Natural and human impact on the land use and soil properties of the Sikkim Himalayas piedmont in India

Natural and human causes of change in land use and soil properties were studied in the Sikkim Himalayas piedmont over the last 150 years, with a special emphasis on the period 1930-2010. Analysis of historical reports, combined with the visual interpretation of topographic maps and satellite images, indicates that the land reforms related to the location of tea gardens caused rapid deforestation of the higher elevated terraces in the late 19th century. Continuous population growth between 1930 and 2010 caused a shift in the major land use changes from the terraces to the floodplains. As a consequence, a gradual extension of tea plantation and forestry development helped in stabilizing the land use of the terraces, while the parallel deforestation of mountain catchments and floodplains for rice cultivation intensified fluvial activity. The enlargement of river-channel area by about 42% between 1930 and 2010 excluded a large part of the floodplains from cultivation and increased risk of soil degradation. The replacement of natural forest by monocultural tea and rice cultivation influenced the physical and chemical properties of the soil. Statistically significant changes were observed only in some chemical properties of the topsoil. Tea cultivation reduced the total carbon content by 26% and total nitrogen content by 33% in the surface soil horizon. The influence of rice tillage on the soil properties is masked by the fluvial activity. The combined effect of flooding and rice cultivation is reflected in the lower content of total carbon and nitrogen in the surface of the soil, namely, 76% and 77% respectively. Taking into account the long-term nature of the plantation, the soil still has the capability to support tea production. The productivity of rice depends partly on fertilization levels and partly on the natural deposition of fresh sediment eroded from mountains.