Effect of heavy metals and storage time on two types of forest litter: basal respiration rate and exchangeable metals

Soil Physicochemical Properties, Metal Deposition, and Ultrastructural Midgut Changes in Ground Beetles, Calosoma chlorostictum, under Agricultural Pollution

Unsustainable agricultural practices that minimize soil organic matter can promote the removal of heavy metal pollutants into the food chain. Such polluted soils can release contaminants into the groundwater, which leads to accumulation in plant tissue that is transferred to animals, birds, insects, and humans. Biomonitors of soil pollution with heavy metals can be identified by the ground beetles Calosoma chlorostictum (Coleoptera: Carabidae) as bioindicators of soil quality and its yield sustainability. The experiment was performed on two sites in Zagazig City (30.62° N, 31.44° E), Egypt. The physicochemical parameters indicated that soil moisture and organic matter had the highest differences in the polluted agricultural soil compared to the reference soil. However, there were no significant differences in chloride content. The atomic absorption analysis exhibited the highest concentration recorded for arsenic (As) and the lowest for selenium (Se) in the polluted soil and the insect’s midgut. Meanwhile, the differences between heavy metal concentrations in the total soil and midgut of C. chlorostictum from current sites indicated that the highest differences were in aluminum (Al) and mercury (Hg), while arsenic (As) and cadmium (Cd) were the lowest. Furthermore, the correlation between heavy metal concentrations in the soil and insect midgut was highest in As, while the lowest correlation was noticed in Al. We used transmission electron microscopy (TEM) that showed a more considerable disturbance in the C. chlorostictum midgut epithelial layer collected from the agricultural area than in the insects collected from the reference area. Evident ultrastructural alterations showed a rupture and distortion of microvilli, destruction of the columnar and regenerative cells, large separation between epithelial cells, and stretching of the cellular axis, as a result of which the lumen became very narrow. Moreover, a lot of vacuoles with little enzyme secretion were observed in the columnar epithelial cells. In addition, other manifestations due to pollution with heavy metals such as a pyknotic nucleus with abnormal chromatin, cytoplasmic vacuolization, disruptions, and vacuolation of mitochondria were detected, as well as the appearance of electron-dense vesicles, a lot of lysosomes, large myelin figures, and dilation of the rough endoplasmic reticulum on account of soil contamination. Potential counteractive health influence in such applications could be avoided if the soil was adequately treated.

Soil processes and tree growth at shooting ranges in a boreal forest reflect contamination history and lead-induced changes in soil food webs

The effects of shooting-derived lead (Pb) on the structure and functioning of a forest ecosystem, and the recovery of the ecosystem after range abandonment were studied at an active shotgun shooting range, an abandoned shooting range where shooting ceased 20 years earlier and an uncontaminated control site. Despite numerous lead-induced changes in the soil food web, soil processes were only weakly related to soil food web composition. However, decomposition of Scots pine (Pinus sylvestris) needle litter was retarded at the active shooting range, and microbial activity, microbial biomass and the rate of decomposition of Pb-contaminated grass litter decreased with increasing soil Pb concentrations. Tree (P. sylvestris) radial growth was suppressed at the active shooting range right after shooting activities started. In contrast, the growth of pines improved at the abandoned shooting range after the cessation of shooting, despite reduced nitrogen and phosphorus contents of the needles. Higher litter degradation rates and lower Pb concentrations in the topmost soil layer at the abandoned shooting range suggest gradual recovery after range abandonment. Our findings suggest that functions in lead-contaminated coniferous forest ecosystems depend on the successional stage of the forest as well as the time since the contamination source has been eliminated, which affects, e.g., the vertical distribution of the contaminant in the soil. However, despite multiple lead-induced changes throughout the ecosystem, the effects were rather weak, indicating high resistance of coniferous forest ecosystems to this type of stress.

Biodiversity and structure of spider communities along a metal pollution gradient

The objective of the study was to determine whether long-term metal pollution affects communities of epigeal spiders (Aranea), studied at three taxonomic levels: species, genera, and families. Biodiversity was defined by three indices: the Hierarchical Richness Index (HRI), Margalef index (D(M)) and Pielou evenness index (J). In different ways the indices describe taxa richness and the distribution of individuals among taxa. The dominance pattern of the communities was described with four measures: number of dominant species at a site, percentage of dominant species at a site, average dominant species abundance at a site, and the share of the most numerous species (Alopecosa cuneata) at a site. Spiders were collected along a metal pollution gradient in southern Poland, extending ca. 33 km from zinc and lead smelter to an uncontaminated area. The zinc concentration in soil was used as the pollution index.The study revealed a significant effect of metal pollution on spider biodiversity as described by HRI for species (p = 0.039), genera (p = 0.0041) and families (p = 0.0147), and by D(M) for genera (p = 0.0259) and families (p = 0.0028). HRI correlated negatively with pollution level, while D(M) correlated positively. This means that although broadly described HRI diversity decreased with increasing pollution level, species richness increased with increasing contamination. Mesophilic meadows were generally richer. Pielou (J) did not show any significant correlations. There were a few evidences for the intermediate disturbance hypothesis: certain indices reached their highest values at moderate pollution levels rather than at the cleanest or most polluted sites.

Tree species effect on the redistribution of soil metals

Phytostabilization of metals using trees is often promoted although the influence of different tree species on the mobilization of metals is not yet clear. Soil and biomass were sampled 33 years after planting four tree species (Quercus robur, Fraxinus excelsior, Acer pseudoplatanus, Populus 'Robusta') in a plot experiment on dredged sediment. Poplar took up high amounts of Cd and Zn and this was associated with increased Cd and Zn concentrations in the upper soil layer. The other species contained normal concentrations of Cd, Cu, Cr, Pb and Zn in their tissues. Oak acidified the soil more than the other species and caused a decrease in the concentration of metals in the upper soil layer. The pH under poplar was lower than expected and associated with high carbon concentrations in the top soil. This might be assigned to retardation of the litter decomposition due to elevated Cd and Zn concentrations in the litter.

Metal toxicity affects fungal and bacterial activities in soil differently

Although the toxic effect of heavy metals on soil microorganism activity is well known, little is known about the effects on different organism groups. The influence of heavy metal addition on total, bacterial, and fungal activities was therefore studied for up to 60 days in a laboratory experiment using forest soil contaminated with different concentrations of Zn or Cu. The effects of the metals differed between the different activity measurements. During the first week after metal addition, the total activity (respiration rate) decreased by 30% at the highest level of contamination and then remained stable during the 60 days of incubation. The bacterial activity (thymidine incorporation rate) decreased during the first days with the level of metal contamination, resulting in a 90% decrease at the highest level of contamination. Bacterial activity then slowly recovered to values similar to those of the control soil. The recovery was faster when soil pH, which had decreased due to metal addition, was restored to control values by liming. Fungal activity (acetate-in-ergosterol incorporation rate) initially increased with the level of metal contamination, being up to 3 and 7 times higher than that in the control samples during the first week at the highest levels of Zn and Cu addition, respectively. The positive effect of metal addition on fungal activity then decreased, but fungal activity was still higher in contaminated than in control soil after 35 days. This is the first direct evidence that fungal and bacterial activities in soil are differently affected by heavy metals. The different responses of bacteria and fungi to heavy metals were reflected in an increase in the relative fungal/bacterial ratio (estimated using phospholipid fatty acid analysis) with increased metal load.