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Soils of the Watershed of Lake Atitlán, Guatemala

其他書名	Their Role in Eutrophication with a Focus on Phosphorus Sorption
出版	University of California, Davis, 2014
ISBN	132136217X9781321362176
URL	http://books.google.com.hk/books?id=DhhhrgEACAAJ&hl=&source=gbs_api

註釋This thesis is divided into two distinct chapters. The first is focused on the extent of phosphorus sorption in the soils surrounding Lake Atitlán, Guatemala. The second chapter is regarding the use of the same soils in a lake based bioassay, determining the extent of nitrate and phosphate release from the soils and the promotion of algal growth. Soils are known for their capacity for phosphorus (P) retention, volcanic soils even more so. This characteristic can be problematic for agriculture, as soil can be deficient in P levels necessary for crop yield. P from subsequent fertilizer additions can then be occluded as well, prompting growers to use higher quantities of fertilizer to overcome the P retention, or sorption. Fertilizer overuse can be detrimental to local waterways, increasing nutrient loads and leading to eutrophication. In addition, agricultural soils can change from a sink to a source of P once introduced into an aquatic ecosystem. The P sorption properties, however, can be used to alleviate a high P load by allowing the soil to remove it from aqueous waste. Lake Atitlán, an endorheic caldera located in the western highlands of Guatemala, is facing eutrophication as the input of nutrients from raw sewage, fertilizer overuse, and general human development has increased over time. The purpose of this study was to determine the extent of P sorption and the fate of sorbed P from soils collected from the near shore watershed of Lake Atitlán. Soil samples collected from various land-uses, stratified by the age of volcanic deposits around the lake, were subjected to P sorption batch assays: samples were flooded with a solution containing phosphate along a concentration gradient from 0 to 1250 mg PO4 -P kg soil−1. After 24 hours this solution was removed and soluble reactive phosphate (SRP) was measured; the difference between the final and the initial concentrations is the sorbed fraction. The same samples were then dried and reflooded with a solution containing no phosphate; after 24 hours they were reanalyzed for SRP and the level of desorption was determined. The data were used to calculate the sorption affinity, the maximum sorption, the equilibrium phosphorus concentration (EPC0), and the phosphate saturation ratio (PSR). Dividing the soils into ages and land-use categories showed a few trends: the younger soils and the cropped and forested soils had higher levels of nutrients than the older soils and disturbed soils. The cropped soils had the lowest sorption affinity with the highest maximum sorption, due to the slash-and-burn agricultural practices. There is a high diversity of P retention in soils from the Atitlán basin, many soils initially desorbed phosphate at low levels, while others sorbed up to 90% of phosphate at the 1250 mg P kg soil−1 level. The desorption test showed that all analyzed soils were efficient in retaining the sorbed fraction, and even continued sorbing additional phosphate. The EPC0 - the concentration at which phosphate is neither sorbed nor desorbed from the soil - ranged from 0 to 6 mg P L−1, with an average of 0.17. Many soils had a zero EPC0; these soils have a high affinity for phosphate and do not readily desorb. The PSR, a measure of sorption capacity and P saturation of the soil under ambient conditions, indicates the soils are not saturated with P; the amorphous aluminum and iron sorption sites are all under 14% saturated. The disturbed soils, mainly landslides, had the highest affinity with the lowest sorption maximum; however, the affinity combined with the low levels of nutrients present in these soils allow the disturbed areas to be candidates for accepting runoff for P removal. But care should be taken to ensure the soil is retained, the EPC0 values indicate that although sorption is high in most soils, once soils are introduced into the water they will desorb P into the lake. The results of this study can also be used to better inform local farmers of best management practices in the application of fertilizer to both increase yield and protect Lake Atitlán from further eutrophication. A bioassay was performed in Atitlán to better understand the role of soils as a nutrient source or sink in fresh water eutrophication. The region is affected by erosion, landslides, and runoff, all of which deposit allochthonous material into the lake. Recently the lake has been experiencing seasonal cyanobacterial blooms of the nitrogen fixing Limnoraphis robusta, and it is highly probable that the allochthonous inputs are contributing to the bloom occurrences. Soils were added to clear plastic bottles filled with lake water at a 7.6 g soil L−1 ratio and incubated at in situ conditions for 4 days. The majority of samples had increased levels of dissolved inorganic fractions of N and P, which in turn increased phytoplankton growth. There was a high range of values for both nitrate and phosphate release. This high variance is reflected in the statistical results - comparing the soils to each other proved non-significant in most cases, but all soil treated bioassays were significantly different from the initial lake water concentrations and the non-soil lake water control. Increased nitrate and phosphate desorbed from soil material promoted the growth of phytoplankton, expressed as the concentration of chlorophyll a. Thus, future erosion of soils into the lake will result in further eutrophication. Results from this experiment can be improved through further testing, using a bioassay with more treatments and analyses to better understand the chemical, physical, and biological factors involved in nutrient cycling in lake water with introduced terrestrial soils.