Soil Conservation and Nitrogen Dynamics in Hillside Maize Cropping in Tropical Mountainous Regions

Land use intensification on steep slopes has led to severe soil degradation by erosion in many tropical countries. This problem has been exacerbated by increased rainfall intensities due to climate change. The overall goal of this study was to assess the short to medium term changes in soil erosion, runoff, nitrogen (N) losses and crop response, as affected by contour barrier/hedgerow and conservation agriculture systems (minimum tillage and retaining crop residues). Particular emphasis was given to changes in the pathways of N loss, for e.g. above vs. below ground losses, and to gaining a better understanding of competition leading to a decline in crop response close to hedges. Finally the magnitude and dynamics of key processes influencing the efficiency of soil conservation measures were assessed by using the WaNuLCAS model (Water, Nutrient and Light Capture in Agroforestry Systems). The study was conducted over a 3-year period (2003-2005) at Ban Bo Muang Noi village in the Loei province of Thailand (17o33' N and 101o1' E, 572 m a.s.l.). The experiment was laid out as a split-plot design, with fertilizer application as the main factor and soil conservation as a subfactor, with two replicates. Plot size was 4 by 18 m with a collection device for runoff water and eroded soil. In all treatments maize (Zea mays L.) was planted along the contours on a moderate slope gradient (21-28%) under minimum tillage conditions and relay cropped with a legume cover crop (Canavalia ensiformis). The two main factor treatments were (i) no fertilizer application and (ii) 60 kg N ha-1 plus 14 kg P ha-1 via split application. Subfactor treatments were: (i) Vetiver grass (Vetiveria zizanioides (L.) Nash) barriers, (ii) Ruzi grass (Brachiaria ruziziensis Germain et Evrard) barriers, (iii) Leucaena (Leucaena leucocephala (Lam) de Wit) hedges, and (iv) a control without hedgerows. Contour barriers/hedgerows combined with minimum tillage and legume relay cropping provide sustainable options for farmers to generate cash income and reduce agricultural risks through a reduction in runoff, soil erosion and soil N loss. However, hedgerows and barriers are probably only required in the establishment phase and thereafter, maize cropping under minimum tillage, combined with legume relay cropping (conservation agriculture) is a viable option, at least for the study areas with moderate slopes