Impact of no-tillage agricultural systems on sediment yield in two large catchments in Southern Brazil

Purpose

The impact of agriculture on water resources has long been a problem associated with the formation of runoff, the siltation of lakes and reservoirs, and overall depletion of water quality. In Brazil, these problems are mainly related to soil degradation by water erosion. However, studies of catchment-scale erosion are still rare particularly in grain-producing regions which have adopted conservative tillage systems for soil protection. In order to contribute to a better understanding of the impact of conservation agriculture on water resources, this study determined the runoff coefficient and sediment yield for two agricultural catchments.

Materials and methods

Hydrological and sedimentological monitoring was conducted in two catchments: the Conceicao catchment is characterized by grain production in weathered soils and a gently sloping landscape, while the Guapore catchment is characterized by heterogeneous soils and topography. Both catchments have problems associated with water erosion.

Results and discussion

The magnitudes of annual runoff coefficients and sediment yield were high, even if compared to similar agricultural regions, including a catchment with widespread adoption of no-tillage. The sediment yield was 140 t km^?2 year^?1, and the runoff coefficient was 14 % for the Conceicao catchment, while the sediment yield was 270 t km^?2 year^?1, and the runoff coefficient was 31 % for the Guapore catchment. The results indicate that problems such as gullies, soil compaction, runoff, floods, siltation, and water quality depletion associated with the misuse of agricultural areas in terms of soil conservation and water use are still evident and important even in regions with widespread adoption of no-tillage systems.

Conclusions

The magnitudes of both runoff and sediment yield clearly indicate the need to adopt complementary practices of soil conservation measures, such as mechanical runoff control.     

坡度与种植方式对紫色土侵蚀与养分流失的影响研究

在涪陵区水土保持监测分站内建立6个径流小区,对紫色土坡耕地水土流失进行试验监测,结果表明:各小区径流量、产沙量总体随降雨量的增大而增大.自然生态修复措施防治水土流失效果显著,3.66 m3的径流量仅有0.41 kg·m3的产沙量,可有效防治紫色土坡耕地的水土流失.采用顺坡耕作措施的小区径流量与降雨量达显著正相关,而产沙量与降雨量未显著相关.径流量和产沙量大小顺序均为25°坡耕地>20°坡耕地>10°坡耕地>15°坡耕地,在15°耕地上径流量与产沙量均为最小,但15°是否是涪陵区最适宜的耕地坡度,仍有待今后收集更多的降雨资料加以分析说明.对一次强降雨进行养分流失观测,养分流失量与径流量及产沙量大小顺序基本一致.开发建设项目弃土弃渣监测点径流量与产沙量均大大高于其他小区,说明开发建设水土保持项目中防治水土流失的重要性.     

Bulk density and content,density and stock of carbon,nitrogen and heavy metals in vegetable patches and lawns of allotments gardens in the northwestern Ruhr area,Germany

Purpose

In view that soils are bodies and that processes such as storage and release of water, carbon, nutrients and pollutants, and aeration and rooting happen in these bodies, it is of interest to know the density of elements and compounds in soils. On the basis of soil bulk and element density of organic carbon (OC), N, and heavy metals in soils and of horizon thickness, stocks of these elements for garden soils were calculated.

Materials and methods

Fourteen gardens in four allotments of the northwestern part of the Ruhr area, Germany were investigated. The research included 14 vegetable patches, 13 lawns, 2 compost heaps, and 1 meadow. Volume samples were taken. The soil analysis included pH, soil bulk density, and OC, N, Pb, Cd, Zn, Cu, and Ni contents.

Results and discussion

The soils were from sandy loam to loamy sand. The pH was slightly acid and C/N ratio about 20. Soil bulk density was between 0.8 and 1.4 g cm^?3 and mean bulk density was 1.1 g cm^?3. Mean OC content was for compost 7.4 %, vegetable patches 5.2 % (0–30 cm depth), and lawns and meadow 5.8 and 5.2 % (0–5 cm depth). OC density for compost was 76 mg cm^?3, vegetable patches 56 mg cm^?3, and lawns 67 mg cm^?3 (0–5 cm). Mean OC stock in 0–30 cm soil depth in vegetable patches was 16.4 kg m^?2, lawns 15.5 kg m^?2, and meadow 11.1 kg m^?2. N contents were between 0.06 and 0.46 %. For compost, the mean was 0.39 %, vegetable patches 0.27 % (0–30 cm), lawn 0.28 %, and meadow 0.26 % (0–5 cm). Mean stock of N in 0–30 cm depth for vegetable patches was 0.84 kg m^?2, lawn 0.76 kg m^?2, and meadow 0.55 kg m^?2. For heavy metals in compost, vegetable patches, lawn and meadow, Cd contents were in the range of 1.7 to 3.0 mg kg^?1, Pb 49 to 152 mg kg^?1, and Zn 52 to 1830 mg kg^?1. The amounts stored per square meters in 30 cm depth were for Cd 0.6–1.1 g, Pb 15–52 g, Zn 41–440 g, Cu 4–39 g, and Ni 1–8 g.

