Human-induced soil fertility decline in a mountain region in Northern Vietnam

For their livelihood, most of the people in mountainous Northern Vietnam depend on slash-and-burn agriculture, and reduced yields as a consequence of soil fertility decline are a dangerous threat. The aim of this study was to examine the degree and the causes of soil fertility decline in a remote mountain area of northern Vietnam. In order to evaluate the impact of deforestation and subsequent cultivation upon soil properties, undisturbed sites under forest were compared with sites of arable land and fallow vegetation. Land use and soils were regionalised with maps, land-use history was registered by way of interviews with farmers, a digital terrain model was established and GIS analyses were carried out. Bulk density, organic matter, total nitrogen, exchangeable cations (Ca2+, Mg2+, K+, Na +) available P and cation exchange capacity were analysed. Total pore volume, air and available water capacity were estimated. Nutrients and organic matter were calculated as stocks [g or kg m-2] for the effective rooting depth and the topsoil. Compared with the reference sites, the examined parameters show a considerable humaninduced soil fertility decline: Soil depth is reduced due to erosion. 60 % of the fallow and cultivated sites are situated on steep slopes with > 30 % inclination. Soil organic matter stocks have declined by 66 %, total N by 67 %, exchangeable Ca2+ by 91 %, Mg 2+ by 94 %, K+ by 73 %, and available P by 75 %. Soil compaction has reached 40 %; pH values decrease by 2.2 units and cation exchange capacity by 56 %. While soil compaction, decline of organic matter and total N is expected not to limit plant growth, loss of basic cations reach a degree which probably leads to plant growth restrictions. The mapped land-use pattern demonstrates that soil fertility improves with increasing distance from the villages, suggesting a less intensive land-use history in remote sites.