Effects of shifting cultivation on soil ecosystems in Sarawak, Malaysia I. Slash and burning at Balai Ringin and Sabal experimental sites and effect on soil organic matter

Experimental shifting cultivation was conducted at two sites, Balai Ringin (B. Ringin) and Sabal in Sarawak, Malaysia. The soils at the B. Ringin site were clayey with strongly acidic nature while those at the Sabal site were sandy with a very low nutrient status. The plots (10 x 10 m(2)) were burned with vegetation biomass (fuel) amounting to 0 (control), 100, 200, and 300 Mg ha(-1) for the cultivation of upland rice. At both sites, the fire severity was found to range from moderate to high for the treatments with 200 and 300 Mg ha(-1) fuel, and from low to moderate for the treatment with 100 Mg ha(-1) fuel. At the B. Ringin site, the content of total C (T-C) of the soils decreased during rice cultivation which could be ascribed to the rapid decomposition of soil organic matter (SOM) and the loss of SOM due to soil erosion. One year after burning, the T-C content returned to the initial level before burning, suggesting that SOM was replenished by the addition of fresh organic matter derived from rice straw and other dead plants. The effects of burning on soil microbial properties which tended to be more pronounced with the increase of the amount of fuel, persisted until 1 year after burning. Burning resulted in a yield of 850 kg ha(-1) of rice on the average, irrespective of the amount of fuel used. On the other hand, at the Sabal site, the T-C content decreased dramatically during rice cultivation. It was assumed that a portion of SOM was washed away by rainfall because of the sandy texture of soil and coarse fragments of SOM. Rice yield was very low due to the low nutrient status of soil and pest infestation.