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Abstract Rill erosion is one of the most important processes affecting soil detachment. Therefore, the quantification of the soil detachment process is important to establish a basic process-based erosion prediction model. The objective of this study was to assess the contributions of the hydraulics parameters and sediment content to the soil detachment rate of purple soil. Purple soil was collected from a typical hilly region in Sichuan, China. Six scouring experiments were conducted in two 1 m × 2 m boxes at six different flow discharge rates (2, 4, 6, 8, 10, and 12 L min−1) in an upslope drainage area (0.30 m2) with a 15° slope. The results show that the soil detachment rate greatly changes during the rill development stage and the soil detachment rate varies at a sediment content of 0.15 g L−1. Furthermore, the soil detachment rate is negatively related to the sediment content. The relationships between the sediment content and hydraulics parameters can be described with a negative power function. The soil detachment rate is positively related to the stream power; the relationship can be described with a linear function. The comprehensive relationship between the soil detachment rate and sediment content and stream power can be described with a binary linear equation. The proportional contribution of the sediment content to the soil detachment rate is 80.65% and that of the stream power is 19.35%. The sediment content has a negative effect on the soil detachment rate in rills. The stream power has a positive effect; it induces soil detachment. The negative effect of the sediment content on the soil detachment rate is influenced by the stream power. This study provides new insights into the effects of the sediment content and hydraulics on the rill erosion process. Highlights • We provided a new insight for revealing the detachment mechanism of rill erosion in purple soil area. • We figured out the contribution rate of sediment content and hydraulics parameters to soil detachment rate. • Using sediment content and stream power to forecast rill erosion intensity was feasible. • Soil detachment process model was established based on sediment content and hydraulics parameters. [ABSTRACT FROM AUTHOR]

Purple soil bunds are embankments constructed along the contour on purple soil sloping farmlands, and play a key role in controlling soil erosion. The soil shear strength makes a significant influence to the bund stability; however, few reports have documented how purple soil shear strength responds to the soil physical properties. The main goal of the present study was to determine how indicators of soil shear strength vary with the soil water content and dry density. In this study, we collected soil samples from 3 purple soil bunds in Zhong County, in the Three Gorges Reservoir Area in China, and performed an unconsolidated undrained triaxial compression test to study the soil shear strength in terms of the cohesion ( c ), internal friction angle ( φ ), and the principal stress difference ( σ 1 – σ 3 ). The test results show that when the dry density was constant and the soil water content increased, the soil cohesion increased and then decreased, and the results fitted to a quadratic curve. As the soil water content increased, the internal friction angle of the soil bunds decreased and displayed a first-order exponential decay. As the soil water content increased and the confining pressure remained constant, the principal stress difference decreased rapidly. When the soil water content was constant and the dry density increased, the soil cohesion, internal friction angle, and the total principal stress difference increased, although at different rates. In general, the water content had a greater effect on the cohesion, internal friction angle, and the principal stress difference than the dry density, but there were little or no interaction between water content and dry density. Furthermore, except when the water content was 6%, the stress–strain characteristics were similar across the range of water contents. For a low water content, fixed confining pressure, and an increasing dry density, the curves gradually changed from the hardening type to the weak hardening type and then to the softening type. In most cases the curves corresponded with the hardening type as the confining pressure increased. [ABSTRACT FROM AUTHOR]

Loess and purple soils are two very important cultivated soils, with the former in the loess region and the latter in southern sub-tropical region of China, featured with high-risks of erosion, considerable differences of soil structures due to differences in mineral and nutrient compositions. Study on soil erodibility ( K r ) and critical shear stress ( τ c ) of these two soils is beneficial to predict soil erosion with such models as WEPP. In this study, rill erosion experimental data sets of the two soils are used for estimating their K r and τ c before they are compared to understand their differences of rill erosion behaviors. The maximum detachment rates of the loess and purple soils are calculated under different hydrodynamic conditions (flow rates: 2, 4, 8 L/min; slope gradients: 5°, 10°, 15°, 20°, 25°) through analytical and numerical methods respectively. Analytical method used the derivative of the function between sediment concentration and rill length to estimate potential detachment rates, at the rill beginning. Numerical method estimated potential detachment rates with the experimental data, at the rill beginning and 0.5 m location. The K r and τ c of these two soils are determined by the linear equation based on experimental data. Results show that the methods could well estimate the K r and τ c of these two soils as they remain basically unchanged under different hydrodynamic conditions. The K r value of loess soil is about twice of the purple soil, whereas the τ c is about half of that. The numerical results have good correlations with the analytical values. These results can be useful in modeling rill erosion processes of loess and purple soils. [ABSTRACT FROM AUTHOR]

