken at 13 points along an alluvial fan transect, and up to a depth of 50-80 cm. Our study examined the vertical distribution of WDC-OC affected by micro-relief. Three colloidal size fractions were identified in all samples: nano-colloids (0.6-24 nm), fine colloids (24-210 nm) and medium colloids (210-300 nm). The vertical contribution of WDC-OC differed distinctively between (i) the active alluvial fan section, (ii) the older inactive alluvial fan section, related to sediment induration and soil crust development, and (iii) the edge between both fan sections. We found that WDC-OC was highest in the active fan with an average of 11.5 mg OC kg^-1 compared with the content found in crust-related older fan (0.24 mg OC kg^-1) or at the edge between the fan sections (0.19 mg OC kg^-1). Notably high WDC-OC in the fan near to the few isolated plant remains were also observed. The increase of biological activities and debris near to the plant contributes to more colloidal-OC (26.8 mg kg^-1). The relatively flat hard impermeable surface of the crust-related old fan section may induce colloids loss during high-intensity rainfalls, e.g. occurring during past El Niño periods. Furthermore, the relative percentage of WDC-OC as a part of the total was highest in the upper layer (0-1 cm) of the active fan (27-48%) and at the edge (69%), while in the older crust-related sections the highest values were observed in the subsurface (5-10 cm) (19%-29%). Near the plant remains, nano-colloids were dominated in the upper soil accounting for 48% of the WDC-OC, whereas medium colloids were predominant in the older crust-related sections (64%). Dust (colloidal-sized) particles may be deposited at the surface and then are easily trapped near plants. We conclude that WDC-OC depth profiles within the hyper-arid Atacama Desert reflects the differential surface characteristics and the age of the fan surface, i.e., the period of geomorphological inactivity. During the extremely rare rainfall events in the Atacama, both factors will lead to differential infiltration rates, which thus in turn affect the size distribution of colloidal-OC with profile depth.