Soil crusting impact on soil organic carbon losses by water erosion

The Sahelian region, characterized by erratic, heavy rainfalls and low soil organic carbon (SOC) stocks, is highly vulnerable to land degradation. While water erosion is recognized as being a main mechanism of SOC losses, little research-has yet been done to investigate the role which soil surface crusting might have on SOC losses. The main objective of this study was to evaluate the impact of soil surface crusting on SOC losses. This study was conducted in Tougou Catchment (37 km(2)), northwest of Burkina Faso, which receives a cumulative mean annual rainfall of 500 mm y(-1). The area is characterized by sandy soils with varying types of surface crusts. The four different crust types studied were: structural crusts (STRU), which were found under cultivated soils, which were plowed annually; perennial desiccation crusts (DES), gravel (GRAV) and erosion (ERO) crusts, generally found in the degraded semi-arid savannas. Three micro-scale runoff plot (1 x 1 m(2)) replicates were installed on each of the different types of surface crusts observed in the catchment. Water and sediment samples were collected from the runoff plots after every rainfall event (n = 10) of the 2011 rainy season. The sediment samples were analyzed for organic carbon (OCsed), while the water samples were examined for dissolved organic carbon (DOC). The average of organic carbon losses with sediment (OC(L)sed), was 037 g C m(-2) y(-1) for ERO, 036 g C m(-2) y(-1) for DES, 024 g C m(-2) y(-1) for STRU and 0.15 g C m(-2) y(-1) for GRAV. DOC accounted for a minute contribution to SOC losses i.e. less than 0.05%. STRU with 10.42 mg C l(-1) showed the highest DOC content, followed by GRAV (6.13 mg C l(-1)), DES (5.06 mg C l(-1)) and ERO (4.92 mg C l(-1)). The OC enrichment ratio (ER) of sediments to that of the 0-0.1 m bulk soil was less than one for DES, GRAV and ERO (0.39, 0.69 and 0.75, respectively) and reached 1.14 for STRU. This pointed to a greater SOC protection from erosion by the perennial crusts of the degraded savannas (DES, GRAV and ERO), as compared to crusts of cultivated fields. Thick, sand-enriched crusts, DES and GRAV, seemed to provide the greatest OC protection. This study pointed out a significant relationship between soils crusting on SOC erosion. It showed that the formation of loose and sandy crusts provides greater SOC protection from water erosion, which in turn may improve SOC stabilization and associated soil functions, such as soil fertility, water-holding capacity and sequestration of atmospheric carbon. (C) 2013 Elsevier B.V. All rights reserved.