Changes in soil nutrient content under shifting cultivation in the Chittagong Hill Tracts of Bangladesh

Abstract

Abstract Abstract In the Chittagong Hill Tracts (CHT) of Bangladesh, shifting cultivation is an age-old practice. Usually tribal people practiced slash-and-burn agriculture in the same area with long fallow periods of 15–20 years. But, with the rapid increase of population in the hills this rotation has come down to 3–5 years, allowing insufficient time for the soils to regain their natural productivity. Although less than 5% of the hill area is used for shifting cultivation each year, 10–20 times of the additional vegetative area beyond the shifting cultivated field can be destroyed by wild fires in relation to the burning process. Thus the combined affect of such cultivation is more extensive than it is usually predicted. The present investigation is a baseline study of the nutrient status in the topsoil and the impact of burning and subsequent runoff on changes in topsoil nutrients following land clearing and burning in a small catchment of 1 ha located on steep to very steep slopes in CHT. Analyses of topsoil samples (0–10 cm) sampled before and after burning showed significant increase in pH, exchangeable base cations (Ca, Mg, K), S, Fe, Mn and Zn but decrease in total C, exchangeable acidity, total N and available P. After having received about ⅔ of the mean anr nual precipitation, 39 ton ha−1 soil was lost by erosion. As these sediments were rich in most nutrients, this runoff removed up to 27% of the nutrient content in the upper 10 cm of soil. Losses of available Cu and P by erosion are especially important as the contents of these nutrients were very low before burning and were even reduced by the burning. Although the short time period of less than a year imposed some limitation on the present investigation, the results indicate that traditional shifting cultivation with a short fallow period may cause severe degradation of those soils on steep to very steep slopes in a very humid climate (∼3000 mm rain per year). Geografisk Tidsskrift, Danish Journal of Geography 100: 37–46.