Ecological risk assessment of heavy metals in riverine sediments of rural area driven by urbanization

Rural revitalization denotes the gathering of large populations in rural areas and the subsequent gradual urbanization. Rural environments have been deteriorated by heavy metals (HMs) over the last few years. Without the existence of large-scale industries, the accumulation of HMs in sediments due to population aggregation in rural environments needs to be scientifically confirmed. Therefore, in this study we first understand the sediment pollution in rural environments in China and across the globe, and subsequently investigate HMs in sediments in rural micro water. The study area, Sichuan Province, China, was divided into two areas, namely, sparsely populated areas (SPA) and densely populated areas (DPA). Eight typical HMs (As, Zn, Ni, Hg, Cd, Cr, Cu, and Pb) were selected to target in riverine sediments, and the content and spatial distribution characteristics were analyzed. The results indicate that As, Hg, Cd, and Pb concentrations in sediments were higher than background values (BVs), with high concentration sample sites located in the DPA. In addition, the geo-accumulation index (Igeo), pollution load index (PLI) and potential ecological risk index (RI) were used to quantitatively evaluate the pollution characteristics of HMs in sediments, revealing that the sediments exhibited high As and Hg pollution in the DPA (PLI = 1.09). In general, mild (RI = 48.76) and moderate (RI = 154.92) HM pollution was observed in the sediments of the SPA and DPA, respectively, based on the high PLI (> 1.0) and RI (> 150) values. Correlation analysis and principal component analysis (PCA) indicate that the Cd in the sediment generally originated from geogenic sources, while the other elements (Zn, As, Cu, Cr, Hg, Ni and Pb) were primarily linked to anthropogenic sources. Finally, the results demonstrate that population aggregation will lead to the enrichment of HMs.

The impact of cattle dung pats on earthworm distribution in grazed pastures

Background

Grazed grassland management regimes can have various effects on soil fauna. For example, effects on earthworms can be negative through compaction induced by grazing animals, or positive mediated by increases in sward productivity and cattle dung pats providing a food source. Knowledge gaps exist in relation to the behaviour of different earthworm species i.e. their movement towards and aggregation under dung pats, the legacy effects of pats and the spatial area of recruitment. The present study addressed these knowledge gaps in field experiments, over 2 years, using natural and simulated dung pats on two permanent, intensively grazed pastures in Ireland.

Results

Dung pats strongly affected spatial earthworm distribution, with up to four times more earthworms aggregating beneath pats, than in the control locations away from pats. In these earthworm communities comprising 11 species, temporally different aggregation and dispersal patterns were observed, including absence of individual species from control locations, but no clear successional responses. Epigeic species in general, but also certain species of the anecic and endogeic groups were aggregating under dung. Sampling after complete dung pat disappearance (27 weeks after application) suggested an absence of a dung pat legacy effect on earthworm communities. Based on species distributions, the maximum size of the recruitment area from which earthworms moved to pats was estimated to be 3.8 m² per dung pat. Since actual grazing over 6 weeks would result in the deposition of about 300 dung pats per ha, it is estimated that a surface area of 1140 m² or about 11% of the total grazing area can be influenced by dung pats in a given grazing period.

Conclusions

This study showed that the presence of dung pats in pastures creates temporary hot spots in spatial earthworm species distribution, which changes over time. The findings highlight the importance of considering dung pats, temporally and spatially, when sampling earthworms in grazed pastures. Published comparisons of grazed and cut grasslands probably reached incorrect conclusions by ignoring or deliberately avoiding dung pats. Furthermore, the observed intense aggregation of earthworms beneath dung pats suggests that earthworm functions need to be assessed separately at these hot spots.

Electronic supplementary material

The online version of this article (10.1186/s12898-018-0216-6) contains supplementary material, which is available to authorized users.