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.

Statistical evaluation of proxies for estimating the rainfall erosivity factor.

Considering the high-temporal-resolution rainfall data requirements for calculating the Rainfall Erosivity factor (that is, the R-factor), studies have developed a large number of proxies for the R-factor (PR). This study aims to evaluate 15 widely used proxies, which were developed in various countries using daily, monthly, or yearly rainfall data, in terms of correlation and statistical equality with the R-factor by using the 6-min pluviographic data from 28 stations in Australia. Meng's test was applied to rank the correlations. Although the Meng's test indicated that the correlation between Rainfall Erosivity (R) and Rainfall Erosivity calculated by the proxy model (PR) generally increased with a finer time resolution of the rainfall data (in the order of year, month, and day), the 15 PRs under examination were all highly correlated with R (r > 0.62, p < 0.004), implying that all of them can be reasonably used as an R predictor. A direct estimation of the R-factor using PRs produced a mean relative error (MRE), root mean square error (RMSE), and Nash-Sutcliffe efficiency coefficient (NSE) with a mean of 50.0%, 1392 MJ mm ha-1 h-1 a-1, and 0.17, respectively. The linear calibrations improved the accuracy of the estimation and produced an MRE, RMSE, and NSE with a mean of 36.0%, 887 MJ mm ha-1 h-1 a-1, and 0.70, respectively. Finally, suitable proxies for instances where only daily, monthly, or yearly rainfall data are available were recommended.

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.

A spatially invariant sediment rating curve and its temporal change following watershed management in the Chinese Loess Plateau.

Sediment rating curves have long been used as a tool for predicting sediment yield. In this study, the spatial and temporal variations of rating curves were examined in the loess area (~15,000km2) of the Wuding River basin of the Chinese Loess Plateau. Data of mean daily water discharge per unit area, Qd (m3s-1km2), and mean daily sediment discharge per unit area, SDd (kgs-1km2), were compiled for three periods of 1959-1971, 1972-1989, and 2006-2013. Soil conservation measures were rarely implemented for the pre-1971 period but were intensively implemented for the latter periods. The proportional function, i.e. SDd=CdQd, fitted the rating curve well for all periods. The coefficient, Cd, is an approximation of the mean sediment concentration of surface runoff (MSCSR). Contrary to the usual belief, Cd was ca. 730kgm-3 whatever the location for the pre-1971 period; examinations at the instantaneous and the event time scales further demonstrated the spatial invariant MSCSR across six orders of magnitude in drainage area sizes (from 0.006 up to 3893km2). The spatial invariant MSCSR forms as a result of the ready availability of mass-wasted materials in headwaters and holds on as a result of the minimal in-stream erosion and sedimentation along the stream channel. For the post-1971 periods, both Cd and its spatial invariability decreased in a non-monotonous way, which was related to the check-dam (i.e., sediment trapping dams) constructions but not to the implementation of revegetation and terracing measures. As a result, the non-monotonous temporal trends of Cd and its spatial coefficient of variation both captured the intermittency of the check-dam construction well. Temporal changes in MSCSR or the sediment rating curve, which is less disturbed by rainfall, provides important insights into anthropogenic impacts on river systems.

Statistical determination of rainfall-runoff erosivity indices for single storms in the Chinese Loess Plateau.

Correlation analysis is popular in erosion- or earth-related studies, however, few studies compare correlations on a basis of statistical testing, which should be conducted to determine the statistical significance of the observed sample difference. This study aims to statistically determine the erosivity index of single storms, which requires comparison of a large number of dependent correlations between rainfall-runoff factors and soil loss, in the Chinese Loess Plateau. Data observed at four gauging stations and five runoff experimental plots were presented. Based on the Meng's tests, which is widely used for comparing correlations between a dependent variable and a set of independent variables, two methods were proposed. The first method removes factors that are poorly correlated with soil loss from consideration in a stepwise way, while the second method performs pairwise comparisons that are adjusted using the Bonferroni correction. Among 12 rainfall factors, I30 (the maximum 30-minute rainfall intensity) has been suggested for use as the rainfall erosivity index, although I30 is equally correlated with soil loss as factors of I20, EI10 (the product of the rainfall kinetic energy, E, and I10), EI20 and EI30 are. Runoff depth (total runoff volume normalized to drainage area) is more correlated with soil loss than all other examined rainfall-runoff factors, including I30, peak discharge and many combined factors. Moreover, sediment concentrations of major sediment-producing events are independent of all examined rainfall-runoff factors. As a result, introducing additional factors adds little to the prediction accuracy of the single factor of runoff depth. Hence, runoff depth should be the best erosivity index at scales from plots to watersheds. Our findings can facilitate predictions of soil erosion in the Loess Plateau. Our methods provide a valuable tool while determining the predictor among a number of variables in terms of correlations.