Conclusions

Allotment gardens have a high capacity to store CO₂ as OC. Roughly, there will be 7–8 million tons of OC stored in the 1.3 million allotment gardens of Germany. The high amount of 8000 kg N ha^?1 could damage the groundwater when released by wrong soil management. Cd, Zn, Pb, Cu, and Ni amounts of 7.8, 1000, 300, 135, and 30 kg ha^?1, respectively, are a lasting burden.

     

Effect of rice straw mulching on migration and transportation of Cd,Cu, Zn,and Ni in surface runoff under simulated rainfall

Purpose

Heavy metals in runoff from contaminated land are becoming a major environmental problem. The presented paper considers the effects of mulching with rice straw on the migration and transportation of heavy metals from the soil into runoff under conditions of simulated rainfall.

Materials and methods

A simulated rainfall experiment was conducted to investigate the impact of rice straw mulching on emissions of sediment and heavy metals in runoff. The soil box was in 20-cm depth with a surface area of 1 m² and the slope was set to 10°. The rainfall intensity was 90 mm h^?1with a 60-min rainfall duration. The study involved samples with different treatments of rice straw mulching: bare soil (BS), low mulching (LM), and high mulching (HM), which had straw contents of 0, 200, and 500 g m^?2, respectively.

Results and discussion

The results showed that compared with BS, the cumulative runoff volume declined by 31 and 50 % and cumulative sediment declined significantly by 93 and 97 % for the LM and HM treatments, respectively. Additionally, with an increase of straw mulching, the concentrations of total heavy metals in the LM and HM treatments declined by 79.90–82.84 and 81.90–90.07 %, and the cumulative total heavy metals decreased significantly by 86.5–87.0 and 90.3–94.6 %, respectively. Particulate-bound heavy metals decreased by 88.1–88.9 % for the LM and 94.5–97.1 % for the HM. Furthermore, Cd, Cu, Zn, and Ni migrated and transported mainly in particulate-bound form and had high enrichment in sediments.

Conclusions

Therefore, straw mulching on soil could reduce the sediment yields, and the loss of both particulate-bound heavy metals, especially for Cd and Ni, and cumulative total heavy metals in runoff. Accordingly, it can be used as an effective measure to control heavy-metal-contaminated soil posing pollution risk to environment through surface runoff.

     

Crop Residue Effects on Total and Dissolved Losses of Fe,Mn, Cu,and Zn by Runoff

Runoff may cause losses of micronutrients from soils. This can result in environmental problems such as contaminant transfers to water or a decrease in soil fertility. Appropriate soil management may reduce these micronutrient losses. This study examined the effect of applying crop residues to the soil surface on iron (Fe), manganese (Mn), copper (Cu), and zinc (Zn) losses by runoff. Runoff and sediment yield were measured on 1-m² plots using a rainfall simulator with constant 65 mm h^?1 intensity. Eight successive rainfall applications were performed at 65 mm each. Corn (Zea mays L.) straw was applied to plots at rates ranging from 0 to 8 t ha^?1. Both total and dissolved concentrations of the micronutrients studied were decreased by corn straw applications. After 520 mm cumulative rainfall, total soil losses ranged from 150 to 15354 kg ha^?1 depending on the amount of corn straw applied. Total micronutrient concentrations in runoff were as follows: Fe from 14.98 to 611.12 mg L^?1, Mn from 0.03 to 0.61 mg L^?1, Cu from 0.10 to 1.43 mg L^?1, and Zn from 0.21 to 5.45 mg L^?1. The relative contribution of the dissolved fraction to the total micronutrient content loss was low, but varied depending on the nutrient, being less than 1 percent for Fe and Mn and almost 10 percent for Zn. Total and dissolved concentrations in runoff of the studied elements decreased exponentially as the rate of applied corn straw increased. In conclusion, the addition of corn straw to soil reduced micronutrient losses.     

