Impact of a WWTP effluent overland flow on the properties of a mediterranean riparian soil.

In this work we aim to assess the impact of a WWTP effluent overland flow on properties and nutrient concentrations of a riparian soil, in order to explore the potential of this practice as a nature-based treatment. We set two study zones of 150 m2 on the field, one control and one that received the WWTP effluent on its surface for one month. Samples were taken before and after the effluent overland flow system, to test the impact of the effluent on soil properties through a BACI design, and after 17 months, to evaluate the recovery of the soil. Two depths were studied: 0-5 cm and 5-20 cm. The effluent overland flow triggered an increase in exchangeable sodium percentage and a decrease in nitrate concentration in both depths, and an increase in ammonium concentration in 0-5 cm depth. After 17 months, there were not found relevant differences among zones. In conclusion, this practice could be used in the purpose to reduce the nutrient concentrations of WWTP effluents. This practice could be relevant for regions where WWTP effluents are discharged in low-flow or intermittent streams, such as semi-arid regions or the Mediterranean region.

Restoration of areas degraded by alluvial sand mining: use of soil microbiological activity and plant biomass growth to assess evolution of restored riparian vegetation.

River or alluvial sand mining is causing a variety of environmental problems in the Itajaí-açú river basin in Santa Catarina State (south of Brazil). When this type of commercial activity degrades areas around rivers, environmental restoration programs need to be executed. In this context, the aim of this study was to assess the evolution of a restored riparian forest based on data on the soil microbial activity and plant biomass growth. A reference site and three sites with soil degradation were studied over a 3-year period. Five campaigns were performed to determine the hydrolysis of the soil enzyme fluorescein diacetate (FDA), and the biomass productivity was determined at the end of the studied period. The variation in the enzyme activity for the different campaigns at each site was low, but this parameter did differ significantly according to the site. Well-managed sites showed the highest biomass productivity, and this, in turn, showed a strong positive correlation with soil enzyme activity. In conclusion, soil enzyme activity could form the basis for monitoring and the early prediction of the success of vegetal restoration programs, since responses at the higher level of biological organization take longer, inhibiting the assessment of the project within an acceptable time frame.

Elemental mercury production from contaminated riparian soil suspensions under air and nitrogen bubbling conditions.

The dynamic change of redox conditions is a key factor in emission of elemental mercury (Hg0) from riparian soils. The objective of this study was to elucidate the influences of redox conditions on Hg0 emission from riparian soils. Soil suspension experiments were conducted to measure Hg0 emission from five Hg-contaminated soil samples in two redox conditions (i.e., treated with air or with N2). In four of the five samples, Hg0 emission was higher in air treatment than on N2 treatment. Remaining one soil, which has higher organic matter than other soils, showed no distinct difference in Hg0 production between air and N2 treatment. In soil suspensions subject to N2 treatment, the dissolved organic carbon (DOC) and Fe2+ concentrations were 3.38- to 1.34-fold and 1.44- to 2.28-fold higher than those in air treatment, respectively. Positive correlations were also found between the DOC and Fe2+ (r = 0.911, p < 0.01) and Hg2+ (r = 0.815, p < 0.01) concentrations in soil solutions, suggesting Fe2+ formation led to the release of DOC, which bound to Hg2+ in the soil and, in turn, limited the availability of Hg2+ for reduction to Hg0 in N2 treatment. On the other hand, for remaining one soil, more Hg2+ might be adsorbed onto the DOM in the air treatment, resulted in the inhibition of Hg0 production in air treatment. These results imply that the organic matter is important to prevent Hg0 production by changing redox condition. Further study is needed to prove the role of organic matter in the production of Hg0.

pH drives the spatial variation of antibiotic resistance gene profiles in riparian soils at a watershed scale.

Owing to convenient water access, riparian areas are often sites for intensive livestock breeding industries and agriculture, which can increase the spread of antibiotic resistance genes (ARGs). However, studies on ARG profiles in riparian soils are limited and there is little information regarding the factors influencing ARGs at a watershed scale. Here, we analyzed ARG profiles, bacterial communities, and soil properties in riparian soils under different land-use types. A total of 124 ARGs and 25 mobile genetic elements (MGEs) were detected in the riparian soils, which covered almost all major classes of antibiotics. Non-metric multidimensional scaling analysis showed that both the distance to the water reservoir and land-use types played important roles in shaping ARG profiles in riparian soils at a watershed scale. Downstream soils harbored three times the abundance of ARGs compared with upstream and midstream soils. Distance-decay analysis indicated that the similarity of ARG profiles and bacterial community composition decreased significantly with the increase of geographical distance (p < 0.001). When taking the land-use type into consideration, the relative abundance and diversity of ARGs and MGEs in orchard and farmland soils were significantly higher than those in wasteland soils. This indicated that anthropogenic activities can also affect ARG patterns in riparian soils. MGE abundance was identified as major driving factors of ARG profiles. In addition, among all the examined soil properties, soil pH was found to be more important than nutrients and organic carbon in shaping ARG profiles. Our findings provide valuable data on ARG distribution in riparian soils in a reservoir catchment and highlight downstream soils is crucial for ensuring water source security.

