Application of Caesium-137 and Berylium-7 to Assess the Effectiveness of Soil Conservation Technologies in the Central Highlands of Vietnam

Over the last decade large areas in Vietnam have become more vulnerable to erosion due to fast economic changes leading to increasing and often unsustainable land use intensification. In this study, soil erosion rates at medium (40-50 years) and short (event based) term have been assessed for steep farmland protected by or without soil conservation measures in the Central Highlands of Vietnam using fallout radionuclide (137Cs and 7Be) based techniques. A 2.5 ha mulberry field with a slope gradient of about 15% was selected as study site in the Lam dong province, which is located in the south-east part of the Central Highlands. Widely spaced shrubby contour hedgerows composed of Leucaena glauca and Lanata camara have been utilized at this site as a soil conservation measure for 22 years. In order to assess the effectiveness of additional novel soil conservation measures, such as the use of vetiver grass barriers (Vetiveria zizanioides L.), additional research plots were installed in the year 2000 on a field with a slope gradient of about 15%, adjacent to the previous study site. On the mulberry field, where widely spaced hedges have been utilized as a soil conservation measure for 22 years, about 55% of the area suffered from erosion with rates varying between 0.6 and 70 t ha-1 a-1 (an average of 31 t ha-1 a-1). Deposition occurred on 45% of the area with deposition rates ranging between 0.2 and 74 t ha-1 a-1, ( an average of 36 t ha-1 a-1), resulting tin a sediment delivery ratio of only 9%. The medium term erosion rate assessed by the use of the 137Cs radionuclide was 1.2 ± 0.6 t ha-1 a-1. The short term soil erosion rate at this field estimated by 7Be in 2006 was 1.5 ± 0.24 t ha-1 a-1, coming close to the medium term soil erosion rate. For the mulberry plot without hedgerows, net erosion rates assessed by the runoff plot method in the period of 2000-2003 ranged between 21 t ha-1 a-1 and 41 t ha-1 a-1, i.e, 18 to 35 times the net erosion rate at the mulberry field with shrubby hedgerows. The 137Cs and 7Be techniques were also used for assessing novel soil conservation measures. For the plot without soil conservation, soil erosion could be observed at all sampled points, with medium term erosion rates ranging between 1 t ha-1 a-1 and 35 t ha-1 a-1 (the average erosion rate was 23 ± 1 t ha-1 a-1). The short term soil erosion rate estimated by the 7Be technique in the year 2005 was 33 ± 6 t ha-1 a-1 for this plot. However, the short term soil erosion rate at the plot with Vetiver grass barriers was for the same year 2 t ha-1 a-1 (estimated by the 7Be technique), and showed that by using this kind of grass barriers as a soil conservation measure soil erosion was almost completed controlled. (author)