Dynamics of soil and vegetation during crop and fallow period in slash-and-burn fields of northern Laos

Slash-and-burn rice production systems in northern Laos are undergoing dramatic changes. Increased population pressure and regulations limiting access to land have resulted in shorter fallow periods. Limited information is available on nutrient dynamics in slash-and-burn systems of Southeast Asia in general and particularly on effects of reduced fallow length. Crop and fallow effects on soil parameters and fallow vegetation were quantified in slash-and-burn fields in Luang Prabang, northern Laos from 1991 to 1994. Over the cropping season from May to October declines of 8, 7, and 3% organic C and 33, 40, and 53% extractable P, were observed for the depth intervals of 0-3, 3-10 and 10-25 cm; respectively. Over the same period extractable K declined by 34% in the 0-3 cm interval and increased by 15 and 17% in the 3-10 and 10-25 cm intervals. The declining trend continued over the 3 year crop-fallow cycle with losses (depth 0-100 cm) of 29 +/- 7.6 t organic C ha(-1), 2.0 +/- 1.1 t total N ha(-1), and 0.7 +/- 0.8 t extractable K ha(-1). At the end of the fallow period the above ground biomass contained 100 kg N ha(-1), 5 kg P ha(-1), and 140 kg K ha(-1). The fallow vegetation was dominated by Chromolaena odorata with a gradual succession towards tree and bamboo species. The nutrients in the above ground fallow vegetation represent only a small fraction of the N and C lost due to mineralization and leaching. With the present no-till system, mineralization losses are far more serious than losses due to soil erosion. Short fallows will result in a fast decline and low equilibrium of soil organic C levels, reducing the potential for rice yields and limiting farmers choice for other land use options which may become available with better market access.