Importance of bank erosion for sediment input, storage and export at the catchment scale

Purpose

The importance of bank erosion was quantified during three periods (October 2006–April 2007, May 2007–April 2008 and May 2008–April 2009) in the 486 km² catchment area of River Odense, Denmark. A catchment sediment budget was established including other sediment sources such as tile drains and surface runoff, in-channel and overbank sinks and storage and the resulting bed load and suspended sediment load exported from the catchment.

Material and methods

Bank erosion and sedimentation were measured using ca. 3,000 erosion pins established in 180 pin plots, each consisting of three vertical lines of pins. Thirty-six representative reaches, each with a length of 100 m, were selected by a stratified random procedure in GIS. Bed load and suspended sediment export from the catchment were measured using a bed load sampler and from continuous measurements of turbidity at the outlet gauging station.

Results and discussion

The gross sediment input from bank erosion during the three study periods amounted to 21,100–25,200 t in the River Odense catchment, which is considerably higher than the estimated input of sediment from tile drains and surface runoff, which amounted to 220–500 t and 0–100 t, respectively. The measured bed load (20–490 t) was five to 60 times lower than the suspended sediment export from the catchment (1,240–2,620 t) during the three study periods, with the largest difference occurring in the driest year. Sediment sinks and storage were of high importance for the catchment sediment budget as the measured in-channel storage of sediment on stream banks was as high as 16,200–20,100 t, and the overbank sediment sink was estimated at 360–3,100 t.

Conclusions

Bank erosion was the dominant sediment source (90–94 %) in the River Odense catchment during the three study years. In-channel and overbank sediment sinks and storage dominated the sediment budget as 79–94 % of the sediment input from all sources was not exported from the catchment during the three study years. Such a large attenuation of sediment in river channels and on floodplains is extremely important for fluvial habitats and ecology. Moreover, it has strong implications for attempts to document changes in sediment export following implementation of mitigation measures.     

Connectivity of sediment transport in a semiarid environment: a synthesis for the Upper Jaguaribe Basin,Brazil

Purpose

Hydrosedimentological studies conducted in the semiarid Upper Jaguaribe Basin, Brazil, enabled the identification of the key processes controlling sediment connectivity at different spatial scales (10⁰–10⁴ km²).

Materials and methods

Water and sediment fluxes were assessed from discharge, sediment concentrations and reservoir siltation measurements. Additionally, mathematical modelling (WASA-SED model) was used to quantify water and sediment transfer within the watershed.

Results and discussion

Rainfall erosivity in the study area was moderate (4600 MJ mm ha^?1 h^?1 year^?1), whereas runoff depths (16–60 mm year^?1), and therefore the sediment transport capacity, were low. Consequently, ～60 % of the eroded sediment was deposited along the landscape, regardless of the spatial scale. The existing high-density reservoir network (contributing area of 6 km² per reservoir) also limits sediment propagation, retaining up to 47 % of the sediment at the large basin scale. The sediment delivery ratio (SDR) decreased with the spatial scale; on average, 41 % of the eroded sediment was yielded from the hillslopes, while for the whole 24,600-km² basin, the SDR was reduced to 1 % downstream of a large reservoir (1940-hm³ capacity).

Conclusions

Hydrological behaviour in the Upper Jaguaribe Basin represents a constraint on sediment propagation; low runoff depth is the main feature breaking sediment connectivity, which limits sediment transference from the hillslopes to the drainage system. Surface reservoirs are also important barriers, but their relative importance to sediment retention increases with scale, since larger contributing areas are more suitable for the construction of dams due to higher hydrological potential.     

Analysis of runoff,sediment dynamics and sediment yield of subcatchments in the highly erodible Isábena catchment,Central Pyrenees

Purpose

The Isábena catchment (445 km²), Spain, features highly diverse spatial heterogeneity in land use, lithology and rainfall. Consequently, the relative contribution in terms of water and sediment yield varies immensely between its subcatchments, and also temporally. This study presents the synthesis of ~2.5 years of monitoring rainfall, discharge and suspended sediment concentration (SSC) in the five main subcatchments of the Isábena and its outlet.

Materials and methods

Continuous discharge at the subcatchment outlets, nine tipping bucket rainfall and automatic SSC samplers (complemented by manual samples), were collected from June 2011 until November 2013. The water stage records were converted to discharge using a rating curve derived with Bayesian regression. For reconstructing sediment yields, the data from the intermittent SSC sampling needed to be interpolated. We employed non-parametric multivariate regression (Quantile Regression Forests, QRF) using the discharge and rainfall data plus different aggregation levels of these as ancillary predictors. The subsequent Monte Carlo simulations allowed the determination of monthly sediment yields and their uncertainty.