Purple soil, a loamy entisol dominating the hilly central Sichuan basin in the upper reaches of Yangtze River, is low in organic matter content and abundant in macropores that form the rapid transport network for water and solutes including pesticides. However, effects of soil's macroporosity on transport of pesticides has not been quantitatively explored and simulated at the column scale. In this study, we explore the transport of 3,5,6-trichloro-2-pyrdionl (TCP), a main pesticide degradation product, in purple soil through experiments and simulations on typical farmland categories, including rice-rape and vegetable farmland. Soil water retention characteristics (SWRCs) and physical properties are measured to learn the hydraulic parameters and porous structures of the soil. Batch experiments are conducted to obtain the adsorption coefficient of TCP on soil, and breakthrough curves of Br − (a conservative tracer) in undisturbed soil columns are applied to evaluate the hydrodynamics of TCP transport. The two-region model is used to simulate the transport processes, and the parameters are obtained by inversion simulation. It is found that TCP transport in purple soil exhibited complex patterns and is strongly affected by both chemical and physical non-equilibrium. A higher pollution risk on water body of rice-rape field is due to the stronger preferential flow and better pore connectivity despite the larger macroporosity observed in vegetable field. These results can provide useful data and modeling approach for evaluating the risk of pesticide leaching toward groundwater. [ABSTRACT FROM AUTHOR]

Agricultural non-point source pollution of nitrogen (N) and phosphorus (P) exerts a key influence on aquatic environment safety. Purple soil and sloping cropland make up the highest proportion of all soil types and land-use pattern in the up and middle reaches of Yangtze River. Therefore, a study on nutrients balance for risk assessment in sloping cropland of purple soil will contribute to the understanding of the causes of eutrophication in aquatic environment in China. The Organization for Economic Cooperation and Development (OECD) have developed the soil surface nitrogen balance indicator to estimate potential N pollution through measuring the difference between agricultural inputs and outputs. Based on this indicator, we established a framework, a methodology, and a database for calculating the N and P balance for typical sloping cropland of purple soil in TGR region, and evaluated the impact of nutrients balance on agricultural production and environment. For the basic watershed of Xinzheng unit, which mainly consists of sloping cropland of purple soil, N and P inputs were 755 and 107 kg ha −1 a −1 , with 68.4% of N and 56.1% of P inputs originating from chemical fertilizer. The N and P outputs were 482 and 60 kg ha −1 a −1 ; the N in crops and P in straw accounted for 23.5% and 45.6% of total outputs, respectively. The N and P surplus intensities were 273 and 47 kg ha −1 a −1 , much higher than the respective risk thresholds of 180 kg ha −1 a −1 and 35 kg ha −1 a −1 , indicating a risk of N and P potential pollution in aquatic environments via cropland discharge. It is essential to utilize proper fertilization methods and improve fertilizer use efficiency in sloping cropland of purple soil in China. [ABSTRACT FROM AUTHOR]

