Impact of agroforestry on soil loss mitigation in the sloping land of Northwest Vietnam

Soil erosion is one of the major threats to soil functions in many parts of the world. To- day’s challenge is to adapt agriculture practices in order to reduce soil erosion and at the same time consider local conditions. One solution that has been proposed for hilly regions is agroforestry, a land-use system in which woody perennials are grown on the same piece of land as agricultural crops and/or animals. This Master thesis was conducted in two sites in Northwest Vietnam, in Son La and Dien Bien province. Northwest Vietnam is characterized by a mountainous landscape and agri- culture is practiced in the hills, often as monoculture of sole crops. This type of land use in combination with seasonal large high-intensity rains aggravates soil erosion. In this study, agroforestry (AF) practices’ capacity for erosion control was evaluated, and compared to sole crops of the dominant species in the area. An Experimental Trial in form of replicated field plots of block design and a larger Exemplar Landscape were used. The Experimental Trials and the Exemplar Landscape had two treatments, one AF system and one control treatment with sole crops. The AF system included crops, fruit trees and grass strips with the aim to form terraces naturally. To evaluate the AF’s capacity for erosion control, average amount of lost soil was es- timated in the Experimental Trials by three methods; erosion traps, erosion pins (both field methods) and a WEPP model. Another function of the erosion pins was to evaluate the movement of soil along the hillside. Soil texture, structure, color and signs of bio- logical activity were analyzed from soil profiles. Soil samples were also taken for soil analyzes of parameters needed for calculations and the WEPP model. A method was also developed to assess the terrace formation over a longer a time scale. Results of this study showed that agroforestry in combination with grass strips perform well in decreasing soil loss in the Son La study site. The soil loss in Son La was found to be 43% less two years after establishment compared to the sole maize system figuring as the control. The results also showed that grass strips have the ability to start forming terraces already one year after establishment of the Experimental Trial in both study sites. The terraces showed further development several years after the establishment at land- scape level