Effects of shifting cultivation on soil ecosystems in Sarawak, Malaysia, 4: Chemical properties of the soils and runoff water at Niah and Bakam experimental sites

Experimental shifting cultivation was carried out at two sites, Niah and Bakam in Sarawak, Malaysia. The plots (10 x 10 m(2)) were treated with burning of vegetation biomass (fuel), amounting to 100, 200 and 300 Mg ha(-1) with an untreated control at the Niah site and 20 and 100 Mg ha(-1) with an untreated control at the Bakam site. At the Niah site, the soils were acidic with a light clay texture. The levels of pH, exchangeable bases and available P of the soils at the depth of 0-5 cm increased by burning while those of exchangeable Al and Al saturation decreased. The changes tended to be more appreciable in the plot treated with fuel at the rate of 300 Mg ha(-1) than in the other plots. Based on a comparison between ash alkalinity and the difference in the content of exchangeable Al at 100 d after burning and before burning, it was assumed that 60% of the alkalinity contained in ash was consumed for inactivating exchangeable Al at the depth of 0-5 cm, which resulted in minimal changes in the soil properties in the deeper layers. The influences of burning persisted at the depth of 0-5 cm in all the burned plots until 1 year after burning, except for exchangeable K and available P. On the other hand, at the Bakam site, the soils showed an acidic nature with a sandy loam to sandy clay loam texture. The changes in the pH values and levels of exchangeable bases and AI were observed only at the depth of 0-5 cm in the plots treated with fuel at the rate of 100 Mg ha(-1). During rice cultivation, these values returned to the levels recorded before burning. Comparison of the amounts of nutrients among soils, ash and runoff water during rice cultivation at the two sites showed that 1) the losses of ash nutrients by runoff water, except for N, were negligible compared with the amounts contained in the soils and ash, 2) the N input from ash and output by runoff water were not significant, compared with the large stock in the soils, and 3) the ash addition significantly contributed to the supply of inorganic bases (Ca, Mg, K and Na) and P.