The susceptibility of purple soil and intensive tillage render the land prone to erosion under heavy precipitations in a sloping cropland in southwestern China. This study aimed to improve the evaluation of the potential benefits of surface protection tillage and organic matter addition to decreasing nutrient losses. A field plot experiment under natural rainfall conditions was conducted, which employed four management practices: conventional downslope tillage system as control (CK), contour tillage (CT) with organic matter addition (CT + OM), CT with wheat straw mulching (CT + SM), and CT combining straw mulching and organic matter addition (CT + OM + SM). Runoff depth, nutrient loads, and soil erodibility were used to estimate the effects of straw mulching and organic matter addition. Results indicated that the runoff depth under CK was largest during the experimental period, with an average of 16.91 mm, and runoff coefficient average was 32%. Compared with CK, the runoff depth under CT + OM, CT + SM, and CT + OM + SM were reduced by 19%, 34%, and 50%, respectively. A significant difference in soil erodibility indicator among the four treatments was indicated (p < 0.05); CK achieved the highest value, whereas CT + OM + SM obtained the least value. In addition, the contour cultivation (i.e., CT approaches) were more sustainable than the downslope tillage system (i.e., CK). Soil erodibility under CK was 9.83 kg ha −1 mm −1 . Meanwhile, soil erodibility under CT + OM, CT + SM, and CT + OM + SM were 8.49, 6.99, and 6.87 kg ha −1 mm −1 , respectively. These values were 14%, 29%, and 30% lower than that of CK, respectively. CK was more susceptible to accelerated erosion compared with the plots with a surface cover or organic addition. This greater erodibility resulted in higher runoff, sediment yield, and associated nutrient loss for CK. The runoff-associated nitrogen losses were mainly controlled by the runoff rate and soil erodibility (p < 0.05). Variations in NO 3 − –N and NH 4 + –N concentration in runoff water were markedly affected by rainfall events and agricultural practice. A significant logarithmic correlation between NO 3 − –N load and runoff depth was identified. NO 3 − –N was proven to be the main form of inorganic nitrogen loss; therefore, fertilizer application of NO 3 − –N should be reduced in the purple soil region. Soil erodibility significantly influenced the available N losses (p < 0.01), which was best described by a positive logarithmic correlation. Soil nutrient concentration also played an important role in nitrogen loss. However, further research is needed to understand the dynamic interactions between soil erodibility as well as soil and nutrient losses. Results indicated that surface protection by CT + OM + SM is one of the good management practices to reduce soil loss by water erosion in regions with intense agricultural activity. [ABSTRACT FROM AUTHOR]

Summary Rills are commonly found on sloping farm fields in both the loess and the purple soil regions of China. A comparative study on rill erosion between the two soils is important to increase research knowledge and exchange application experiences. Rill erosion processes of loess and purple soils were determined through laboratory experiments with the volume replacement method. Water was used to refill the eroded rill segments to compute eroded volume before sediment concentration distribution along the rill was computed using the soil bulk density, flow rate, and water flow duration. The experimental loess soil materials were from the Loess Plateau and purple soil from the southwestern part of China, Chongqing City. A laboratory experimental platform was used to construct flumes to simulate rills with 12.0 m length, 0.1 m width, and 0.3 m depth. Soil materials were filled into the flumes at a bulk density of 1.2 g cm −3 to a depth of 20 cm to form rills for experiments on five slope gradients (5°, 10°, 15°, 20°, and 25°) and three flow rates (2, 4, and 8 L/min). After each experimental run under the given slope gradient and flow rate, the rill segments from the upper slope between 0–0.5, 0.5–1, 1–2, 2–3, …, 7–8, 8–10, and 10–12 m were lined with plastic sheets before be re-filled with water to determine sediment concentration after the eroded volumes was measured. Rill erosion differed between the two soils. As purple soil started to erode at a higher erosive force than loess soil, it possibly exhibits higher resistance to water erosion. The subsequent erosion process in the eroding purple rill was similar to that in the loess rill. However, the total erosion in the eroding loess rill was more than that in the eroding purple rill. The maximum sediment concentration transported by the eroding purple rills was significantly lower, approximately 55% of those transported by the loess rills under the same flow rate and slope gradient. Hence, less purple sediments can be transported compared with loess sediments. These findings were validated through comparison with existing studies. The results indicate that the research methods and experiences on rill erosion can be inter-exchanged between the loess and purple soils. [ABSTRACT FROM AUTHOR]

