Above-ground earthworm casts affect water runoff and soil erosion in Northern Vietnam

This manuscript focuses on the effects of above-ground earthworm casts on water runoff and soil erosion in steep-slope ecosystems in Northern Vietnam. We investigated the effects of Amynthas khami, an anecic species producing above-ground casts of prominent size, on water infiltration and soil detachment along a land-use intensification gradient: a cultivation of cassava (Mahinot esculenta; CAS), a plantation of Bracharia (Bracharia ruzziziensis; BRA), a fallow (FAL), a fallow after a forest of Eucalyptus sp. (EUC) and a plantation of trees (Acacia mangium and Venicia Montana; FOR). Two scales of studies were considered: (i) at the structure scale (cm(2)), a water runoff simulation was used to differentiate the effects of casts, free biogenic aggregates that previously belong to casts, and free pbysicogenic aggregates; (ii) at the station levels, 1-m(2) plots were used to determine runoff and soil detachment rates during the rainy season in 2005. A. khami was sensitive to land-use management. Earthworm density was low in all the fields (0-1 ind m(-2)). The highest densities were found in EUC and FOR and no individual was found in CAS. As a consequence, soil surface in EUC and FOR was covered with casts and free biogenic aggregates (approximately 22 and 8 kg m(-2), respectively). In FAL and BRA, casts covered the soil only sparsely with < 3 kg m-2. In CAS, soil surface was characterized by free physicogenic aggregates that might be produced by human activity or cridogeic earthworms through tillage (approximately 1 kg m-2). Water runoff simulation clearly showed an enhancement of water infiltration with earthworm casting activity. Water runoff was more decreased with casts (R-2 = 0.26) than free biogenic aggregates (R-2 = 0.49). Conversely, physicogenic aggregates were not associated with higher water infiltration. Analyses of runoff and soil detachment rates during the rainy season underlined that the more land-use type have aggregates on soil surface and the less important is surface runoff (R-2 = 0.922). Conversely, no relation occurred between aggregates and soil detachment rate. While above-ground casting activity decreased surface runoff, they were not involved in soil detachment, and therefore soil erosion. (c) 2007 Elsevier B.V. All rights reserved.