Characterisation of soil erosion and overland flow on vegetation-growing slopes in fragile ecological regions: A review.

Grasses and shrubs occupy large areas of fragile ecosystems following ecological restoration. Therefore, it is increasingly important to assess and monitor the environmental safety of pastures. However, previous studies on this topic lacked systematicity and directionality. In this study, we reviewed the literature on runoff and erosion to summarise the core issues for future research, resolve the current research bottleneck, and promote the balance of soil, water, and energy in fragile ecological areas. The results of the review indicate that coverage remained the main vegetation feature considered when characterising rangeland slope erosion and runoff. Erosion energy should be comprehensively considered based on the influence of vegetation on rainfall distribution and runoff erosivity. Rangeland slope erosion and runoff changes can only be explained by integrating the above- and below-ground characteristics of vegetation. Additionally, the impact of vegetation on runoff separation and the sediment transport processes at different erosion stages under rainfall conditions vary. Therefore, studying the comprehensive indicators of vegetation at different erosion stages in response to erosion and runoff will be vital in rangeland erosion research. From the perspective of disciplinary development, this study can promote the further development of soil erosion, ecology, soil science, hydrology, hydraulics, and other disciplines.

Optimizing the Dryland Sheet Erosion equation in South China.

Optimisation of models applied in sheet erosion equations could facilitate effective management of sheet erosion in the field, and sustainable agricultural production. To optimise the characterisation of sheet erosion on slope farmland in South China, the present study conducted field simulation rainfall experiments with vegetated and fallow soils. According to the results, sheet erosion rate first increased with an increase in rainfall duration and then stabilised. Exclusive P. vulgaris planting and P. vulgaris in combination with earthworms could reduce sheet erosion by 10-60%, and the combined method could better control sheet erosion. There were significant differences in erosion rate between mild and steep slopes, and light and heavy rain conditions. The influence of rain intensity on sheet erosion was greater than that of slope. Soil organic matter (SOM), rain intensity, and slope can be used to optimise sheet erosion equations of exposed slopes, and SOM and hydraulic parameters can be used to optimise sheet erosion equations in vegetated slopes. The results of the present study could facilitate the reduction of the time and space variability errors in the establishment of sheet erosion models for vegetated slopes.

[Temporal and Spatial Variation Characteristics and Source Analysis of Agricultural Non-point Source Pollution Load in Guangdong During the Past 20 Years].

The agricultural non-point source pollution (ANPSP) load in Guangdong province is very large and has a serious impact on the regional ecological environment. Inventory analysis was used to assess and analyze the temporal and spatial variation characteristics of the ANPSP load of Guangdong province during 1999-2019, and the sources of ANPSP were discussed, as well as the relationship between them. The results showed that, during the past 20 years, the total ANPSP pollution loads of Guangdong province decreased by 6.08%, and the pollution loads of chemical oxygen demand (COD), total nitrogen (TN), and total phosphorus (TP) increased by -11.88%, 4.99%, and 26.17%, respectively. The input intensity of chemical fertilizers and pesticides increased by 112.19% and 60.38%, respectively. The Pearl River Delta had the highest ANPSP loads in Guangdong province, followed by those in northern, western, and eastern Guangdong. Livestock and poultry breeding were the main sources of COD, the total percent fertilizers and livestock and poultry breeding were the main sources of TN, and livestock and poultry breeding and aquaculture were the main sources of TP. In addition, the contribution of pollutants discharged from aquaculture showed an obvious increasing trend. There were certain differences in the pollution sources in different regions. In western Guangdong, northern Guangdong, and eastern Guangdong, livestock and poultry breeding were the main sources of COD and TP, and fertilizer was the main source of TN; by contrast in the Pearl River Delta, aquaculture had become the main source of TN and TP pollution loads. The correlation results showed that the decline in the total ANPSP in Guangdong province was mainly due to the increase in high urbanization rate and the decrease in the proportion of rural population. In general, there were stage changes in the time and differences in spatial characteristics and sources of ANPSP in Guangdong province. A combination of comprehensive treatment and targeted pollution treatment should be adopted, and fertilizer reduction measures and pollution treatment in the aquaculture should be strengthened in an all-around way, focusing on strengthening the treatment of pollution from aquaculture in the Pearl River Delta region and the treatment of rural life pollution in northern Guangdong.

Effects of biochar application on the loss characteristics of Cd from acidic soil under simulated rainfall conditions.