The relationship between soil fauna groups and different fertilization regimes has been of continuous concern. Little attention has been paid to the response of soil fauna to changes in soil fertility. Thus, the aim of this study was to investigate the response of soil fauna communities to fertilizer management practices, to explore the inter-relationships between soil fauna communities and fertilization regimes so as to identify soil fauna species or groups that are sensitive to changes in soil fertility. This long-term fertilization experiment was conducted with a no fertilizer control and six fertilization regimes: CK (control, no fertilizer), N (synthetic N fertilizer), NPK (synthetic fertilizer: nitrogen, phosphorus and potassium), OM (pig manure), OMNPK (pig manure plus nitrogen, phosphorus and potassium), RSD (crop residues returned) and RSDNPK (crop residues returned with nitrogen, phosphorus and potassium). The application of organic fertilizers (i.e., pig manure or crop residues) promoted the diversity of the soil fauna communities due to the abundant organic matter the fertilizers supplied for the communities’ survival and development. Furthermore, organic–inorganic compound fertilizers (i.e., treatment of OMNPK or RSDNPK) were beneficial for richness and diversity of soil fauna communities due to their abundance of organic matter and supply of nutrients. Indices of soil fauna communities (i.e., TI, the DG (density-group) index, the number of individuals of Nemata, Lumbricida, Collembola and Oribatida) showed the same trend with an increase in the soil organic matter. Therefore, indices of soil fauna communities can be applied to indicate certain features of soil fertility, such as soil organic matter content, but they cannot be used to reflect integrated assessment for soil fertility. [ABSTRACT FROM AUTHOR]

Abstract: Cadmium (Cd), through human activities, has severely accumulated in purple soil. Purple soil is considered as one of the most important types of agricultural soil in Southern China. In this study, we investigated the release of Cd from 13 purple soils with a wide range of total Cd content collected from vegetable fields and forestlands in Sichuan Basin. Soil batch extraction and column leaching experiments were conducted to determine the influence of chemical speciation, soil properties, soil water contact time, and soil/water ratio on the release characteristics of Cd from soil. Results indicate that Cd release was primarily present in exchangeable, carbonate-bound, and organically bound fractions. Moreover, precipitation-induced, long-term saturated conditions enhance the release of Cd from soil to water. Multiple regression analysis also shows that the release of Cd from soil is related to pH level, organic matter amount, and total Cd content in native soil. This finding explains the Cd release variation of 86% and 95% during batch extraction and column leaching experiments, respectively. Finally, this investigation provides a more convenient way of evaluating potential ecological risks of Cd in purple soil by determining the properties of several common soil types. [Copyright &y& Elsevier]

Abstract: Soil aggregation is a crucial soil property that affects a wide range of physical and chemical processes in soil ecosystems. Arbuscular mycorrhizal (AM) association is recognised as an important promoter of soil aggregation through the action of individual roots, mycelia and an insoluble, glue-like and hydrophobic proteinaceous substance, which is (at least partly) of AM fungi origin, named glomalin-related soil protein. Considering the increasing application of commercial AM inoculants, we addressed how the soil aggregates respond to the hyphal functions of AM inoculation in the field with a resident AM community. To this end, we introduced a new system in which the hyphae were separated by mesh and regular rotation to break the ingrowing hyphae as a control and to demonstrate the causal link between the hyphae and soil aggregates under conditions simulating natural parameters. The results showed the following: (i) the hyphal length was positively correlated with the mean weight diameter (r = 0.384, P < 0.05), geometric mean diameter (r = 0.257, 0.05 < P < 0.10) and easily extractable glomalin (r = 0.296, P < 0.05); (ii) the colonisation rate of the roots in the cores was increased by constantly severing the extraradical mycelium and (iii) the colonisation rate of the control plant roots (approx. 10%) was significantly lower compared to those inoculated with AM fungi (ranging from 34% to 54%). It was concluded that the hyphal networks of AM inoculations can promote the formation and stability of soil aggregates under conditions that closely simulate those occurring in nature. [Copyright &y& Elsevier]