Results and discussion

The stage–discharge rating curves showed wide credibility intervals for the higher stages, with great uncertainties associated with the discharge rates, especially during floods. The water yield of the subcatchments differed considerably. The entire catchment’s output was dominated by the northernmost subcatchment (~360 mm year^?1). The smaller, southern subcatchments featured much higher variability and lower runoff rates (55–250 mm year^?1). The SSCs exhibited a wide range and can exceed 100 g l^?1 for the central subcatchments, where most of the badlands are located. For the reconstruction of the sedigraphs, the QRF method proved suitable with Nash–Sutcliffe indices of 0.50 to 0.84. The specific sediment yield ranges from relatively low (32 t km^?2 year^?1) in the highly vegetated north to high values (3,651 t km^?2 year^?1) in areas with many badland formations.

Conclusions

The Isábena catchment shows high erosion dynamics with great variability in space and time, with stark contrasts even between adjacent subcatchments. The natural conditions make water and sediment monitoring and instrumentation very challenging; the measurement of discharge is particularly prone to considerable uncertainties. The QRF method employed for reconstructing sedigraphs and monthly yields proved well suited for the task.     

金沙江干热河谷典型区段水土流失特征

以金沙江干热河谷典型区段不同处理坡面和林地、农地集水区为研究对象,采用坡面径流小区和集水区卡口站三角形薄壁堰实测法,在坡面径流小区和集水区土地利用现状调查基础上,通过降雨、径流及泥沙观测,分析金沙江干热河谷典型区段水土流失特征。结果表明:(1)不同处理坡面径流小区产流量、产沙量大小依次为坡耕地>自然坡面>水平阶整地>反坡台整地,表明反坡台和水平阶造林整地以及植被良好的自然坡面对地表径流的拦蓄和对泥沙的削减都起到较好效果;(2)大暴雨对集水区尺度产流产沙贡献值大,产流降雨量在4.8～27.6mm区间,农地集水区径流深小于林地集水区,大于27.6mm时,农地集水区径流深反超林地集水区,而产沙量不受降雨量影响,为林地集水区低于农地集水区;(3)径流小区尺度与集水区尺度雨季水土流失起止时间一致,集水区径流系数小于径流小区,集水区产沙量大于径流小区。     

Hydrosedimentology of nested subtropical watersheds with native and eucalyptus forests

Purpose

Information on the effects of eucalyptus forests on hydrosedimentological processes is scarce, particularly at the catchment scale. Monitoring and mathematical modeling are efficient scientific tools used to address the lack of information for natural resource management and the representation and prediction of those processes. This study evaluates the effects of eucalyptus cultivation on hydrosedimentological processes in watersheds and to use the Limburg soil erosion model (LISEM) to represent and predict hydrological processes.

Material and methods

The study was conducted in two forested watersheds: the main watershed (94.46 ha) and a nested sub-watershed (38.86 ha), both cultivated with eucalyptus and residual riparian native forest, located in southern Brazil. Hydrosedimentalogical monitoring was conducted from 16th February 2011 to 31st December 2012, and LISEM model calibrations were performed on the bases of six storms events.

Results and discussion

The sediment yield for 2011 was 41.6 Mg km^?2 and 38.5 Mg km^?2 for the watershed and sub-watershed, respectively. An extreme event in 2012 provided greater sediment yield for the sub-watershed (99.8 Mg km^?2) than that for the watershed (51.7 Mg km^?2). Rainfall events with a greater maximum intensity generated rapid discharge and suspended sediment concentration responses in the sub-watershed due to the smaller drainage area and steeper landscape. In the main watershed, the accumulation of flood waves occurred for most events, with less steep hydrographs, and a later occurrence of the discharge peak after that of the sub-watershed. The LISEM adequately reproduced the peak discharge and runoff for the calibrated events; however, the peak time and the shape of the hydrograph were not adequately represented.

Conclusions

The hydrosedimentological patterns of the watershed and sub-watershed, both cultivated with eucalyptus, was characterized by sedimentographs preceding hydrographs during rainfall–runoff events where scale effects occur, with maximum discharge and specific sediment yield greater in the watershed than that in the sub-watershed. Empirical models based on hydrologic variables may be used for estimating the suspended sediment concentration and sediment yield. Therefore, LISEM may be used for the prediction of hydrological variables in these forested watersheds.     

