¹³C isotopic discrimination: A starting point for new insights in competition for nitrogen and water under contour hedgerow systems in tropical mountainous regions

Competition for nutrients and water between crops and associated hedgerows reduces overall performance of contour hedgerow systems and hampers its acceptance by rural communities in tropical mountainous regions. Therefore, it is imperative to better understand competition leading to a decline in crop response close to hedges. In the highlands of North East Thailand spatial variability in grain yield of maize (Zea mays L., cv. Suwan 1) was assessed for two contour hedgerow systems based on Brachiaria ruziziensis Germain et Evrard (Ruzi grass) barriers or Leucaena leucocephala (Lam) de Wit hedges without or with fertilizer (60 kg N ha^-1 and 14 kg P ha^-1). Available NO₃^- - N was analyzed across the slope. In addition, shoot N concentration and δ¹³C values in leaves were measured for maize plants in the center of the alley and in the row next to and at the upper side of barriers or hedges. Despite variable field conditions, δ¹³C values were significantly (p < 0.05) less depleted close to the barriers or hedges, except for 2 out of 16 plots, suggesting that water deficiency was not the main driver for spatial variability along the alleys. The negative correlation between ¹³C isotopic discrimination and available NO₃^- - N in the soil, with R ² ranging from 0.5 (p < 0.10) to 0.9 (p < 0.01), assigned a major role to N availability in the reduced crop response towards the barriers. The proposed framework of ¹³C isotopic discrimination, together with plant and soil N data, is a new approach and was shown to be suitable to determine N and water competition between hedgerows and crops grown in alleys under field conditions.