This study provides a combined dataset on N loss pathways and fluxes from sloping cropland in the purple soil area, southwestern China. A lysimeter experiment was conducted to quantify nitrate leaching (May 2004–May 2010) and N2O emission (May 2009–May 2010) losses. Nitrate leaching was the dominant N loss pathway and annual leaching fluxes ranged from 19.2 to 53.4 kg N ha−1, with significant differences between individual observation years (P < 0.05). Direct N2O emissions due to N fertilizer use were 1.72 ± 0.34 kg N ha−1 yr−1, which corresponds to an emission factor of 0.58 ± 0.12%. However, indirect N2O emissions caused by nitrate leaching and surface runoff N losses, may contribute another 0.15–0.42 kg N ha−1 yr−1. Our study shows that nitrate leaching lowered direct N2O emissions, highlighting the importance for a better understanding of the tradeoff between direct and indirect N2O emissions for the development of meaningful N2O emission strategies. [Copyright &y& Elsevier]

Summary: Bioavailable phosphorus (P) losses due to agriculture activity in a purple soil watershed in the Sichuan Basin of Southwestern China were monitored to define the hydrological controls of P transport. Our results indicate that the proportion of P that was transported in particulate form increased in the rainy season, and that the mass of total bioavailable P (BAP) loads exhibited seasonal fluctuations, wherein the majority (over 90%) was observed to have been exported between June and September. The proportion of bioavailable dissolved P (BDP) in the BAP discharge budget in the watershed varied between 11% and 15% during the monitoring period. The bioavailable particulate P (BPP) and BDP concentrations of stream water under rainstorm events increased by over 40% in comparison to their annual mean concentrations, and the annual BAP load was primarily dominated by the loads that occurred during rainstorm events in the study year. BAP concentration in groundwater significantly fluctuated with the seasons, and the ratio of total BAP in groundwater to that in surface water gradually increased during the rainy season. Thus, the impact of agriculture on the water quality of this watershed becomes clearly evident. [ABSTRACT FROM AUTHOR]

Abstract: The potential for using 7Be measurements to document soil redistribution associated with a heavy rainfall was estimated using 7Be method on a bare purple soil plot in the Three Gorges Reservoir region of China. The results were compared with direct measurement from traditional approaches of erosion pins and runoff plots. The study shows that estimation of soil losses from 7Be are comparable with the monitoring results provided by erosion pins and runoff plots, and are also in agreement with the existing knowledge provided by 137Cs measurements. The results obtained from this study demonstrated the potential for using 7Be technique to quantify short-term erosion rates in these areas. [Copyright &y& Elsevier]

Abstract: To characterize the phytoextraction efficiency of a hybrid poplar (Populus deltoids × Populus nigra) in cadmium contaminated purple soil and alluvial soil, a pot experiment in field was carried out in Sichuan basin, western China. After one growing period, the poplar accumulated the highest of 541.98±19.22 and 576.75±40.55μg cadmium per plant with 110.77±12.68 and 202.54±19.12g dry mass in these contaminated purple soil and alluvial soil, respectively. Higher phytoextraction efficiency with higher cadmium concentration in tissues was observed in poplar growing in purple soil than that in alluvial soil at relative lower soil cadmium concentration. The poplar growing in alluvial soil had relative higher tolerance ability with lower reduction rates of morphological and growth characters than that in purple soil, suggesting that the poplar growing in alluvial soil might display the higher phytoextraction ability when cadmium contamination level increased. Even so, the poplars exhibited obvious cadmium transport from root to shoot in both soils regardless of cadmium contamination levels. It implies that this examined poplar can extract more cadmium than some hyperaccumulators. The results indicated that metal phytoextraction using the poplar can be applied to clean up soils moderately contaminated by cadmium in these purple soil and alluvial soil. [Copyright &y& Elsevier]