Testing the influence of sediment granulometry on heterotrophic respiration with a new laboratory flow-through system

Purpose

Increased sedimentation due to land use intensification is increasingly affecting carbon processing in streams and rivers around the globe. This study describes the design of a laboratory-scale flow-through incubation system as a tool for the rapid estimation of sediment respiration. The measurements were compared with those obtained using an in situ closed chamber respiration method. The influence of sediment size on respiration rates was also investigated.

Materials and methods

Measurements were conducted on a pre-alpine gravel-bed river sediment separated into the following grain size fractions: > 60 mm (14.3%), 60–5 mm (60.2%), 5–2 mm (13.7%), 2–0.063 mm (11.1%) and <0.063 mm (0.6%). Concurrently, in situ and laboratory measurements were carried out on a naturally heterogeneous sediment. In situ respiration was determined in closed chambers as O₂ consumption over time, while in the laboratory, respiration was determined using flow-through respiration chambers. Oxygen concentrations were measured using a fibre-optic oxygen meter positioned at the inflow and outflow from the chamber.

Results and discussion

The mean respiration rates within naturally mixed riverbed sediments were 1.27 ± 0.3 mg O₂ dm^?3 h^?1 (n = 4) and 0.77 ± 0.1 mg O₂ dm^?3 h^?1 (n = 3) for the flow-through chamber system and closed chamber system, respectively. Respiration rates were statistically significantly higher in the flow-through chamber system (t test, p < 0.05), indicating that closed chamber measurements underestimated the oxygen consumption within riverbed sediments. Sediment grain size was found to significantly affect respiration rates in both systems (ANOVA, p < 0.001) with the fine sediment fraction (particle size <0.063 mm) having the highest respiration rate (r_flow-through = 51 ± 23 mg O₂ dm^?3 h^?1). The smallest fractions (2–0.063 and <0.063 mm), which represent approximately 12% of total sediment volume, contributed 60% of total respiration.

Conclusions

The study demonstrated that flow-through respiration chambers more accurately estimate the respiration rate within riverbed sediments than in situ closed chambers, since the former experiment imitates the natural conditions where continuous interstitial flow occurs in the sediments. We also demonstrated that fine sediments (<5 mm) substantially contribute to heterotrophic respiration in the studied gravel-bed river.

     

北京山区河岸植被的水土保持效益

通过野外简易径流小区的放水冲刷试验,研究北京山区河岸带不同植被类型水土保持效果差异。试验设计4,6,8L/min 3种上方来水流量以及6种植被类型小区坡面,冲刷历时31min,试验过程收集径流泥沙样,分析不同径流小区的产流产沙特征。结果表明:(1)各植被类型小区平均总产流产沙量(43.29L/m~2,0.71kg/m~2)均显著小于裸坡下平均总产流产沙量(65.86L/m~2,3.21kg/m~2)(p0.05)。植被具有明显的减流减沙效益,且减沙效益(78.66%)大于减流效益(36.16%);(2)不同植被类型间减流减沙效果不同,随着放水流量的上升,坡面产流速率、产沙速率不断上升,其中以狗牙根小区平均产流速率(6.16L/min)、平均产沙速率(55.50g/min)最小,水土保持效益较好;(3)不同植被类型小区稳定入渗速率差异显著(p0.05),均大于裸坡条件下的稳定入渗速率(42mm/h),其中以狗牙根小区坡面的径流稳定入渗速率最大(129.0mm/h)。综上所述,植被具有明显的蓄水保土效益,且不同的植被类型间减流减沙效果差异显著。研究结果为北京山区河岸带进行生态环境安全建设以及生态修复提供了基础理论依据,并且为河岸边坡的植被修复物种选择提供了参考依据。     

Superabsorbent polymers influence soil physical properties and increase potato tuber yield in a dry-farming region

Purpose

Superabsorbent polymers, new water-saving materials and soil conditioners, are used widely in dry-farming agriculture. However, little is known about their effects on the soil physical properties under dry-farming conditions. To elucidate the effects of two SAPs (Wote and microbe) at different doses on the soil bulk density, water status, potato growth, yield, and economic benefit in a dry-farming region, we conducted a 2-year fixed field position experiment in the semiarid drought-prone area of Ningxia, China.

Materials and methods

The two SAPs were diluted 1:10 (product:soil) and applied at different rates before planting, i.e., Wote SAP 30 kg ha^?1, Wote SAP 60 kg ha^?1, Wote SAP 90 kg ha^?1, microbe SAP 30 kg ha^?1, microbe SAP 60 kg ha^?1, and microbe SAP 90 kg ha^?1. The treatment without SAP was used as the control.

