Most of the thousands of substances and species that risk assessment has to deal with are not investigated empirically because of financial, practical, and ethical constraints. To facilitate extrapolation, we have developed a model for accumulation kinetics of organic substances as a function of the octanol-water partition ratio (Kow) of the chemical and the weight, lipid content, and trophic level of the species. The ecological parameters were obtained from a previous review on allometric regressions. The chemical parameters, that is, resistances that substances encounter in water and lipid layers of organisms, were calibrated on 1,939 rate constants for absorption from water for assimilation from food and for elimination. Their ratio was validated on 37 laboratory bioconcentration and biomagnification regressions and on 2,700 field bioaccumulation data. The rate constant for absorption increased with the hydrophobicity of the substances with a Kow up to about 1,000 and then leveled off, decreasing with the weight of the species. About 39% of the variation was explained by the model, while deviations of more than a factor of 5 were noted for labile, large, and less hydrophobic molecules as well as for algae, mollusks, and arthropods. The efficiency for assimilation of contaminants from food was determined mainly by the food digestibility and thus by the trophic level of the species. A distinction was made between substances that are stable, that is, with a minimum elimination only, and those that are labile, that is, with an excess elimination probably largely due to biotransformation. The rate constant for minimum elimination decreased with the hydrophobicity of the substance and the weight of the species. About 70% of the variation was explained by the model, while deviations of more than a factor of 5 were noted for algae, terrestrial plants, and benthic animals. Labile substances were eliminated faster than isolipophilic stable compounds, but differences in laboratory elimination and accumulation were small compared with those in field accumulation. Excess elimination by vertebrates was faster than by invertebrates. Differences between terrestrial and aquatic species were attributed to water turnover rates, whereas differences between trophic levels were due to the food digestibility. Food web accumulation, expressed as organism-organic solids and organism-food concentrations ratios could be largely explained by ecological variables only. The model is believed to facilitate various types of scientific interpretation as well as environmental risk assessment.

The power of size. 1. Rate constants and equilibrium ratios for accumulation of organic substances related to octanol-water partition ratio and species weight

It is suggested that compounds with traditional structures for humic substances (HS) do not exist in soils but that soil organic matter consists instead of mixtures of plant and microbial constituents plus the same constituents in various stages of degradation. That is, HS are mixtures of plant carb

Are the traditional concepts of the structures of humic substances realistic

Stable carbon isotope ratios of roots and soil organic matter were measured in Curlew Valley, Utah to determine if changes in the relative dominance of two shrub species had occurred in this salt-desert community. Measurements were made on soil cores along transects stretching from monospecific stands of Ceratoides lanata, a C3 shrub, to monospecific stands of Atriplex confertifolia, a C4 shrub. δ13C values of roots and soil organic matter under Ceratoides cover appeared to be in equilibrium with the current plant community. By contrast, δ13C values of roots and soils under Atriplex portions of the transects were more negative than would be expected for a C4-dominated community. These results indicate that a change in relative C3/C4 dominance has occurred, and suggest that the C4 shrub Atriplex confertifolia is increasing in importance in this salt-desert community.

/pdf/carbon-isotope-ratios-of-soil-organic-matter-and-their-use-39z245lo42.pdf

Carbon isotope ratios of soil organic matter and their use in assessing community composition changes in Curlew Valley, Utah

Secondary forests are becoming increasingly widespread in the tropics, but our understanding of how secondary succession affects carbon (C) cycling and C sequestration in these ecosystems is limited. We used a well-replicated 80-year pasture to forest successional chronosequence and primary forest in Puerto Rico to explore the relationships among litterfall, litter quality, decomposition, and soil C pools. Litterfall rates recovered rapidly during early secondary succession and averaged 10.5 (± 0.1 SE) Mg/ha/y among all sites over a 2-year period. Although forest plant community composition and plant life form dominance changed during succession, litter chemistry as evaluated by sequential C fractions and by 13C-nuclear magnetic resonance spectroscopy did not change significantly with forest age, nor did leaf decomposition rates. Root decomposition was slower than leaves and was fastest in the 60-year-old sites and slowest in the 10- and 30-year-old sites. Common litter and common site experiments suggested that site conditions were more important controls than litter quality in this chronosequence. Bulk soil C content was positively correlated with hydrophobic leaf compounds, suggesting that there is greater soil C accumulation if leaf litter contains more tannins and waxy compounds relative to more labile compounds. Our results suggest that most key C fluxes associated with litter production and decomposition re-establish rapidly—within a decade or two—during tropical secondary succession. Therefore, recovery of leaf litter C cycling processes after pasture use are faster than aboveground woody biomass and species accumulation, indicating that these young secondary forests have the potential to recover litter cycling functions and provide some of the same ecosystem services of primary forests.

https://www.fs.fed.us/global/iitf/pubs/ja_iitf_2008_ostertag001.pdf

Litterfall and Decomposition in Relation to Soil Carbon Pools Along a Secondary Forest Chronosequence in Puerto Rico

Soil profiles under beech, spruce and a grassland have been analysed to study the evolution of natural n-alkanes in pollution-free ecosystems. The soils had all developed on granitic bedrock, at an altitude of 1300-1500 m in the region of Mont-Lozere (southern Massif-Central, France). In contrast to the grassland soil, the two forest soils both possessed a well-developed acidic moder humus-type horizon. This could be subdivided as follows: fresh litter (OL), fragmentation (OF) and humification (OH) layers; two litters, one fresh (OL1) and one old (OL2) could actually be distinguished in the beech forest soil. The n-alkane signature of the parent plants was preserved in the top litter. Immediately underneath, in the OF layer(s) the original il-alkane signatures were progressively but rapidly replaced by a common signature composed of n-C-27 and n-C-25 With larger proportions of the former than of the latter. These two hydrocarbons were most probably produced in situ by fungi. These results appear to illustrate the action of soil microorganisms which metabolize the inherited n-alkanes and produce new compounds of the same family. Unlike the alkanes and the low molecular weight fatty acids less than or equal to C-20 (which increase greatly in the OL2 layer under beech as a result of intense microbial activity), the heavy fatty acids (>C-20) show no significant change in the organic horizon.

n-Alkanes and free fatty acids in humus and a1 horizons of soils under beech, spruce and grass in the Massif-Central (Mont-Lozère), France

A survey of the literature pertaining to soil fats, waxes, and resins (often called bitumens or simply lipids) indicates that these substances are probably the least studied of soil organic matter components. Soil organic matter chemists have largely ignored these materials in preference to studies on the true humic materials. This neglect is probably based on the fact that fats, waxes, and resins comprise but a small percentage of the total organic matter of mineral soils (1 to 5%). However, 10 to 20% of the total organic matter of organic soils may come under the lipid classification. Three review articles deal with the soil lipid fraction:HOWARD and HAMER [1960], STEVENSON [1966], and MORRISON [1969]. WOLLRAB and STREIBL [1969] also review the literature on peat and lignite waxes.

R. H. Miller

Papers

Fats, Waxes, and Resins in Soil