Fog has been shown to be an important water source for coastal vegetation, but the extent to which fog influences soil processes and hydrology has not been well studied. Precipitation inputs from fog and rain, along with soil moisture and throughfall under Bishop pine (Pinus muricata) canopy, were measured at two site pairs on Santa Cruz Island, California. Each pair consisted of a forested site which received fog drip and had an isomesic soil temperature regime and a grass site which did not receive fog drip and had a thermic soil temperature regime, on two parent materials, chlorite schist (WS) and rhyolitic tuff/breccia (SB).
Fog drip measured as throughfall provided an average of 10 mm (SB) to 20 mm (WS) additional water per month to the pine sites during the summer when no rainfall is available. Soil moisture sensors at 5 cm incremental depths to about 20 cm recorded infiltration of water from fog events, confirming that fog drip can infiltrate to the rooting zone. Modeled water balances for each site indicate that actual evapotranspiration (AE) nears zero in July at the grass sites, but soil moisture continues to be utilized throughout the summer at the pine sites.
Organic matter decomposition rates are influenced by climatic factors, organic matter composition, and soil microbial communities. Both temperature and moisture can be limiting factors in organic matter decomposition, but the role of fog in determining decomposition rates has not been well studied. In a litterbag decomposition experiment performed over 3 years, decomposition rates of both pine and grass litter were highest at the pine sites, suggesting that moisture is a limiting factor in this environment. The additional moisture received as fog drip at the pine sites may accelerate decomposition processes regardless of temperature. Decomposition rates of both litter types decreased with time at the pine sites, while rates for both litters remained relatively constant at the grass sites over the duration of the experiment.
Pine forest soils were Typic Haplustalfs (WS) and Ultic Paleustalfs (SB), and grassland soils were Typic Haplustalfs (WS) and Ultic Argixerolls (SB). Soils formed under pine canopy tended to be deeper, have greater total clay, lower pH and base saturation than soils formed under grass or shrub vegetation. Higher Feo/Fed ratios from selective dissolution indicate that iron oxides in the pine soils may be less crystalline than in the grass soils, corresponding to decreased drying of the pine soils due to summer fog drip. On a total profile basis, total pedogenic iron (Fed) and Feo/Fed ratios also indicate that the pine soils are more weathered than the grass soils, and that the schist soils may be more weathered that the rhyolitic tuff/breccia soils. The apparent ratio of kaolinite to smectite, based on XRD peak intensity, is highest in the upper horizons and decreases with depth. A similar pattern is found in the WS grass soils. The SB pine soils are dominated by quartz and feldspar in the silt and sand fractions, and kaolinite in the fine clay fraction, with smectite found in the lower horizons of the pedon. The SB grass soils are dominated by smectite in the fine clay fraction, with minor amounts of kaolinite.
Fog events occurred dominantly during summer months, and were generally enriched in isotope values compared to rain events, which occurred mainly during winter months. Isotope values of soil solution indicate that grass sites are influenced by evaporation, while pine canopy sites show minimal evaporation effects. Mineralogical, chemical, and oxygen and hydrogen isotope compositions of the <0.2 μm fraction of phyllosilicate samples from a surface and subsurface horizon at each site are presented in this study. Fog influence is expected to be greatest near the soil surface at the pine site soils. Soils contained a mixture of 1:1 (kaolinite) and 2:1 (smectite) phyllosilicates, with kaolinite decreasing in proportion to smectite with profile depth. The δD values of the phyllosilicates range from -64.3 to -94.3, and δ18O values range from 19.8 to 25.1. The estimated temperatures of crystallization range from 7 to 22°C, with grass sites showing a greater difference in temperature than pine sites between surface and subsurface horizons. Grass site temperatures of formation are higher for surface horizons than for subsurface horizons, confirming the influence of evaporation. (Abstract shortened by UMI.).
|Commitee:||O'Geen, Toby, Rolston, Dennis, Zierenberg, Robert|
|School:||University of California, Davis|
|Department:||Soils and Biogeochemistry (formerly Soil Science)|
|School Location:||United States -- California|
|Source:||DAI-B 71/11, Dissertation Abstracts International|
|Subjects:||Soil sciences, Environmental science|
|Keywords:||California, Fog, Pedogenesis, Pinus muricata, Santa Cruz Island|
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