Biochar is widely used for immobilizing heavy metals in soil as a kind of high-effective passivator. This research conducted incubation and simulated rainfall experiments to study the effects of biochar application on the loss characteristics of runoff and sediment, as well as the transportation of the Cd during the water erosion process. Two rainfall intensities (60 and 120 mm h-1) and five biochar application rates (0%, 1%, 3%, 5%, and 7%) were considered in the experiment. The result showed that slaking had a greater effect than mechanical stirring in aggregate breakdown of the soil, and the addition of biochar generally increased the sensitivity of the soil to wet stirring, while had no obvious influence on the resistance to slaking. The H2O and CaCl2 extractable Cd in soil significantly decreased with the increase of biochar application rate. The runoff yields decreased with the increase of biochar application rate at both the two rainfall intensities, while the eroded sediment generally decreased at the 120 mm h-1 rainfall intensity. The addition of biochar tended to increase the loss of the middle-sized (1-0.05 mm) aggregates at the 60 mm h-1 rainfall intensity, whereas reduced their loss at the 120 mm h-1 rainfall intensity. Biochar application could significantly reduce the concentration of Cd in the runoff and decreased the total loss amount of Cd (sediment+runoff) in most of the cases. Excessively high level (7%) of biochar application may aggravate soil erosion and result in more Cd loss.

Assessment of the long-term leaching characteristics of cement-slag stabilized/solidified contaminated sediment.

The use of stabilized/solidified (S/S) soils and sediments as sustainable construction materials is a global concern due to the potential risk of contaminant leaching including potentially toxic elements. The long-term leachability of four metals (Zn, Pb, Cd and As) in sediments mixed with OPC (Ordinary Portland Cement) and OPC/GGBS (Ground Granulated Blast Furnace Slag) binders were investigated through a combination of tank leaching tests and kinetic leaching models, with varying ranges of curing ages and ambient pH. The leaching data revealed that both binder compositions offer an excellent immobilization capacity for the four metals, while their releases are strongly pH-dependent and are a complex function of curing time. The partial replacement of OPC by GGBS is more effective for fixing Zn and As at pH of 1, Pb at pH of 7, Zn and Pb at pH of 10. Controlling leaching mechanisms and leachability indices were determined using nonlinear regression analysis and kinetic leaching models. The first-order diffusion model (FRDM) was the most applicable model for describing the leaching characteristics of these metals under the investigated cases, the leaching rate is controlled by surface wash-off initially and then by diffusion. The leachability indices indicate that the cement-slag S/S sediment can be regarded as an environmentally sustainable material with potential beneficial uses in construction.

Effects of soil particle size on the adsorption, distribution, and migration behaviors of heavy metal(loid)s in soil: a review.

In recent years, toxic pollution from heavy metal(loid)s in soil has become a severe environmental problem worldwide. The migration and transformation of heavy metal(loid)s in soil have become hot topics in the field of environmental research. Soil particle size plays an important role in influencing the environmental behavior of heavy metal(loid)s in soil. This review collates and synthesizes the research on the adsorption, distribution, and migration of heavy metal(loid)s in soil particles. There is no unified method for soil particle separation, since the purposes of different studies are different. Regardless of adsorption or distribution characteristics, fine soil particles generally exhibit a higher capacity to combine heavy metal(loid)s; however, certain studies have also observed a contrary phenomenon, according to which heavy metal(loid)s were more enriched in coarser particles. The adsorption and distribution of heavy metal(loid)s in soil particles were essentially determined by the physicochemical properties of the soil particles. Land use obviously affected the distribution of heavy metal(loid)s in the soil particles. Organic matter had an important influence on the distribution and availability of heavy metal(loid)s in agricultural and forest soils, while for urban soils and sediments, clay minerals or metal (hydr)oxides may play the dominant role. Preferential surface migration of fine particles during erosion processes did not always lead to the enrichment of heavy metal(loid)s in the lost soil. Further research should be conducted to explore the relationships among the soil aggregates, organic matter, heavy metal(loid)s, and soil microorganisms; the association between the distribution and availability of heavy metal(loid)s and the properties of soil particles; and the migration patterns of heavy metal(loid)s in soil particles at different scales.

Removal of phenanthrene and pyrene from contaminated sandy soil using hydrogen peroxide oxidation catalyzed by basic oxygen furnace slag.