Results and discussion

The tilth soil bulk density decreased under different SAP doses compared with the control, and the soil total porosity improved greatly, where the Wote SAP treatments had the greatest effects. The soil bulk density (0–60 cm) under Wote SAP 90 kg ha^?1 was significantly decreased by 6.4% compared with the control. The Wote SAP treatments had the greatest effects on water conservation during the critical potato growth stage, where the soil water storage (0–100 cm) was significantly higher than the control. The Wote SAP treatments promoted potato growth in the later period, where the plant height and stem diameter were higher than the control. Higher yield and commodity rate improvements were achieved by the application of Wote and microbe SAP compared with the control, where the optimum dose was 60–90 kg ha^?1 for Wote SAP. The application of Wote SAP 90 kg ha^?1 significantly increased crop water use efficiency compared with no SAP, and the commodity rate was highest with Wote SAP 60 kg ha^?1. The mean potato yield, commodity rate, and net income increased significantly using Wote SAP at 60 and 90 kg ha^?1, i.e., by 38.2 and 50.5%, 18.5 and 14.1%, and 28.5 and 35.0%, respectively, compared with no SAP.

Conclusions

The application of SAPs can decrease soil bulk density and significantly improve soil porosity and soil water conservation capacity, thereby promoting potato growth. The application of Wote SAP 60–90 kg ha^?1 significantly increased potato yield and net income in a dry-farming region of Ningxia, China.

     

Modeling sediment sources and yields in a Pyrenean catchment draining to a large reservoir (Ésera River,Ebro Basin)

Purpose

The study aimed to use the Soil and Water Assessment Tool (SWAT) model to simulate erosion processes in an alpine–prealpine catchment in order to provide data and information that may be relevant for managers so as to minimize reservoir siltation and water quality degradation. The main objective was to assess sediment production across the catchment and sediment supply to the main reservoir.

Materials and methods

The Barasona reservoir catchment (1,509 km²) is located in the Central Spanish Pyrenees, in the Ebro Basin. This catchment was selected for the case study given the regional significance of the Barasona reservoir and its siltation problems. The catchment has a mountain climate, with strong altitudinal and north–south gradients. The catchment is characterized by heterogeneous topography and lithology, resulting in a varied mosaic of slopes, soil types, and land covers. The Jueu karst system and two small headwater reservoirs were parameterized and calibrated in the model. The SWAT model sediment calibration for the catchment was based on a prior monthly hydrologic calibration, and the model validation was based on the sediment depositional history of the Barasona reservoir.

Results and discussion

The simulation period (2003–2006) and the validation period (1993–2002) produced average sediment yields to the reservoir of 643,000 and 575,000 t year^?1, respectively. Large variations in sediment production were found between the subcatchments in the Barasona catchment due to differences in rock outcrops, land cover, and slope gradient. Sediment loss in the Jueu karst system was 15,500 t and the two small headwater reservoirs retained 31,200 and 50,300 t. Sediment production in relation to precipitation showed high temporal variability, with specific sediment yields to the Barasona reservoir ranging from 2.74 to 8.25 t ha^?1 year^?1. Strong lithological control was observed for sediment production in the subcatchments. The main sediment sources were located in the badlands developed on marls in the middle part of the catchment (internal depressions).

Conclusions

The proposed model has proved useful for identifying areas where significant erosion processes take place in large alpine–prealpine catchments at a regional level and also for assessing discharge losses by the karst system and the sedimentary role of the small reservoirs. The information obtained through this research will be of interest in assessing the spatial distribution of sediment sources and areas of high sediment yield, which will be useful to establish criteria for remediation strategies.     

Soil crusting and infiltration on steep slopes in northern Thailand

Predicting the rate at which rain infiltrates on steep slopes is very uncertain. There is no consistent information in the literature. We have therefore related infiltrability to slope gradient under field conditions by experimenting on a gravelly loamy soil occupying the upper half of a cultivated convex hill in northern Thailand. Fifteen 1 m × 1 m plots with slope gradients ranging from 16 to 63% were established, and simulated rain was allowed to fall on them at controlled rates and for fixed times. We obtained the following results. The surface fell 0.4–7.2 mm due to compaction and soil loss. The proportions of crust (0–40%) and embedded gravel (10–60%), the runoff coefficient (0.05–0.78 mm mm^?1), the mean sediment concentrations (0–5.6 g l^?1), and soil detachment (10–313 g m^?2) were more pronounced on the gentle slopes than on the steep ones. The steady final infiltration rate (1–107 mm hour^?1) increased sharply with increasing slope gradient. Microaggregates tended to behave like sand and become tightly packed on gentle slopes (packing crust). These results suggest that the vertical component of kinetic energy, which is greater on gentle slopes, has a dominant role. Nevertheless, the differences in compaction and in sediment concentration could not be ascribed to the vertical component of kinetic energy alone. On steep slopes the horizontal component of the kinetic energy is transformed into shear stress, hampering the development of crusts so that water can still infiltrate. On steeper slopes, the water film was thinner, thereby limiting the role of splash. We conclude that the relationship between slope gradient and infiltrability depends on the nature of the soil and must be examined in the light of surface crusting processes.     