Abstract: Nutrient loss from purple soils has been reported to increase pollution of the Yangtze River. However, few studies have addressed the variations of nutrient concentration in runoff during natural rainstorms in the regions. Nitrogen and phosphorus concentrations in runoff waters from a small agricultural watershed, in the purple soil region of southwest China, were investigated for four natural rainstorms occurred in a conventional double cropping system (wheat–corn) and another six rainstorms in a new triple cropping system (wheat–corn–sweet potato). The NO3 − concentrations in runoff for the observed rainstorms generally varied from 1.0 to 3.5gm−3, which were noticeably affected by flow rates. A significant logarithmic correlation between NO3 − concentrations and flow rates for each rainstorm was identified. In contrast, the concentrations of NH4 + and dissolved reactive phosphorus (DRP) in runoff fluctuated substantially without a noticeable trend for each rainstorm. Positive linear correlation between the concentrations of DRP and sediment for each rainstorm tested was found under the circumstances of double cropping system. In addition, the ratios of NO3 − to NH4 + for the loss amount in 10 rainstorms varied from 1 to 7 for the triple cropping system and 16–29 for the double cropping system. Furthermore, the ratios of the sum of NO3 − and NH4 + to DRP for the loss amount in 10 rainstorms ranged from 12 to 79 depending on the cropping systems. Nitrate nitrogen was proved to be the main form of inorganic nitrogen loss in runoff water in the purple soil region. Compared with the conventional double cropping system, the new triple cropping system tends to cause more NH4 + loss. These findings would help develop the effective erosion control strategies and select a suitable cropping system to reduce potential pollution hazards. [Copyright &y& Elsevier]

Abstract: Water and soil loss are the major causes of the land degradation in the hilly area of the Sichuan Basin, China, where purple soil (FAO, Calcaric Regosols) is widely distributed. Little research has been conducted on changes in overland flow and sediment during rain events on sloping cropland in this area. In this study, 45 artificial rainfall events were applied to five runoff plots with slopes of 9%, 18%, 27%, 36%, and 47%, respectively. Results showed that erosion processes on purple soil of sloping cropland are complex, because the sediment concentration changes with time under different situations. According to correlations between runoff discharge and sediment concentration, all erosion events were classified into three types: (1) positive correlation; (2) negative correlation; and (3) no correlation. Positive correlations were dominant, accounting for 60% of all events. Rainfall intensity, slope gradient, and tillage treatment were the main factors that influenced erosion events. [Copyright &y& Elsevier]

Abstract: Recently the effect of plant hedgerows on controlling soil and water loss has been well recognized, and this technology has been widely applied in the world. However, there are few studies on hedgerows’ effect on soil fertility of sloping lands. With an 8-year fixed field experiment, we investigated the effect of two different hedgerows on soil fertility through comparing with the control. Our results showed that along contour lines across the field, clay particles tended to accumulate above plant hedgerows but to be eroded downward below hedgerows. Except for potassium (K), all plant nutrients and soil organic matter showed the same distribution pattern as clay particles. K, however, was evenly distributed in the field without any noticeable influence from hedgerows. Since the beginning of our field experiment, soil phosphorus (P) kept accumulating, while soil organic matter and K were in depletion. Taken together, our results suggest that better nutrient management for the sloping lands should reduce P but increase farm manure and K. As far as the whole sloping field is concerned, special attention in nutrient management should be paid to the soil stripes below hedgerows, the portions suffering from more serious soil erosion. [Copyright &y& Elsevier]

Abstract: Determining soil nutrient distribution is critical to identify sites which are at risk of N and P loading. Equally important are determining factors that influence such distribution (e.g. land use, land management, topography, etc.). In this research, soil nutrient distribution and its influencing factors were studied in a small-scaled watershed in the purple soil region of Sichuan Province, China. The watershed is 1.3km2 with a complex land management system including agriculture and forestry. Surface soil samples (0–20cm) from 48 sites within the watershed were collected in the spring of 1999 and analysed for chemical properties. When spatial distribution patterns of soil organic matter and soil nutrients were considered, several patterns were evident. Soil organic matter and total nitrogen coincided with high soil nutrients in the center of the watershed. Total phosphorus was linked to low soil nutrient contents on hilltops, while total potassium was characterized by low variability and high soil nutrient content throughout the watershed. Water-soluble nutrients in soils were highly variable throughout the watershed. Results of this study indicate that land use, topography and other variables play important roles in controlling the spatial distribution of most soil nutrients. However, the relative roles of these indices were nutrient specific. The relationship was therefore complex. Land use, silt, and sand content contributed to a high level of heterogeneity for soil organic matter and total nitrogen with the former being the dominant factor. Land use, particle size distribution (silt and sand) and topographic factors (slope and elevation) contributed to the variability of total phosphorus. However, particle size distribution and topographic factors were the dominant factors affecting total phosphorus. Elevation influenced the concentration of total potassium and land use controlled the concentration of dissolved phosphorous. No significant relationship was found between the studied characteristics and nitrate nitrogen. [Copyright &y& Elsevier]