[Effects of Biochar Addition on Soil Nitrogen Mineralization and Leaching Characteristics in Riparian Zone of Taihu Lake].

Nitrogen mineralization in riparian soil changes the migration and utilization efficiency of nitrogen, which is closely related to the control of water eutrophication. The differences in soil properties caused by land use alter nitrogen retention and transport capacity. Therefore, the soil of three land use types (woodland, grassland, and cultivated land) in the western riparian zone of Taihu Lake were selected for research on the dynamic changes in nitrogen mineralization amount using an incubation experiment and a leaching characteristics by soil column leaching experiment, and their environmental effects were also studied under different biochar addition conditions (0%, 1%, and 5%). The results showed that, in general, the addition of biochar inhibited nitrogen mineralization in forest land and grassland soil, whereas the effect of biochar on nitrogen mineralization in cultivated land was promoted in low concentrations but inhibited in high concentrations. Leaching experiments showed that the biochar addition reduced the loss of soil mineral nitrogen, and the reduction rate in ammonia nitrogen was 23.28%-39.79%, whereas there was little difference between the three land use types. The nitrate decreased by 17.20%-44.49%, and the reduction rate of cultivated land was smaller than that of forest land and grassland. In conclusion, the input of biochar into grassland and cultivated land can better maintain soil fertility and reduce soil nitrogen loss in riparian soil.

Towards a better understanding of linear species distribution in river valleys: The abundance of river corridor plants is linked to soil exchangeable potassium and pH.

The phenomenon that some vascular plant species grow mainly or exclusively in the large river valleys of Central Europe constitutes a long-standing distribution puzzle. In our study, we focused on determining which physicochemical properties of soil affect the distribution of river corridor plant (RCP) species. Research that included RCP coverage, the physicochemical properties of soil as well as several topographic and spatial variables were conducted in 10 transects perpendicular to the San River (SE Poland). The sampled plots were located both in close proximity to the river, where the greatest number of RCP populations are concentrated, and along anthropogenic linear landscape elements which have proven to be important for RCP occurrence in areas located away from the riverbed. Spearman rank correlation matrices were constructed to observe the general relationships between particular variables and Boosted Regression Tree models were used for explaining RCP coverage. Our study indicated that in the case of plots located in proximity to the river, the expected coverage of RCP species was highest in plots characterized by a higher soil exchangeable potassium (K) content, as well as in those located closer to the San River and at lower altitudes. In plots situated along anthropogenic linear landscape elements, the expected coverage of RCPs was highest in plots with a high soil exchangeable K content and in those with a high soil pH. The analyses clearly showed that the physicochemical properties of soils indeed affect RCP species occurrence and they require soils with a high exchangeable K content and a high pH. Since these two soil parameters are closely related to soil fertility, and additionally K enhances root development, we suppose that both factors strengthen the competitiveness of RCPs in floodplain ecosystems.

Assessment of the contamination of riparian soil and vegetation by trace metals--A Danube River case study.

The aim of this study was to assess the spatial distribution of arsenic and heavy metals (Cd, Cr, Cu Hg, Ni, Pb and Zn) in a riparian area influenced by periodical flooding along a considerable stretch of the Danube River. This screening was undertaken on soil and plant samples collected from 43 sites along 2386 km of the river, collected during the international Joint Danube Survey 3 expedition (ICPDR, 2015). In addition, data on the concentration of these elements in river sediment was used in order to describe the relationship between sediment, riparian soil and riparian plants. A significant positive correlation (Spearman r, for p<0.05) was found for trace metal concentrations in river sediment and soil (r=0.817). A significant correlation between soil and plants (r=0.438) and sediment and plants (r=0.412) was also found for trace metal concentrations. Elevated levels of Cd, Cr, Cu, and Ni were found at certain sites along the Serbian stretch, while elevated concentrations of Hg were also detected in Hungary, of Pb along the Romanian stretch and of As along the Bulgarian stretch (the Lower Danube). These results point to the presence of naturally-occurring metals derived from ore deposits in the Danube River Basin and anthropogenic metals, released by mining and processing of metal ores and other industrial facilities, which are responsible for the entry of metals such as Cu, Ni and Zn. Our results also indicated toxic Cd and Zn levels in plant samples, measured at the Hercegsznato site (Middle Danube, Hungary), which highlighted these elements as a potential limiting factor for riparian vegetation in that area. The distribution of the analysed elements in plant material also indicates the species-specific accumulation of trace metals. Based on our results, the Lower and Middle Danube were found to be more polluted in terms of the analysed elements.