Effects of shifting cultivation on soil ecosystems in Sarawak, Malaysia II. Changes in soil chemical properties and runoff water at Balai Ringin and Sabal experimental sites

Experimental shifting cultivation was conducted at the Balai Ringin (B. Ringin) and Sabal sites in Sarawak, Malaysia. At the sites, plots (10 x 10 m(2)) were burned with the fuel of aboveground biomass amounting to 0 (control), 100, 200, and 300 Mg ha(-1). At the B. Ringin site, the soils were clayey and strongly weathered with a strongly acidic characteristic. Ash addition enabled to alleviate the soil acidity and to increase the amounts of nutrients of the soils, especially the surface soils. It was indicated that 1) N addition from ash to the soils was negligible, 2) the losses of nutrients by runoff water were not substantial compared with the amounts of nutrients contained in ash, 3) ash alkalinity seemed to be consumed for inactivating exchangeable Al mainly in the surface soils, and 4) development of variable negative charges could contribute to the retention of inorganic bases derived from ash. After harvest of upland rice, the soil chemical properties in the plots treated with 100 and 200 Mg ha(-1) fuel returned to the levels before burning, indicating the rapid loss of nutrients due to leaching and erosion as well as the uptake by plants. However, the soils treated with 300 Mg ha(-1) fuel still showed high contents of exchangeable bases and a low content of exchangeable Al. On the other hand, the soils at the Sabal site were sandy and were characterized by a very low nutrient status. The changes in the amounts of nutrients by ash addition were similar to those at the B. Ringin site. However, the changes in the level of exchangeable Al which were not appreciable were probably due to the low Al content. It was postulated that because of the sandy texture and low CEC of the soils, inorganic bases contained in ash were only suspended in the soil solution. Taking into account the low yield of rice and low level of secondary biomass, it appeared that most of the nutrients were lost downward in soils by leaching.