Impact of topography and sedentary swidden cultivation on soils in the hilly uplands of North‐East India

Abstract

A hilly catchment (4 km2) was selected to quantify the changes in physico‐chemical soil properties when traditional shifting cultivation is converted to sedentary swidden cultivation (elements of slash and burn cultivation with short fallow period), locally called bun (cropping in rows of raised beds formed from soil), in the central part of the Meghalaya Plateau. Assuming that the monsoonal climate, deep‐weathered granites, and time are relatively uniform over the small catchment, the differences in soil were compared on two contrasting landforms (flat ridges and steep slopes) and three land use types (natural deciduous forest, sedentary swidden cultivation of potatoes, and fallow land with pine forest) within each landform. In contrast to previous studies in region, soil fertility indices calculated in the present study indicated that the bun system can also improve soil quality. Soil response significantly varied at spatial scales, however, and in terms of the possibility of continuing sustainable cultivation in the future. At a local scale on flat terrain upon granites, soil under swidden cultivation had the higher fertility indices, than fallow land and natural forest, whereas steep slope cultivated soil had the lowest fertility indices, following fallow land and natural forest. Therefore, bun system can be efficient in tropical regions with gentle terrain and limited forest resources. At a regional scale, low potato yield combined with a growing food demand forces farmers to expand potato cultivation on steep slopes, the dominant landform in the Meghalaya, increasing the risk of soil degradation. Introducing agroforestry with pine trees into bun cultivation can mitigate the rates of soil degradation on steep slopes.

Response of Water Chemistry to Long-Term Human Activities in the Nested Catchments System of Subtropical Northeast India

“…In effect, the population density reached an average of 400 inhabitants km −2 in the studied region, and almost 1500 inhabitants km −2 locally in small catchments [40]. Old deforestation and demographic explosion meant that present-day vegetation is restricted to the mosaic of natural semi-evergreen deciduous forest, secondary pine forest, and grassland interspersed with settlement and cultivated land ( Figure 1C) A shortage of agricultural land due to population growth caused the development of intensive agriculture, combining elements of traditional shifting cultivation with sedentary intensive cultivation [30]. This approach is based on natural soil fertilization (without the use of chemical fertilizers) by burning biomass under soil cover and tillage twice a year.…”

“…Within grasslands, vegetation biomass with dense roots is sufficiently high to increase the ion pool in the topsoil more than in forest or cultivated land (Table 2). In the Nongkrem catchment, grasslands are used as pastures and for the collection of biomass for burning on cultivated fields [30]. Grasslands are also densely covered by granite outcrops (boulders).…”

“…Forest was not associated with chemical elements in either season. Forest soils of the Nongkrem catchment are poor in chemicals [30] Table 2). Fast litter decomposition and nutrient uptake lead to the ion stock being held mainly in living biomass [62].…”

“…Most naturally occurring Al resides in feldspar, chlorite, and biotite of granites in the Nongkrem catchment [33,34]. Their weathering and alteration products lead to a high content of Al in soils [29,30] (Table 2). Additional anthropogenic sources of Al include remnants of iron smelting slag [37].…”

“…It was hypothesized that the low ion pool previously observed in old and highly weathered soils of this region [29,30] should be reflected in natural low ion concentrations in stream water. It was further suggested that changes in dissolved solute loads under the anthropogenic impact (i.e., various population densities and different LULCs) would be readily observed in small catchments with homogenous climates, geologies, topographies, and soils, as well as similar atmospheric deposition due to their small geographical area.…”