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Soil Erosion and Nitrogen Leaching in Northern Vietnam: Experimentation and Modelling

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Soil Erosion and Nitrogen Leaching in Northern Vietnam: Experimentation and Modelling Soil erosion and nitrogen leaching in northern Vietnam: Experimentation and modelling Promotor: Prof. dr. ir. H. van Keulen Hoogleraar bij de leerstoelgroep Plantaardige Productiesystemen Wageningen Universiteit Co-promotoren: Dr. R.P. Roetter Senior onderzoeker, Soil Science Centre, Alterra Green World Research, Wageningen Universiteit en Researchcentrum Dr. R. Hessel Senior onderzoeker, Soil Science Centre, Alterra Green World Research, Wageningen Universiteit en Researchcentrum Promotiecommissie: Prof. dr. C.J. Ritsema (Wageningen Universiteit) Dr. C.T. Hoanh (IWMI-SEA, Malaysia) Dr. ir. M.E.F. van Mensvoort (Wageningen Universiteit) Dr. ir. C.M.M. Mannaerts (ITC, Enschede) Dit onderzoek is uitgevoerd binnen de C.T. de Wit onderzoekschool: Production Ecology and Resource Conservation. Soil erosion and nitrogen leaching in northern Vietnam: Experimentation and modelling Mai Van Trinh Proefschrift ter verkrijging van de graad van doctor op gezag van de rector magnificus van Wageningen Universiteit, prof. dr. M.J. Kropff, in het openbaar te verdedigen op dinsdag 17 april 2007 des namiddags te half twee in de Aula. Mai Van Trinh (2007) Soil erosion and nitrogen leaching in northern Vietnam: Experimentation and modelling. Mai Van Trinh –[S.l.:s.n.]. Ill. PhD thesis Wageningen University. –With ref.– With summaries in English, Dutch and Vietnamese. ISBN: 90-8504-605-x Abstract Mai Van Trinh, 2007. Soil erosion and nitrogen leaching in northern Vietnam: Experimentation and modelling. PhD thesis, Wageningen University, Wageningen, The Netherlands. With summaries in English, Dutch, and Vietnamese, 182 pp. Two years research has been conducted in Tam Duong district, Vinh Phuc province, upstream in the Red River Delta in northern Vietnam, comprising three geographical regions, i.e. mountainous areas, terraces in hilly land and flat land. The extent of soil degradation in the district was delineated, using the Red/Near-Infrared band ratio of satellite images, calibrated on the basis of soil maps and field checks. Results showed strong dynamics in soil degra- dation with the largest area of degraded soil of 3280 ha in 1992, associated with a substantial reduction in forest cover and expansion of the agricultural area from the mid-1980s. Subsequently (1996), re-forestation, particularly planting of eucalyptus plantations, led to a reduction, followed (2000) by expansion again, as a consequence of harvesting of production forests. In the mountainous and hilly areas, soil erosion is the dominant degradation process, very severe at individual plot scale, but far less at sub-watershed and watershed scales (i.e. measured soil losses over eight events, with the same total rainfall, were 1360 and 773 kg ha–1 in a cassava and an eucalyptus plot, while it was 45 in the sub-watershed and 125 in the main watershed). Performance of the soil erosion model LISEM was evaluated in the study area; results showed differences in performance at different spatial scales. In the main watershed, simulated total runoff and soil loss were underestimated, because of storage and release in terraces and rice fields. In the upland sub-watershed, total soil loss was overestimated due to overestimation of sediment concentration, as a result of high detachment and transport capacity. In addition to soil erosion from the terraces, nitrogen leaching from the root zone in these sandy soils contributes to negative environmental impact of agriculture. A nitrogen balance model was developed, and calibrated on the basis of measured soil nitrogen concentrations in different cropping systems. Results from the model showed increased nitrogen leaching with increasing fertilizer doses, i.e. in a rice-rice-maize rotation, the traditional land use pattern in the district, annual nitrogen leaching losses varied from 52 to 60 kg ha–1, 56 to 114 kg ha–1, and 58 to 154 kg ha–1 for low, intermediate, and high fertilizer inputs, respectively. In the lowland area, agriculture has diversified and intensified, and high value crops are cultivated with very high doses of fertilizer. In these cropping systems, nitrogen leaching is particularly high. Annual leaching losses were calculated of up to 193 kg ha–1 in flowers and 115 kg ha–1 in vegetables compared to about 50 kg ha–1 in rice. From a set of point measurements, spatial distributions of nitrate- and ammonium nitrogen for a small region were predicted, using regression block kriging. The method was based on a stepwise backward linear regression, combined with expert knowledge, taking into account the weighted influences of various explanatory variables, i.e. elevation, soil type, and land use. Temporal variability in nitrogen concentrations in the groundwater were mainly the result of variations in rainfall and in land use, characterized by different irrigation and fertilizer regimes. For exploration of the consequences of introduction