Phosphorus losses from two representative small catchments in the Mediterranean part of Spain

Purpose

Information about phosphorus (P) losses from agricultural catchments in Mediterranean environments is scarce. In this work, P losses in overland flow from two representative small Mediterranean catchments, one dominated by Alfisols and the other by Vertisols, were studied.

Materials and methods

At the lowest level of each catchment, overland flow was measured and several runoff samples taken in each runoff event during two growing seasons (2001–2002 and 2002–2003). After centrifugation, total P in sediments and total and molybdate reactive P in supernatant were determined. Different chemical extraction methods were used to quantify the forms of P in soils and sediments.

Results and discussion

Total P losses in the studied catchments ranged between 0.5 and 3.2 kg ha^?1 year^?1, losses higher than 2 kg P ha^?1 being observed in one event. Phosphorus was mainly lost in the sediments, the ratio of total dissolved P to particulate P being higher in the Alfisol than in the Vertisol catchment. Phosphorus concentration in sediments from the Vertisol catchment was similar to that in the source soils, whereas sediments in the Alfisol catchment had 2.1 times more inorganic P and 9 times more organic P (OP) than the source soils. In the latter catchment, there was an enrichment in the more labile inorganic P forms in the sediments relative to the source soils, which corresponds to a relative enrichment in iron (Fe) oxides by a factor of 2.4. Alfisols had lower TP contents and exhibited lower erosion rates than Vertisols in the studied period but they posed a greater environmental risk than the latter soils because their sediments were richer in P and had a higher proportion of P in forms bound to the Fe oxides compared to the source soils—these P forms can be easily released with the onset of reducing conditions at the bottom of waterbodies.

Conclusions

A study of the P enrichment ratios and the dominant P forms in eroded sediments is therefore necessary to predict the impact of P losses from soils on the ecological quality of waterbodies.     

Impacts of natural polymer derivative neutral polysaccharide Jag S and cationic hydroxypropyl polysaccharide Jag C162 on rainfall infiltration on an experimental loess hillslope

ABSTRACT

Developing effective measures to improve soil structure and increase soil infiltration in the Loess Plateau located in arid and semiarid areas is important for soil and water conservation. Simulated rainfall experiments were conducted to determine the effects of two new natural polymer derivatives, namely neutral polysaccharide (Jag S) and cationic hydroxypropyl polysaccharide (Jag C162), on rainfall infiltration and their underlying mechanisms. The proportions of different sizes of water-stable soil aggregates were analyzed after spraying four different concentrations (0, 1, 3, and 5 g m^?2) of Jag S and Jag C162 under rainfall intensities of 1, 1.5, and 2 mm min^?1 and a slope gradient of 15°. Treatments with Jag S and Jag C162 significantly improved the rainfall infiltration rates (IRs) compared with the control. Moreover, applying 1 and 3 g m^?2 Jag S effectively increased the IRs by 22.81% and 13.69%, respectively. Treatment with Jag C162 also increased the rainfall IRs by 39.47%, 46.59%, and 46.50%. Furthermore, the content of >0.25 mm water-stable soil aggregates increased from 27.19% to 90.42% before rainfall and from 9% to 50% after rainfall. Compared with Jag C 162, treatment with Jag S was less effective on improving rainfall infiltration and aggregate content. In particularly, application of 5 g m^?2 Jag S improved the soil aggregate content but weakened rainfall infiltration because of the higher viscidity and consistency of the Jag S solution. Overall, spraying appropriate amounts of Jag C162 and Jag S on the loess slope surface can increase the water-stable soil aggregate content, resulting in improved rainfall infiltration and reduced soil erosion. Thus, application of two new natural polymer derivatives is a possible alternative conservation practice in the Loess Plateau.     

