Changes in the relationship between soil erosion and N loss pathways after establishing soil conservation systems in uplands of Northeast Thailand

Slow establishment of green barriers together with competition for nutrients and water between crops and contour hedges hamper their acceptance by rural communities in tropical mountainous regions. Alternatively, a combination of hedges/barriers and minimum tillage may shift the pathway of N losses from water erosion towards leaching. In Northeast Thailand, run-off, soil loss, N leaching (by resin cores) and crop response were monitored in grass barriers (Vetiveria zizanioides, Brachiaria ruziziensis) and hedgerow (Leucaena leucocephala) based soil conservation systems in fertilized/unfertilized treatments from their establishment in 2003 to 2005. In all treatments, maize was grown on a moderate slope gradient (21–28%) under minimum tillage conditions and relay cropped with a legume cover crop (Canavalia ensiformis). After 3 years, maize grain yields increased from 1.5 and 3.2 to 3.8 and 5.5 Mg ha−1 in the unfertilized and fertilized control plots. Over the same period, yield increases were lower for soil conservation treatments reaching yields of 2.0–2.7 Mg ha−1 without fertilizer and 3.9–4.2 Mg ha−1 with fertilizer. After 3 years, runoff (190–264 m3 ha−1) and soil loss (0.2–1 Mg ha−1) in fertilized plots with barriers showed an average decrease of 72% and 98%, respectively, compared to 2003, the reduction being lower in unfertilized plots. The control had a much higher soil loss in the first year (24.5 Mg ha−1), but also showed much reduced erosion (1.6–2.5 Mg ha−1) in the third year, partly due to reduced rainfall but also due to the combined effects of minimum tillage and surface mulch. Runoff, however, did not decrease on the control plots over the years in the same way as it did under soil conservation (runoff only after >12 mm day−1). Average cumulative N losses by runoff, soil loss and leaching were reduced from 55 kg N ha−1 in the control to 37–40 kg N ha−1 in the barrier treatments. The dominant N loss pathway shifted from above ground N losses to leaching with the establishment of barriers and hedges. Due to the positive maize yield development and partial control of soil loss, minimum tillage combined with legume relay cropping under the trial conditions indicates a potential alternative to contour barrier/hedgerow systems for soil conservation on moderate slopes in tropical mountainous regions.