of alternative land use types and crop management, a spatial dynamic model was developed to simulate nitrogen dynamics at the scale of a sub-region, consisting of a mosaic of different soil and land use types. The model was calibrated and validated on the basis of two years of measured mineral nitrogen (both NO3 and NH4) concentrations under different land use types with different irrigation and fertilizer regimes. Simulated results showed annual leaching losses varying from 88 to 122 kg N ha–1 in flowers, 64 to 82 in vegetables of the ‘cabbage group’(i.e. paprika, cabbage, eggplant, kohlrabi), 51 to 76 in chili, 56 to 75 in vegetables of the ‘squash group’(i.e. cucumber, tomato, pumpkin), and 36 to 55 in rice. The model, after further calibration and validation, is a useful tool for regional environmental assessment, and management support. The study has indicated that current agricultural developments in Tam Duong district, that are strongly influenced by (short-term) economic drivers, present a serious threat for the quality of the natural resource base, particularly soil and water and thus for the sustainability of the production systems. The obvious conflicts between the various objectives need to be addressed through integrated analysis in which the various stakeholders are involved. Keywords: Soil degradation, remote sensing, watershed, soil erosion model, paddy fields, terraces, water balance model, nitrogen balance model, geostatistics, rice-based systems, spatial dynamic model. Preface In 2002, I joined the Systems Research for Integrated Resource Management and Land Use Analysis in East and Southeast Asia (IRMLA) project that aimed at combining research into future-oriented design of agro-technologies with evaluation of stakeholder-negotiated choices for sustainable land use options at different decision levels (farm, district, and province) and supportive policy measures. I would like to thank Tran Thuc Son and Chu Thai Hoanh for introducing me to this project and the associated Wageningen scientists. Through this project I met Herman van Keulen and Reimund Roetter, who became my promotor and co-promotor, respectively. Thank you Herman, for your warm and patient support and for finding time to comment on both the English language and the substance of my draft chapters, until they finally were combined in this thesis. I would not have finished my thesis without your help. Many thanks go to Reimund Roetter for his guidance and comments. The discussions with you helped to keep my work on track, which greatly improved the quality and relevance of my research. I also want to thank Coen Ritsema, who first suggested the use of a soil erosion model and introduced me to his skilful and kind colleague, Rudi Hessel, who also became my co-promotor. I want to thank him for his guidance and the contributions to the thesis. Thank you very much Rudi, you are very kind and enthusiastic. I benefited a lot from discussions with Chu Thai Hoanh during formulation of my project proposal, and on soil erosion and water balance modeling. He always motivated and inspired me to continued dedication to my research. I also want to express many thanks to Don Acton and Thai Phien for their strong support to my research. From Alterra, I would like to thank Erik van den Elsen for helping me in ordering measuring equipment and Jannes Stolte for the many helpful suggestions about field measurements. Let me express my thanks also to Joost Wolf for his interest in my work and the many suggestions and discussions that helped in creating a clearer picture of my study, and to Tommie Ponsioen for sharing his technical coefficient generator and his knowledge of the study area. This study involved many trips to Tam Duong, and the people there helped me in making the trips productive and pleasant. The IRMLA team in Vietnam provided valuable material and information on the study area. Mr. Dang Hai Trieu, vice-director of Vinh Phuc Soil and Fertilizer Research Center helped me in identifying the study area. Many thanks to Mr. Pham Dinh, who was a great help in taking care of the rain gauge and data logger, and in taking soil and water samples. Many thanks to Mr. Nguyen Hue, the Head of Quan Dinh cooperative, who intensively supported me in selecting the watershed and the terraces for the experiments, provided me with expert information on soils, crops, and farming activities in the study area, and especially for the hospitality shown by him and his family, with very good food and an excellent atmosphere during my visits. Also many thanks to Mr. Nguyen Van Ke and his son for helping me in intensive soil and water sampling during two years in his own and his neighbour’s fields. I am indebted to Gon van Laar for doing a wonderful job in editing this thesis. I have appreciated very much your help with critical comments and ready and transparent answers. Ken Giller, many thanks for your hospitality and guidance during lunch meetings and in meetings of the soil fertility discussion group. Thanks to Ria van Dijk and Charlotte Schils for providing excellent administrative support.
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