Ph.D. - Tropical Plant and Soil Sciences
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ItemPredicting Sediment Export into Tropical Coastal Ecosystems to Support Ridge to Reef Management([Honolulu] : [University of Hawaii at Manoa], [May 2016], 2016-05)Sediment, a principal land-based source pollutant, can negatively affect coral reef ecosystems and impact the services they provide to society, such as recreation and fishing. Watershed managers interested in evaluating options and trade-offs require decision support tools that predict sediment export and retention. However, sediment load has previously been difficult to model in Hawaii because 1) observed annual sediment load data is not widely available and 2) Hawaii’s volcanic hydrology makes it difficult to use models designed on the continent. One method to estimate annual sediment loads is by measuring deposition rates in reservoirs. As of 2013, Hawaii had 140 regulated water storage reservoirs that perform as retention basins, yet the extent of their functionality had not been analyzed. I conducted a state-wide survey of reservoir capacity. Results demonstrated that only 15% of the total water storage capacity of Hawaii’s reservoirs is currently used, and 40% of all reservoirs are affected by sedimentation, representing a significant maintenance cost and potential risk. I also investigated spatial patterns of a broader array of ecosystem services including sediment retention, nitrogen retention, water yield, carbon sequestration, and agricultural production in West Maui. Between 1778 and 1920, I determined that sediment export increased by 18 times, while nitrogen export increased by 11 times over the same period. I demonstrated that past impacts from agriculture were more severe than predicted future development and climate change. Lastly, to calculate the first state-wide estimates of annual sediment export, I used a RUSLE-based model coupled with an estimate of the sediment delivery ratio (SDR), and adapted input datasets, including the erodibility and cover factors. To calibrate the model, I analyzed 60 watersheds with observed data for annual sediment load. I calculated that only 42% of the total land area contributes to sediment export, and that Hawaii and Maui export 57% of the total sediment load. Annual specific sediment yields ranged from 26 to 273 tons km-2 yr-1 across all islands, with Kahoolawe and Maui having the highest rates. These projects combined provide a base for watershed managers to identify management strategies to mitigate sediment export within an ecosystem context.
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ItemCharacterization of Dormancy, Establishment and Seed Production of Waltheria indica and Panicum torridum([Honolulu] : [University of Hawaii at Manoa], [August 2015], 2015-08)
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ItemBiochar as an Amendment to Acid Soils([Honolulu] : [University of Hawaii at Manoa], [May 2015], 2015-05)The capacity of biochar to improve acid soil productivity and enhance nutrient retention was the main focus of this study. The specific objectives were to characterize six wood-derived biochars, to assess biochars’ liming effects on Hawaiian and Indonesian acid soils, and to study nutrient retention of biochars. Six and another two biochars were collected, characterized, and then were used to evaluate their liming effect on a Hawaiian and two Indonesian acid soils with Desmodium intortum and soybean (Glycine max) as test plants, respectively. Two biochars in combination with two composts (both at 2%) as nutrient sources were used to investigate their nutrient retention with pak choi (Brassica rapa) as the test plant. The results showed that six wood-derived biochars were different in their properties, including ash content, pH, cation exchange capacity (CEC), CaCO3 equivalent, basic cations and surface functional groups. Based on their CaCO3 equivalent, leucaena (Leucaena leucocephala) and lac tree (Schleichera oleosa), Hilo mixed wood and she oak (Casuarina junghuhniana), and mahogany (Sweitenia mahagoni) and mountain gum (Eucalyptus urophylla), were grouped into the highest, moderate, and lowest liming potential biochars, respectively. Additions of six biochars at 2% and 4% with or without 2 cmolc/kg of lime to a Hawaiian acid soil increased soil pH and CEC, reduced exchangeable Al, enriched plant nutrients and enhanced Desmodium growth with lac tree and leucaena being most effective, followed by she oak and Hilo mixed wood biochars. Similar results were obtained from lac tree wood and rice husk biochars (4 and 8%) applied to two Indonesian acid soils. Addition of lac tree wood and Hilo mixed wood biochars in combination with vermicompost or thermocompost to a Ultisol and a Oxisol of Hawaii showed a positive interaction effect on EC, P and K, cabbage fresh and dry matters. Biochars increased soil pH, plant tissue Ca, retention of K, Ca and Mg, and reduced exchangeable Al in both soils. Overall, the liming capacity and nutrient retention potential of selected biochars have been positive.