Phosphorus fractions in preferential flow pathways and soil matrix in hillslope soils in the Thuringian Forest (Central Germany)

Phosphorus (P) is primarily transported in soil through preferential flow pathways (PFP), which can rapidly move water and matter bypassing large portions of the soil. This study investigated the composition of P forms in PFPs and soil matrix in two profiles at a forested hillslope in the Thuringian Forest (Central Germany), in order to evaluate (1) the effect of PFPs on the distribution of P fractions in forest soils, and (2) how hillslope position influences P fractions and other chemical parameters. To characterize water and mass fluxes in the profiles, flow pathways were visualized using dye tracer experiments. Stained and unstained soil material was sampled to assess differences of chemical parameters in the PFPs and soil matrix, and tested for correlations between chemical parameters to determine the factors influencing P fractions in soils. The results revealed significantly higher P contents (total P and most P fractions) in the upslope profile compared to the downslope profile. This accumulation effect in the upper profile was also observed for C, N, Fe, and Mn. The distribution of flow patterns also differed between the two profiles with stronger vertical infiltration into mineral soil and more preferential flow along stones and roots in the upslope profile compared to the downslope profile. However, the observed difference could not be addressed to hillslope effects as both test plots were located in mid‐slope position, but were strongly influenced by spatial heterogeneity (e.g ., micro‐relief). Furthermore, no statistically significant accumulation effect of P or other elements in PFPs compared to soil matrix was found. At the test site, the combination of high stone content with low potential for P sorption, and predominance of near‐surface lateral flow, appears to have hampered the development of gradients in chemical parameters between PFPs and soil matrix.     

山地土壤优先流路径的染色示踪研究

在山地林区开展土壤大孔隙及优先流的实验研究,可深化对森林土壤涵养水源机理的认识,为山区的水土流失防治和植被恢复提供依据。利用剖面染色与图像分析相结合的方法,沿长江三峡大老岭-邓村一线,对山地不同垂直带内土壤优先流特征进行了调查,分析了优先流路径对剖面水分入渗过程的影响。结果显示,中山常绿落叶针阔混交林-山地黄棕壤、低山暖性针叶林-山地黄壤中大孔隙孔径大、分布广,有利于优先流的形成和入渗。受耕作扰动的弃耕土壤中大孔隙结构遭到破坏,优先流路径与森林土壤不同,且渗流强度较弱,染色区域较浅。森林土壤各发生层内优先流特征差异显著,腐殖质层内以洞穴流为主,水分与土壤基质域交换较少,多通过大孔隙快速下渗。淋溶淀积层内洞穴流消失或减弱,侧向渗透增强。低山暖性针叶林-山地黄壤质地较粗,出现以裂隙为主的大孔隙,优先流表现为裂隙流。弃耕土壤各发生层都表现为指流,染色面积随深度减小,侧向渗透基本稳定。植被-土壤垂直地带性分布是山地土壤优先流存在差异的主要原因,山区生态环境建设中应促进土壤优先流路径的发育。     

土壤污染物优势流的数学模拟

A simple modeling approach was suggested to simulate preferential transport of water and contaminants in soil.After saturated hydraulic conductivity was interpolated by means of Krige interpolation method or scaling method,and then zoned,the locations where saturated hydraulic conductivity was larger represented regions where preferential flow occurred ,because heterogeneity of soil,one of the mechanisms resulting in prefeential flow,could be reflected through the difference in saturated hydraulic conductivity,The modeling approach was validatd through numerical simulation of contaminant tansport in a two-dimensional hypothetical soil profle.The results of the numerical simulation showed that the approach suggested in this study was feasible.     