Sediment delivery ratio of single flood events and the influencing factors in a headwater basin of the Chinese Loess Plateau.

Little is known about the sediment delivery of single flood events although it has been well known that the sediment delivery ratio at the inter-annual time scale is close to 1 in the Chinese Loess Plateau. This study examined the sediment delivery of single flood events and the influencing factors in a headwater basin of the Loess Plateau, where hyperconcentrated flows are dominant. Data observed from plot to subwatershed over the period from 1959 to 1969 were presented. Sediment delivery ratio of a single event (SDRe) was calculated as the ratio of sediment output from the subwatershed to sediment input into the channel. It was found that SDRe varies greatly for small events (runoff depth <5 mm or rainfall depth <30 mm) and remains fairly constant (approximately between 1.1 and 1.3) for large events (runoff depth >5 mm or rainfall depth >30 mm). We examined 11 factors of rainfall (rainfall amount, rainfall intensity, rainfall kinetic energy, rainfall erosivity and rainfall duration), flood (area-specific sediment yield, runoff depth, peak flow discharge, peak sediment concentration and flood duration) and antecedent land surface (antecedent precipitation) in relation to SDRe. Only the peak sediment concentration significantly correlates with SDRe. Contrary to popular belief, channel scour tends to occur in cases of higher peak sediment concentrations. Because small events also have chances to attain a high sediment concentration, many small events (rainfall depth <20 mm) are characterized by channel scour with an SDRe larger than 1. Such observations can be related to hyperconcentrated flows, which behave quite differently from normal stream flows. Our finding that large events have a nearly constant SDRe is useful for sediment yield predictions in the Loess Plateau and other regions where hyperconcentrated flows are well developed.

Identification of factors influencing the restoration of cyanobacteria-dominated biological soil crusts.

Biological soil crusts (BSCs) cover >35% of the Earth's land area and contribute to important ecological functions in arid and semiarid ecosystems, including erosion reduction, hydrological cycling, and nutrient cycling. Artificial rapid cultivation of BSCs can provide a novel alternative to traditional biological methods for controlling soil and water loss such as the planting of trees, shrubs, and grasses. At present, little is known regarding the cultivation of BSCs in the field due to lack of knowledge regarding the influencing factors that control BSCs growth. Thus, we determined the effects of various environmental factors (shade; watering; N, P, K, and Ca concentrations) on the growth of cyanobacteria-dominated BSCs from the Sonoran Desert in the southwestern United States. The soil surface changes and chlorophyll a concentrations were used as proxies of BSC growth and development. After 4 months, five factors were found to impact BSC growth with the following order of importance: NH4NO3 ≈ watering frequency>shading>CaCO3 ≈ KH2PO4. The soil water content was the primary positive factor affecting BSC growth, and BSCs that were watered every 5 days harbored greater biomass than those watered every 10 days. Groups that received NH4NO3 consistently exhibited poor growth, suggesting that fixed N amendment may suppress BSC growth. The effect of shading on the BSC biomass was inconsistent and depended on many factors including the soil water content and availability of nutrients. KH2PO4 and CaCO3 had nonsignificant effects on BSC growth. Collectively, our results indicate that the rapid restoration of BSCs can be controlled and realized by artificial "broadcasting" cultivation through the optimization of environmental factors.

Temporal variations of ecological security with soil and water loss stress in black soil region of northeast China: a case study on Baiquan County.

The deterioration of ecological situation with serious soil and water loss in black soil region of northeast China has attracted more attention due to its significant role on food security of China. To investigate the temporal characteristics of ecological status in typical black soil areas, Baiquan County is selected. Based on the model of Press-Status-Response (P-S-R), indicators are established and the ecological security situations with soil and water loss of Baiquan County are evaluated for the years of 1979, 1990, 2000 and 2005. The results show that the ecological insecurity indicator changes from 0.701 to 0.435 from 1979 to 2005, with a decrease of 37.9% for Baiquan County. And the contributions of physical and human factors to the temporal variations of the ecological security are discussed in detail. Moreover, several problems are recognized to be the potential threats to the ecological security in Baiquan county, including reduction of the effective thickness, excessive application of the fertilizer and low efficiency of the agricultural irrigation system. It is found that effective soil and water loss control actions have made great contribution to the improvement of the ecological security in Baiquan county. All these results and discussions are very helpful for the further investigation on the quantitative relationship between soil and water loss and ecological security in black soil region of northeast China.