Soil contamination with polycyclic aromatic hydrocarbons (PAHs) is a serious problem in Northeast China, especially in the steel industrial area. The objective of this study was to evaluate the feasibility of using basic oxygen furnace (BOF) slag to activate the Fenton-like remediation of PAH-contaminated soil to achieve the objectives of "waste control by waste" and "resource recycling" in Chinese steel industry. The effects of BOF slag dosages, H2O2 concentrations, and exothermicity-driven evaporation were evaluated with respect to the removal efficiencies of phenanthrene (Phe) and pyrene (Pyr). Results indicated that PAH oxidation was proportional to the BOF slag dosages and was increased exponentially with H2O2 concentrations. Evaporation due to increasing temperature caused by exothermic reaction played an important role in total soil PAH losses. The sequential Fenton-like oxidation with a 3-times application of 15% H2O2 and the same BOF slag repeatedly used were able to remove 65.87% of Phe and 58.33% of Pyr, respectively. Soluble iron oxides containing in BOF slag were reduced, while amorphous iron oxide concentration remained stable during the repeated Fenton-like process. Column study mimics real field applications showing high removal efficiencies of Phe (36.05-83.20%) and Pyr (21.79-68.06%) in 30-cm depth of soil profile. The tests on soluble heavy metal concentrations after the reactions with high slag dosage or high H2O2 concentration confirmed that BOF slag would not cause heavy metal contamination. Consequently, BOF slag may provide an efficient way for enhancing the Fenton-like based remediation of heavily PAH-polluted soil with little risk on collateral heavy metal contamination. However, an external gas collection and purification equipment would be essential to eliminate the evaporated PAHs.

Effect of moisture condition on the immobilization of Cd in red paddy soil using passivators.

In this study, the immobilization of cadmium (Cd) was evaluated in red paddy soil amended with four different passivators (biochar, lime (CaO), calcium dihydrogen phosphate (Ca(H2PO4)2) and zeolite (modified with NaOH)) for three moisture regimes (70% water-holding capacity (WHC), continuous flooding (CF) and wetting-drying cycle (WDC)). The results showed that: the pH of the soil was obviously influenced by the moisture regimes and decreased in the general order of CF > WDC > 70% WHC, and the addition of lime and Ca(H2PO4)2 could obviously enhance and reduce the values to some extent, respectively. Flooding condition could enhance soil pH and increase the content of free Fe-oxides, amorphous Fe-oxides and water-soluble organic/inorganic carbon in paddy soil. The efficiency in decreasing the exchangeable Cd of different moisture regimes followed the order: CF > 70%WHC > WDC. CF combined with lime was the most effective treatment for reducing exchangeable Cd. The transformation of Cd fractions mainly occurred between the acid-extractable and reducible fractions in the immobilization experiments. The most effective passivator is Ca(H2PO4)2, and CF is the most favorable regime in promoting the transformation of Cd in red paddy soil from acid-extractable to reducible fraction.

Loss characteristics of Cd in soil aggregates under simulated rainfall conditions.

Soil particles exert a significant influence on the migration of heavy metals in soil and water environments. In this study, a simulated rainfall experiment was conducted to investigate the loss characteristics of cadmium (Cd) from a red soil surface. Two rainfall intensities (50 and 100 mm h-1) were considered and the distribution of Cd in different fractions (>1, 1-0.25, 0.25-0.05 and <0.05 mm) of aggregates lost from the soil was examined. The result showed that in the artificially polluted soil, the Cd content in different aggregates decreased with particle size, with the lowest Cd content in the <0.05 mm fraction aggregates. In the rainfall experiment, the runoff rate generally increased with the rainfall duration, while the sediment yield rate first increased and then remained stable. The loss rates of various fractions of aggregates in the sediment generally followed the order of (<0.05) mm > 1-0.25 mm > 0.25-0.05 mm > (>1) mm. The proportions of the <0.05 mm-fraction aggregates were the highest in the loss sediment throughout the entire rainfall process. The lost Cd was transported mainly in the sediment-bound form, while only a very small portion of Cd was lost in the water phase of runoff. High rainfall intensity led to greater Cd loss in runoff. In the lost sediment, the concentration of Cd was higher in the macroaggregates. The phenomenon of Cd enrichment in the fine particles was not observed. Although the concentration of Cd in the <0.05 mm fraction was the lowest, the contribution of this fraction to the total Cd content in the sediments was the highest.

Distribution characteristics of heavy metal(loid)s in aggregates of different size fractions along contaminated paddy soil profile.

Soil aggregates exert a significant influence on the retention and availability of heavy metal(loid)s in soil. In this study, the concentration distribution and chemical forms of heavy metals (Cu, Zn, Cd, Pb, and Hg) and a metalloid (As) in different aggregate-sized fractions (2-0.25, 0.25-0.05, 0.05-0.002, and < 0.002 mm) along the profile (0-1, 1-5, 5-15, and 15-25 cm) of a contaminated paddy field were investigated. The results showed that the values of pH, free Fe oxides (Fed), bulk density, and catalase activity gradually increased, whereas the soil organic matter (SOM), cation exchange capacity (CEC), electrical conductivity (EC), microbial biomass carbon (MBC), and urease activity decreased with depth. Long-term heavy metal pollution might impact the catalase activity but showed no obvious influence on the urease activity. Additionally, there was a notable difference in physicochemical properties among the soil aggregates of various particle sizes. The 2-0.25-mm fraction aggregates contained more organic matter, whereas the highest values of CEC and Fed were observed in the < 0.002-mm fraction. The concentrations of all six heavy metals/metalloid decreased with depth. In different layers, Cu and Cd showed the highest concentrations in the 2-0.25 mm-fraction, followed by the < 0.002-mm fraction, whereas the highest concentrations of Zn, Pb, and As appeared in the < 0.002-mm fraction. No obvious distribution regularity was observed for Hg among the aggregates. All of the metal(loid)s had lower activity in the deeper soil layers, except for Hg. Furthermore, Cu and Cd displayed more stable forms in the < 0.002-mm fraction aggregates.