Abstract: Bedload sediment is not only a major pollutant, but can also act as a transporter of pollutants. Sediment flow behavior often reflects the characteristics of a watershed, such as land use and management. To clarify the behavior of sediment flow under different spatial scales and storm types in the Chinese purple soil area, the sediment flow behavior in three small agro-watersheds with different catchment areas, physical characteristics and land use types was investigated during natural rainfall events from 1999 to 2000. The results showed that there were three types of sediment hydrographs, waveform or linear with a low sediment concentration (1.13±0.58gl−1), logarithmic decrease (3.41±4.13gl−1), and compound linear and logarithmic decrease (9.58±10.90gl−1). During PP-type rainfall events (high-intensity rainfall occurred during the prophase of the rainfall process) and ML-type rainfall events (high intensity rainfall occurred during the middle of the rainfall event), the waveform or linear type sediment hydrographs or the logarithmic decrease type hydrographs were generated in each of the three agro-watersheds. However, sediment hydrographs produced by IM-type rainfall events (discontinuous rainfall with obvious segments) depended on the spatial scale of the watershed. As the spatial scale decreased, the hydrographs were transformed from the logarithmic decrease type to the compound type. In addition, it was found that the S = aQ b model could effectively simulate the sediment hydrologic process of a single rainfall event. The information provided in this study will help evaluate and prevent soil and water loss and non-point source pollution in the purple soil region. [Copyright &y& Elsevier]

Abstract: Most of the studies reporting on nitrogen (N) loss from purple soil in the Three Gorges Reservoir region in China have focused only on soil losses and surface slope hydrological processes. Few investigations have addressed the fact that the N losses are associated with the dominant runoff process, and in particular with subsurface flow pathways. A simulation study was conducted in an area of purple soil near the Three Gorges Reservoir to evaluate the N losses associated with surface runoff and subsurface runoff under artificial rainfall simulation. Three rainfall intensities (low [0.5mmmin−1], moderate [1.0mmmin−1] and high [2.0mmmin−1]) and two fertilizer treatments (control [no fertilizer input] and fertilized [200kgNhm−1]) were tested. The results showed that only subsurface flow was observed following the low intensity rainfall events. During moderate and high rain intensities, both surface flow and subsurface flow were observed. A prolonged low intensity rainfall is much more likely to cause subsurface flow and lead to larger volumes of subsurface runoff than a short high intensity rainfall. Generation of subsurface flow by means of macropores resulted in large N losses, especially of nitrate N (NO3–N) which was mainly transferred by the subsurface flow, rather than the surface flow, during all rainfall events. In the control treatment, the average NO3–N concentration in the subsurface flow was six times greater than that in the surface flow. In the 200kghm−1 N input treatment, the average NO3–N concentrations in the subsurface flow from plots 1 and 2 were 23.83 and 25.62mgl−1, respectively, which were more than twenty times as much as that in the surface flow. The fertilizer treatment greatly increased the loss of NO3–N by subsurface flow. The average NO3–N concentrations in the subsurface flow from the fertilized plots were over five times greater than from the non-fertilized plot. Under low intensity rainfalls, N loss, especially in the form of NO3–N, was higher than under the other two rain intensities as a result of macropore flow. Up to a half of the total NO3–N load was attributed to subsurface runoff from almost all the rainfall events (except for high intensity rainfall in plot 3). However, the application of urea fertilizer tended to have little effect on NO3–N losses in the surface flow. Rainfall events after urea application probably enhanced NH3–N loss in the surface hydrological pathway. In the purple soil region, the N loss is primarily governed by hydrological processes especially the subsurface flow, and local agricultural practices. [Copyright &y& Elsevier]