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Differences in soil water characteristics of monoculture oil palm plantations, agroforestry oil palm plantations and natural forest

Oosterhout, M.J. van (2022) Differences in soil water characteristics of monoculture oil palm plantations, agroforestry oil palm plantations and natural forest.

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Abstract:Due to a growing population, more stress is put on food production all over the world. This causes the need for more agricultural land, and more productive agriculture. The first need can result into deforestation all over the world. While the second needs research into how more productive agriculture could be reached. Forest makes place for monoculture plantations, which have a negative impact on the climate and ecosystem of an area. Therefore, more sustainable options should be found for this problem. University Gadjah Mada (UGM) in Indonesia works together with the Kehati Foundation to transform monoculture oil palm plantations in Jambi province, Sumatra, into agroforestry plantations which are expected to have a better influence on the climate and ecosystem. However, this has not been proven yet. Therefore, this research focusses on soil water characteristics, namely water retention curves and infiltration rates, one of the important factors which can help provide more insight in the impact of the change of monoculture plantations to agroforestry plantations, but also the differences compared to natural forest. The impact of these different land uses on the soil-water characteristics can then be linked to the growth of vegetation. To answer this question, first the research areas were investigated: natural forest, old agroforestry (more than 10 years), new agroforestry (around 2,5 years), and monoculture. Data was gathered on mostly visual features of the different areas like soil colour, vegetation density, organic material in the surface layer and the bulk density. This was done to make a fair comparison between the different land use types and explain the differences between them. From this research there was found that the vegetation density of the forest is the highest followed by the new agroforestry plot. The old agroforestry and monoculture both had a really low vegetation density. This corresponded with the thickness of the organic matter layer which was the largest for the natural forest. The average bulk density was 1,15 g/cm3 for the natural forest and around 1,56 g/cm3 for the other land use types. This difference can be explained by the compaction of plantation plots, while preparing or working on the plots. Two different soil water characteristics were investigated. The first characteristic, water retention curves, was tried to be established by making use of the filter paper method. However, these results turned out to be not useable. This could have been caused by several reasons but based on literature research the unusable results were most likely caused by the use of wrong equipment. For the second characteristic the infiltration rates were determined for the different land use types by using a double infiltrometer. On the measured data an infiltration model was fitted, namely the Horton equation. It was found that there was a lot of variability within one land use type for all land use types, but this variability was hard to explain. The constant infiltration rate for the monoculture and old agroforestry was really low, with means of 0,38 cm/h and 0,56 cm/h respectively. The average constant infiltration rates for the forest and new agroforestry were higher with values of 10,37 cm/h and 5,76 cm/h respectively. As the water retention curves could not be used, the porosities of the land use types were compared. This showed that both agroforestry plots had the lowest porosity (porosity new agroforestry = 0,28, porosity old agroforestry = 0,29). The monoculture plot had a similar porosity of 0,31, while the forest had a way higher porosity of 0,44. This could be partly explained by the bulk density found. The parameter values of the Horton equation on the infiltration rates, namely the k-value and constant infiltration rate, were compared. A lot of factors can influence the initial infiltration rate; therefore, this factor was not compared between different land use types. For the constant infiltration rate, it turned out there is a significant difference between the values of the forest and new agroforestry and the values of the old agroforestry and monoculture, despite the big variability within each individual land use type. Both the natural forest and new agroforestry have significantly higher constant infiltration rates compared to the other two land use types. The differences between the old agroforestry and monoculture are not significant. This is the same for the new agroforestry and natural forest. The decay factor gained from the Horton equation was also compared between land use types. For this there was only a significant difference found between the new agroforestry plot and the other three land use types. The decay factor for the agroforestry is lower, which means in general it takes longer for the soil to reach a constant infiltration rate. The methods chosen and choices made during the project could have influenced the outcomes of the research. One of the points up for discussion is the available data. There was almost no data available on other factors which could influence the ground water characteristics investigated (for example on soil texture, amount of organic matter, etc.), which made it hard to explain the results. Besides, the precision of the double infiltrometer set up and measurements could have given slight deviations in the actual infiltration rates, due to human factors as well by factors caused by the procedure. Generally, the conclusion can be made that the natural forest has the most optimal soil water characteristics, namely the highest constant infiltration rate and the highest porosity, followed by agroforestry (with new agroforestry having a way more positive effect than old agroforestry). Monoculture as a negative effect on the soil water characteristics investigated. To further substantiate this conclusion more research into this subject is recommended. Even though the porosity might say something about the water retention curves, more research should be done in the actual water retention curves of the different land use types. Also, more data should be gathered on the different land use types and their features to explain the differences in soil water characteristics. Lastly more data on infiltration rates and water retention curves at other locations outside the research area of this report can be investigated to use the outcomes of the research in a broader scale.
Item Type:Essay (Bachelor)
Faculty:ET: Engineering Technology
Programme:Civil Engineering BSc (56952)
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