Rannikkodyynien hiekan hydrofobisuus, esimerkkinä Vattajanniemen ja Yyterin dyynialueet

In this Master's thesis the incidence of soil water repellency on coastal dune sands in boreal environment is investigated. The focus is on evaluating the severity of occurring hydrophobicity and identifying the factors causing it. Soil water repellency is a reduction in the rate of wetting caused by the presence of hydrophobic coatings on soil particles. Hydrophobic organic compounds derive from certain fungal species, surface waxes of plant leafs, exudates produced by plant roots and soil microbes, and decomposing organic litter. Surfaces with a surface free energy less than the surface tension of water, are hydrophobic in character. Weak infiltration leads to increased surface runoff, which in turn enhances erosion by water and wind. The research areas are Vattajanniemi in Lohtaja and Yyteri in Pori. Both dune areas are located on the western coast of Finland. In total 230 soil samples were collected from the research areas. The samples were dried in 50 - 55 °C temperature and analyzed in laboratory. The samples were divided in subgroups based on the vegetation growing on the sample sites. On the investigated white dunes the dominating species was sea lyme grass and on the grey dunes either sea pea or lichens. On the forested dunes the vegetation was either pine tree forest or fertile deciduos forest. A few samples were also taken from deflation surfaces between the dunes. Additionally, some of the grey dunes were recently burned at the time of the sampling, and the research is also focusing on the effect of fire on soil water repellency. The samples from Vattajanniemi were collected from the surface layer of the soil and the samples from Yyteri were collected from different layers of the soil profile. The sampling depths in Yyteri were: 0 - 5 cm, 5 - 10 cm, 10 - 20 cm, 20 - 30 cm, 30 - 40 cm and 40 - 50 cm. The severity of water repellency was assessed by measuring Water Drop Penetration Time (WDPT) in seconds. The organic carbon content of the soil samples was measured using loss on ignition analysis. Part of the samples were additionally sieved, and grain size parameters were calculated from the results. Soil water repellency was a common feature on the investigated areas. Especially strong water repellency was found on dunes covered by lichens. The surface soil samples taken from the forested dunes and burned areas were also all water repellent. The bare deflation surfaces and the white dunes covered by sparse vegetation were mostly hydrophilic or slightly hydrophobic in character. The organic carbon content in the soil samples was principally less than 1 %. There was a strong positive correlation between water repellency and the amount of organic matter, but the best individual explaining variable according to regression analysis was the type of vegetation. The best results were gained with a general linear model when the explaining variables were the type of vegetation, the amount of organic carbon as first and second degree equation and the covariation between these variables. The model explains approximately 75 % of the variation in the WDPT test results. The texture was very similar between the samples. Standard deviation was the only statistically significant sand-sorting characteristic, and it had a weak correlation with the WDPT test results. Water repellency decreases with increasing depth principally because the amount of organic matter decreases. The general linear model explained 97 % of the variation in the WDPT test results when the explaining variables were the type of vegetation, the amount of organic carbon, the sampling depth and the covariation between the variables.