Silicon is a beneficial element for rice plants and is one of the major factors affecting the sustainability of rice production. We investigated silica (Si) availability and dynamics in soils of sawah, other land uses, and also in river and canal water in two watersheds in Citarum and Kaligarang, Java Island, Indonesia. The term sawah refers to a leveled and bounded rice field with an inlet and an outlet for irrigation and drainage, respectively. In the present study, we examined Si content in soils, plants and river water in relation to factors influencing the Si content, such as parent material and land use. The available Si content in sawah was found to be deficient at two sites and low at 10 sites out of 16 sites investigated in the Citarum watershed. In the Kaligarang watershed, no sawah site was classified as deficient and nine out of the 15 sawah sites were determined to be low for rice plant growth. A survey of Si content in rice flag leaves in some selected rice fields showed that seven out of 12 samples had contents less than 125 g SiO 2 kg -1 ; these rice samples with low Si contents were those in sawah classified as low in Si contents. In the Citarum watershed, sawah soils developed from the accumulation of lake and clay sediment contained relatively little available Si, while sawah soils in the Kaligarang watershed that were mainly developed from tuff and volcanic ash contained relatively more available Si. In the Citarum watershed, the type of land use influenced Si availability in the soils via a large amount of litter accumulation of pine trees in the case of pine plantations, and acidification in the soils in the case of tea plantations and maize fields. In general, the Si content in river and canal water was higher in the Kaligarang watershed than in the Citarum watershed, and this appears to be affected by the type of parent material. In addition to the type of parent material, Si depletion occurring in dams might also influence Si content in the lower stream of river or canal water in the Citarum watershed.
Quantitative evaluation of soil erosion rates provides important baseline data to investigate, manage and improve land use systems. However, soil erosion analyses have not been sufficiently conducted in Indonesia. In the present study, we investigated the spatial distribution of soil erosion rates in relationship to land use patterns in the Sumani agricultural watershed, the primary rice-producing region in West Sumatra. The soil erosion rate was estimated applying the Universal Soil Loss Equation (USLE) using representative soil sur vey data obtained in the watershed. Sediment deliver y ratio (SDR, i.e.sediment yield / soil erosion rate) was determined from sediment yield data, which was generated in a previous study. Soil erosion rate in the Sumani watershed was estimated as . Mg ha -y -in and . Mg ha -y -in (annual averages).These values are far greater than the tolerable soil erosion rates (TER) for Indonesia i.e. Mg ha -y -. Natural factors, including heavy rainfall and local soil properties in a landscape susceptible to soil erosion were the fundamental factors responsible for the high soil erosion in the watershed. In addition to these causes, changes in land use accelerated soil erosion. From to , the soil erosion rate showed a % increase due to forest conversion to agricultural fields. SDR in five sub-watersheds exhibited relatively small values ranging from % to %, indicating an accumulation of eroded soil particles in flat areas in the lower part of the watershed where the land is primarily allocated to sawah.
The addition of beneficial nutrients as silicon (Si) has become a concern in rice cultivation in some countries. However, it is not fully adopted in Indonesia. It is known that Si plays an important role on improving plant resistance on biotic and abiotic stresses. A pot experiment was conducted to study the response of rice plant growth on Si addition in Oxisols. A completely randomized design was set with eight treatments as varying Si rates (0, 100, 200, 300, 400, 500, 600 and 700 kg SiO2 ha−1) and three replications. Some variables were observed as plant height, number of tillers, stem strength, lodging resistance, biomass and yield components. Related to stem strength and lodging resistance, the observation was conducted prior to harvest. The results showed that initial soil Si available was 264 mg SiO2 kg−1 and categorized below the critical level. Si application showed significant effect on increasing number of tillers, stem strength, lodging resistance and the yield which occurred at 700 kg SiO2 ha−1 treatments. The rice yield increase by 34.66% at 700 kg SiO2 ha−1 treatments compared to control. This current result showed that addition of Si fertilizer has showed the effect on improving rice growth and yield in Oxisols.
The conversion of peat soils to agricultural uses has been thought to increase CO2 emission due to several factors, including fertilization. However, evidence on the effect of fertilization on CO2emissionsfrompeat soils is rareand often inconsistence. We measured the effects of different types of fertilizer, including N, P and K sources, and clay as an ameliorant on CO2 emission from a bare peat soil in Lubuk Ogong, Riau Province. Nutrients were added in the following combinations: 0 (unfertilized plot), N source (urea), slow-release N (slow release urea), N and Psource (Urea+SP-36), N, P and K sources (urea+SP-36+KCl) and combined NPK-Clay. Fertilization resulted in a decreasein CO2 emissions compared to that prior to fertilization except when slow-release urea was applied. Decreasing of CO2 emissions was probably due to pH-related effects because the pH in the N treatment was lower than in both the control and the unfertilized plot. A decreasein the level of CO2 emissions among the treatments followed the order NPK-Clay>NP>NPK>urea>slow-release urea. Covariance analyses showed that the difference in CO2 emissions prior to treatment was not significant. The application of individual and combined treatments of N, P, K and NPK mixed with 5 Mg ha-1 clay led to significantly reduced CO2 emissions from bare peat soil in Lubuk Ogong, Riau Province. In addition to fertilization, the water table depth was the only parameter that significantly affected the CO2 emissions (P<0.05). We conclude that the application of nutrient combinations, including N, P, K and clay, could reduce CO2 emissions because these treatments maintain a balanced nutritional condition in the soil with respect to the microbial activity.Keywords: Amelioration, CO2 emission, fertilization, tropical peat soils
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