Occurrence, fate and ecological risk of chlorinated paraffins in Asia: A review.

Chlorinated paraffins (CPs), complex mixtures of polychlorinated alkanes, are widely used in various industries and are thus ubiquitous in the receiving environment. The present study comprehensively reviewed the occurrence, fate and ecological risk of CPs in various environmental matrices in Asia. Releases from the production and consumption of CPs or CP-containing materials, wastewater discharge and irrigation, sewage sludge application, long-range atmospheric transport and aerial deposition have been found to be most likely sources and transport mechanisms for the dispersion of CPs in various environmental matrices, such as air, water, sediment, soil and biota. CPs can be bioaccumulated in biota and biomagnified through food webs, likely causing toxic ecological effects in organisms and posing health risks to humans. Inhalation, dust ingestion and dietary intake are strongly suggested as the major routes of human exposure. Research gaps are discussed to highlight the perspectives of future research to improve future efforts regarding the analysis of CPs, the environmental occurrence and elimination of CPs, the total environmental pressure, and the risks to organisms and populations.

Estimation of agricultural water consumption from meteorological and yield data: a case study of Hebei, North China.

Over-exploitation of groundwater resources for irrigated grain production in Hebei province threatens national grain food security. The objective of this study was to quantify agricultural water consumption (AWC) and irrigation water consumption in this region. A methodology to estimate AWC was developed based on Penman-Monteith method using meteorological station data (1984-2008) and existing actual ET (2002-2008) data which estimated from MODIS satellite data through a remote sensing ET model. The validation of the model using the experimental plots (50 m(2)) data observed from the Luancheng Agro-ecosystem Experimental Station, Chinese Academy of Sciences, showed the average deviation of the model was -3.7% for non-rainfed plots. The total AWC and irrigation water (mainly groundwater) consumption for Hebei province from 1984-2008 were then estimated as 864 km(3) and 139 km(3), respectively. In addition, we found the AWC has significantly increased during the past 25 years except for a few counties located in mountainous regions. Estimations of net groundwater consumption for grain food production within the plain area of Hebei province in the past 25 years accounted for 113 km(3) which could cause average groundwater decrease of 7.4 m over the plain. The integration of meteorological and satellite data allows us to extend estimation of actual ET beyond the record available from satellite data, and the approach could be applicable in other regions globally where similar data are available.

[Characteristics and simulation of heat and CO2 fluxes over a typical cropland during the winter wheat growing in the North China Plain].

In order to study the surface energy budget of the cropland in North China Plain, this paper discussed the characteristics of heat and CO2 fluxes of the cropland during the winter wheat growing, and then simulated the dynamic change of the flux of heat and carbon by SiB2 (simple biosphere model Version2) based on the observational data from 2005-10-10 to 2006-06-10 of Weishan experimental station. The results showed that the heat and CO2 fluxes put up obvious inter-daily variations in the course of the wheat growing and their maximum appeared at around midday. The diurnal variations of them are obvious too, and the net radiation, latent heat flux and the heat flux of underlaying surface during the winter were minimum while they were maximum in the anthesis and maturation of wheat; and the sensible heat flux during the winter was maximum and it was minimum in the shooting and heading period of wheat; while the CO2 flux during the shooting and heading period of wheat was the maximum and it was minimum during the winter. Finally, SiB2 model was used to simulate the heat and CO2 fluxes and the surface temperature based on the data of Weishan observation station and the results showed that the model was good at simulating, the simulated net radiation, latent heat flux, sensible heat flux, the heat flux of underlaying surface, CO2 flux and surface temperature were shown to be basically in agreement with observations with their square of related coefficient being 0.985, 0.637, 0.481, 0.725, 0.499 and 0.877 respectively. In addition, the average simulated value of net radiation, latent heat flux and the heat flux of underlaying surface were lower than the observed value and others were higher than the observed ones. In addition, the fluxes and surface temperature were simulated by SiB2 in the different stages of winter wheat and the results showed that the accuracy of the model in the shooting and heading period of wheat was better than the other two stages and the leaf area index was sensitive to the model.