Sediment denitrification in waterways in a rice-paddy-dominated watershed in eastern China

Purpose

Rice-paddy-dominated watersheds in eastern China are intensively cultivated, and lands with two crops receive as much as 550–600 kg?ha^–1?year^–1 of nitrogen (N), mainly through the addition of N-based fertilizers. However, stream N concentrations have been found to be relatively low. Waterways in the watersheds are assumed to be effective “sinks” for N, minimizing its downstream movement. We directly measured net sediment denitrification rates in three types of waterways (ponds, streams/rivers, and a reservoir) and determined the key factors that control net sediment denitrification. Such information is essential for evaluating the impact of the agricultural N cycle on the quality of surface water.

Materials and methods

The pond–stream–reservoir continuum was sampled every 2 months at nine sites in an agricultural watershed between November 2010 and December 2011. Net sediment N₂ fluxes/net sediment denitrification rates were determined by membrane inlet mass spectrometry and the N₂/Ar technique. A suite of parameters known to influence denitrification were also measured.

Results and discussion

Net denitrification rates ranged between 28.2?±?18.2 and 674.3?±?314.5 μmol N₂–N?m^–2?h^–1 for the streams, 23.7?±?23.9 and 121.2?±?38.7 μmol N₂–N?m^–2?h^–1 for the ponds, and 41.8?±?17.7 and 239.3?±?49.8 μmol N₂–N?m^–2?h^–1 for the reservoir. The mean net denitrification rate of the stream sites (173.2?±?248.4 μmol N₂–N?m^–2?h^–1) was significantly higher (p?<?0.001) than that of the pond sites (48.3?±?44.5 μmol N₂–N?m^–2?h^–1), and the three types of waterways all had significantly higher (p?<?0.01) mean net denitrification rates in summer than in other seasons. Linear regression and linear mixed effect model analysis showed that nitrate (NO₃ ^?–N) concentration in surface water was the primary controlling factor for net sediment denitrification, followed by water temperature. Using monitoring data on NO₃ ^?–N concentrations and temperature of the surface water of waterways and an established linear mixed effect model, total N removed through net sediment denitrification in the pond–stream–reservoir continuum was estimated at 46.8?±?24.0 t?year^–1 from July 2007 to June 2009, which was comparable with earlier estimates based on the mass balance method (34.3?±?12.7 t?year^–1), and accounted for 83.4 % of the total aquatic N. However, the total aquatic N was only 4.4 % of the total N input to the watershed, and thus most of the surplus N in the watershed was likely to be either denitrified or stored in soil.

Conclusions

High doses of N in a rice-paddy-dominated watershed did not lead to high stream N concentrations due to limited input of N into waterways and the high efficiency of waterways in removing N through denitrification.     

Phytoremediation of Pb in the sediment of a mangrove ecosystem

Purpose

Coal-fuelled power plants can discharge hazardous materials, particularly heavy metals such as lead (Pb). An alternative way of reducing Pb concentration from contaminated sediments is through phytoremediation. Presently, there are few research findings on the phytoremediation potential of mangroves on metals like Pb. The study was conducted to survey and identify mangroves that thrive near the coal-fired power plant and to assess the phytoremediation potential of mangroves on Pb in sediment.

Materials and methods

The study sites were located in the mangrove ecosystems of Sitio Oyon and Sitio Asinan in Masinloc, Zambales, Philippines. The first stage of our study was to survey and identify the mangrove species. The second stage was to assess the levels of Pb in the sediments, water, and tissues of mangrove trees. The diversity assessment of the mangrove species was done through the use of 10?×?12 m quadrat technique. Water and sediment samples from each mangrove ecosystem were collected using composite sampling methods.

Results and discussion

Three mangrove species were identified in the study sites: Avicennia marina, Rhizophora stylosa, and Sonneratia alba. The order of importance of the mangrove trees in the two sampling locations, based on an importance value index (IVI), were as follows: SA (IVI?=?171.20)?>?AM (77.79)?>?RS (51.01). The total uptake of Pb from sediments near the power plants varied significantly (p?≤?0.001) among the three mangrove species. S. alba had the highest Pb uptake of 48.4 kg ha^?1 followed by A. marina (23.1 kg ha^?1), and R. stylosa (2.4 kg ha^?1). These three mangrove species have the potential to phytoremediate Pb in the sediment.

Conclusions

The three mangrove species present in the coastal ecosystem near the electric power plant—A. marina, R. stylosa, and S. alba—were potential phytoremediators of sediment Pb. The present study indicated that the mangroves possess beneficial characteristics that remove Pb from contaminated sediments in areas directly affected by coal-fired power plants, and thus have potential phytoremediation properties.