Organic horizon

O, organic horizons A, surface mineral horizons 1-3, subhorizons. 

Fig. 2. Relationship between log Koc of phenanthrene and the fraction of aromatic carbon in the humic acids extracted from soil horizons as determined by 13C NMR. O, organic horizons A, surface mineral horizons 1-3, subhorizons. Modified from Xing (2001).

Krauss and Wilcke examined the TiC -photocatalyzed oxidation of 12 PCB congeners [and 20 polycyclic aromatic hydrocarbons (PAHs)] on various soil samples (four mineral topsoil horizons, six organic horizons, and four particle-site fractions in three different soils) [107]. When the Ti02/soil mixture was irradiated in the absence of H2O, no photooxidation of the chlorobiphenyls occurred. When slurried with water, however, chlorobiphenyl concentrations decreased by 40-50% after 48 hours of irradiation, while the PAH concentrations were unchanged. By way of contrast, PAHs and PCBs doped onto quartz sand diminished by 95-100% after 8 hours of photolysis. The pollutants are clearly more accessible to hydroxyl radicals on sand than on soil. It is also clear that the photooxidation occurred in the soil and not in solution. Thus, OH is generated on one surface (Ti02), diffuses in the water to the other surface (soil), where the oxidation occurs. 

In general, the DOC of streams in forested catchments is formed in soil organic horizons, while minimal amounts of stream DOC are accounted for by through fall or atmospheric deposition (Schiff et al., 1990). Riparian flow paths can account for the greatest proportion of DOC exported to headwater streams (Hinton et al., 1998). The 6 C of stream DOC can also serve to distinguish between terrigenous-derived DOC and that derived in situ in streams in cases where decomposition of phytoplankton is a major DOC source (Wang et al., 1998). 

As decomposition proceeds, the C N ratio declines as organic carbon is lost from the system as CO2, and the C N ratio of stabilized SOM approaches the C N ratio of the microbial biomass. In a forest soil, Scheu and Parkinson (1995) found that the C N ratio dropped from 37.5 in fresh litter to 20.7 in the organic horizons and to 9.3 in the first mineral horizon. 

Sample Collection and Worioip. Samples were collected from a watershed in the Penobscot Experimental Forest, Bradley, Maine. Organic horizon material was collected from a deciduous and coniferous she. Approximately 12 L of water was taken from the Blackman stream. Finally, a wetland sanq>le was taken from a sedge marsh bordering the stream. Holes 20 cm wide were dug whbh promptly filled whh sediment laden water which was collected for analysis. 

In Ihe early literature dealing with soil acidity, soils were characterized by their percent base saturation at specified pH levels. Soils with low percent base saturation values were considered to be dominated by kaolinite and hydrous oxide minerals, but soils of high percent base saturation were considered to be dominated by 2 1-type minerals, such as montmorillonite, vermiculite, chlorite, and the micas. Base saturation is a criterion of soil taxonomy in the U.S. soil classification scheme. Fifty percent base saturation (based on soil CEC at pH 7) is one criterion for distinguishing between mollic epipedons (dark, high organic horizons) and their umbric (low organic) counterparts. 

Kruyts N and Delvaux B (2002) Soil organic horizon as a major source for radiocaesium biorecycling in forest ecosystems. J Environ Radioact 58 175 — 190. 

Overlying organic horizons 1.1 humous without admixtures of ... 

In the spring water, except for U, Sr, B and F, trace element concentrations are low, usually lower than in the local rain water, suggesting that these elements were retained in the soil. On the Ca-Si-Mg diagram (Fig. 8C) the soil horizons of the aquifer, from the bottom to the top of the profile, become progressively enriched in Si relative to Ca and Mg (except the two uppermost organic horizons). The waters typically become enriched in Ca, Na, F and SO4. The latter is usually attributed to the oxidation of FeS2 present in small amounts (Pfeifer et al., 1992). 

The sampling was performed at the end of September 1998. At each of the three sampling sites, three Populus tremuloides were uprooted carefully (for a total of nine trees). The three trees are the field replicates used to establish site variability. All soil samples are taken from the upper B horizon (15-20 cm under the organic-mineral interface). This horizon enables the collection of enough roots to provide sufficient rhizosphere mass for chemical analyses, while being deep enough to avoid the lack of contrast between the rhizosphere and the bulk soil that is found in organic horizons. The root diameter was between 0.5 mm and 1 cm. 

---