Effect of petroleum on decomposition of shrub-grass litters in soil in Northern Shaanxi of China

Dynamics of the remediating effects of plant litter on the biological and chemical properties of petroleum-contaminated soil

To investigate the comprehensive remediating effects of plant residues on biological and chemical properties and the long-term dynamics of these effects, litter from Caragana korshinskii (caragana) or Ziziphus jujuba var. spinosa (jujube) was mixed with three types of soil that were contaminated with 12.49, 27.54, and 45.37 g kg^-1 of petroleum. The mixtures were incubated at 20-25 °C with consistent soil moisture for 360 days. Subsequently, the litter impacts on the soil microbial population, the activities of 12 types of soil hydrolytic, and redox enzymes related to the cycling of C, N, and P, and the available N, P, and K contents were determined during the incubation. The results indicated that both types of litter significantly accelerated the reproduction of soil microbes and significantly increased the activities of most of the hydrolytic enzymes and the available nutrient contents after the short-term treatments, while the litters usually simultaneously depressed the activities of polyphenol oxidase and peroxidase in the slightly and moderately contaminated soils. However, the comprehensive remediating effects of the litters on the lightly contaminated soil significantly decreased over time while it recovered to some extent at the end of the experiment. The remediating effects on the seriously contaminated soil exhibited the opposite trend, and their remediating effects on the moderately contaminated soil exhibited continuous weakening. Generally, the remediating effects of the caragana litter were more noticeable than those of the jujube litter, except for the effect on the slightly contaminated soil after 180 days of treatment.

Changes in nutrients and decay rate of Ginkgo biloba leaf litter exposed to elevated O3 concentration in urban area

Ground-level ozone (O₃) pollution has been widely concerned in the world, particularly in the cities of Asia, including China. Elevated O₃ concentrations have potentially influenced growth and nutrient cycling of trees in urban forest. The decomposition characteristics of urban tree litters under O₃ exposure are still poorly known. Ginkgo biloba is commonly planted in the cities of northern China and is one of the main tree species in the urban forest of Shenyang, where concentrations of ground-level O₃ are very high in summer. Here, we hypothesized that O₃ exposure at high concentrations would alter the decomposition rate of urban tree litter. In open-top chambers (OTCs), 5-year-old G. biloba saplings were planted to investigate the impact of elevated O₃ concentration (120 ppb) on changes in nutrient contents and decomposition rate of leaf litters. The results showed that elevated O₃ concentration significantly increased K content (6.31 ± 0.29 vs 17.93 ± 0.40, P < 0.01) in leaves of G. biloba, significantly decreased the contents of total phenols (2.82 ± 0.93 vs 1.60 ± 0.44, P < 0.05) and soluble sugars (86.51 ± 19.57 vs 53.76 ± 2.40, P < 0.05), but did not significantly alter the contents of C, N, P, lignin and condensed tannins, compared with that in ambient air. Furthermore, percent mass remaining in litterbags after 150 days under ambient air and elevated O₃ concentration was 56.0% and 52.8%, respectively. No significant difference between treatments was observed in mass remaining at any sampling date during decomposition. The losses of the nutrients in leaf litters of G. biloba showed significant seasonal differences regardless of O₃ treatment. However, we found that elevated O₃ concentration slowed down the leaf litter decomposition only at the early decomposition stage, but slightly accelerated the litter decomposition at the late stage (after 120 days). This study provides our understanding of the ecological processes regulating biogeochemical cycles from deciduous tree species in high-O₃ urban area.

Impacts of soil petroleum contamination on nutrient release during litter decomposition of Hippophae rhamnoides

Petroleum exploitation causes contamination of shrub lands close to oil wells. Soil petroleum contamination affects nutrient release during the litter decomposition of shrubs, which influences nutrient recycling and the maintenance of soil fertility. Hence, this contamination may reduce the long-term growth and stability of shrub communities and consequently, the effects of phytoremediation. Fresh foliar litter of Hippophae rhamnoides, a potential phytoremediating species, was collected for this study. The litter was placed in litterbags and then buried in different petroleum-polluted soil media (the petroleum concentrations were 15, 30, and 45 g kg(-1) dry soil, which were considered as slightly, moderately and seriously polluted soil, respectively) for a decomposition test. The impacts of petroleum contamination on the release of nutrients (including N, P, K, Cu, Zn, Fe, Mn, Ca and Mg) were assessed. The results showed that (1) after one year of decomposition, the release of all nutrients was accelerated in the slightly polluted soil. In the moderately polluted soil, P release was accelerated, while Cu, Zn and Mn release was inhibited. In the seriously polluted soil, Cu and Zn release was accelerated, while the release of the other nutrients was inhibited. (2) The effect of petroleum on nutrient release from litter differed in different periods during decomposition; this was mainly due to changes in soil microorganisms and enzymes under the stress of petroleum contamination. (3) To maintain the nutrient cycling and the soil fertility of shrub lands, H. rhamnoides is only suitable for phytoremediation of soils containing less than 30 g kg(-1) of petroleum.