Vertical distribution and environmental significance of PAHs in soil profiles in Beijing, China.

Vertical distribution of both the concentration and composition of polycyclic aromatic hydrocarbons (PAHs) in ten profiles in Beijing has been investigated. The results showed that PAH concentrations and compositions in topsoil from different sampling sites were different. PAH concentrations were much higher in topsoil of the investigated urban area, industrial region, and paddy field with wastewater irrigation than in other areas. Moreover, PAH concentrations in topsoil were much higher than those at greater depth, where the concentrations were relatively consistent in most soil profiles. The fingerprints of PAHs in the samples from topsoil (0-30 cm) in the same profiles were similar and were obviously different from those at greater depth, suggesting that PAH sources were consistent in topsoil samples and were discriminating between topsoil and deeper soils. PAHs in topsoil mainly arose from mixed sources of combustion of liquid fuel, coal, and/or wood, as well as wastewater irrigation, while those at greater depth were derived from soil genesis and the process of soil formation.

[Effects of Organic Carbon Content on the Residue and Migration of Polycyclic Aromatic Hydrocarbons in Soil Profiles].

The effects of total organic carbon content (TOC) on the migration of polycyclic aromatic hydrocarbons (PAHs) in the soil were investigated. This study analyzed the vertical properties of the concentrations and distributions of PAHs and TOC at various soil profiles from functionally different environmental regions including nature reserves, ploughs, orchards, farmlands, metropolitan areas, and industrial parks. The vertical migration properties of PAHs in soils were examined by conducting leaching experiments in soil columns. The concentrations of PAHs varied from region to region and showed strong, positive correlations with TOC in the same region. Furthermore, based on the leaching experiments, the transport abilities of PAHs were significantly influenced by TOC, although they could all be transported to the deep layers by TOC in soil columns. The downward migration of PAHs decreased with the increase in TOC and vice versa. The properties of the composition and structure of PAHs also had an obvious influence on their residues and migration in soil profiles at the same TOC conditions. In addition, the transport of PAHs was related to the amount of leaching water, the leaching time, and the additional PAHs.

Seasonality, Rather than Nutrient Addition or Vegetation Types, Influenced Short-Term Temperature Sensitivity of Soil Organic Carbon Decomposition.

The response of microbial respiration from soil organic carbon (SOC) decomposition to environmental changes plays a key role in predicting future trends of atmospheric CO2 concentration. However, it remains uncertain whether there is a universal trend in the response of microbial respiration to increased temperature and nutrient addition among different vegetation types. In this study, soils were sampled in spring, summer, autumn and winter from five dominant vegetation types, including pine, larch and birch forest, shrubland, and grassland, in the Saihanba area of northern China. Soil samples from each season were incubated at 1, 10, and 20°C for 5 to 7 days. Nitrogen (N; 0.035 mM as NH4NO3) and phosphorus (P; 0.03 mM as P2O5) were added to soil samples, and the responses of soil microbial respiration to increased temperature and nutrient addition were determined. We found a universal trend that soil microbial respiration increased with increased temperature regardless of sampling season or vegetation type. The temperature sensitivity (indicated by Q10, the increase in respiration rate with a 10°C increase in temperature) of microbial respiration was higher in spring and autumn than in summer and winter, irrespective of vegetation type. The Q10 was significantly positively correlated with microbial biomass and the fungal: bacterial ratio. Microbial respiration (or Q10) did not significantly respond to N or P addition. Our results suggest that short-term nutrient input might not change the SOC decomposition rate or its temperature sensitivity, whereas increased temperature might significantly enhance SOC decomposition in spring and autumn, compared with winter and summer.

[Vertical distribution characteristics and composition of saturated hydrocarbon in soil profiles of Beijing].

Soil samples were collected from ten soil profiles with different environmental conditions in Beijing for saturated hydrocarbons (SHs) analysis. The vertical distribution of the both concentration and composition of SHs in ten soil profiles were investigated. Concentration of SHs in different profiles is significantly different, ranged from 1.5 microg x g(-1) to 54.1 microg x g(-1). The higher concentrations are found in the samples from B7, B9 and B10. A series of SHs including n-alkanes, isoprenoid alkanes, terpenoids and steranes, alkyl hexamethylene were detected in all samples. The order from higher content to lower in most profiles (except B7) is: n-alkanes, isoprenoid alkanes, terpenoids and steranes, alkyl hexamethylene, and the relative content of n-alkanes is obviously dominant in the upper horizons. Concentrations of SHs and content of soil organic carbon in each profile show similar trend with depth, declined rapidly down to 30 cm and trend consistent in the deep part( > 40 cm). CPI1, CPI2, (C25 + C27 + C29 + C31) / Sigma alkanes, terpenoids and steranes, and biomarker parameters suggest that pollutants are mainly from fossil fuel in B7, high plant-derived n-alkanes dominated in other profiles and fossil fuel contamination to different extent. The sources of n-alkanes in deep part of soil profiles are different from those in topsoil samples, which are related to the soil itself, such as soil parent material and process of soil formation, but the sources of alkyl hexamethylene, terpenoids and steranes between topsoil and deep part are constant.