三峡库区花岗岩林地土壤特性与“优先路径”的关系

 “优先路径”是指土壤中植物根系穿插而产生的根孔、动物的运动通道和居穴、由于土壤膨胀收缩形成的裂隙,以及因湿润锋不稳定所形成的指状渗透通道等。通过在长江三峡曲溪小流域花岗岩林地坡面开挖土壤剖面,测定了“优先路径”在剖面的分布状况,并采取土壤样品,对土壤的物理特性用筛分法进行了室内测定。结果表明:剖面中距地表以下82～110cm范围内,土壤密度最小,其毛管孔隙度也较小,显示出其土壤中存在有较多非毛管孔隙构成的“优先路径”。“优先路径”在距地表以下80～100cm范围内,分布比较集中。该范围内含有较多粗颗粒物质,有利于“优先路径”的形成。“优先路径”的存在,使该层土壤稳渗速率较其他土层得以大幅度提高,土层内快速运动的水流,又加速了“优先路径”的形成与发展。“优先路径”的存在,影响着土壤物理特性,而土壤物理性质,对“优先路径”的形成与发展,也有较大影响。     

Fate of heavy metals in a strongly acidic shooting‐range soil: small‐scale metal distribution and its relation to preferential water flow

To assess the mobility of Pb and associated metals in a highly contaminated shooting range soil (Losone, Ticino, Switzerland), we investigated the spatial distribution of the metals and their relation to preferential water flow paths. A 2.2 m² plot located 40 m behind the stop butt was irrigated with a solution containing bromide and Brilliant Blue, a slightly sorbing dye. A soil profile 50 cm in width was sampled down to 80 cm with a spatial resolution of 2.5 cm, resulting in 626 samples. Concentrations of elements (12 ≤ Z ≤ 92) were determined by energy‐dispersive Xray fluorescence spectrometry, and Brilliant Blue concentrations were determined with a chromameter. In the acidic (pH 3), organic matter‐rich, well drained Dystric Cambisol, maximum concentrations of 80.9 g kg^‐1 Pb, 4.0 g kg^‐1 Sb, and 0.55 g kg^‐1 Cu were measured in the topsoil. Within 40 cm soil depth, however, Pb, Sb, and Cu approached background concentrations of 23 mg kg^‐1, 0.4 mg kg^‐1, and 9.4 mg kg^‐1, respectively. The even horizontal distribution and the steep gradient along soil depth indicate tight metal binding in the topsoil, and a fairly homogeneous transport front. In contrast, water flow through the profile was highly heterogeneous. In the uppermost 20 cm, preferential flow was initiated by heterogeneous infiltration at the soil surface, but had no influence on metal distribution. Below 20 cm, however, preferential flow originated from larger tree roots, and metal concentrations were significantly elevated along these macropores. Spatial distributions of Pb, Sb, and Cu were similar, suggesting that all three metals are strongly retained in the topsoil and transported along preferential water flow paths in the subsoil.     

采用染色示踪技术的土壤优先流自动分割与量化系统

针对土壤染色图像色度不一致、染色/非染色区域对比度低的特点,以及现有土壤染色图像分割方法自动化程度弱的问题,该研究提出一种土壤优先流自动分割与量化系统。该系统采用基于H分量改进的模糊C均值方法（Fuzzy C-Means Based on H Component and Morphology,HM-FCM）实现染色区域的自动分割,运用数学统计法提取总染色面积比、基质流深度、优先流比等特征参数,实现对土壤染色区域的量化分析,以揭示优先流的发育程度。并基于2种林地染色图像验证了系统性能。试验结果表明：1）HM-FCM法对于天然次生林和榛子林图像均具有最佳分割效果,其分割准确率为87.9%和83.3%,调和平均值为90.5%和80.3%;2）2种林地土壤染色区域总体集中于0～50 cm土层,优先流具有不同发育程度（P<0.05）。该系统可为优先流路径的空间演变提供技术支持和理论依据。     

土壤优先流模型理论与观测技术的研究进展

优先流是土壤中常见的和重要的水流运动和溶质运移形式。由于土壤优先流的形成和影响因素众多、表现形式多样,加之土壤优先流的快速非平衡特征明显以及土壤高度的空间变异性,准确描述和模拟土壤优先流的时空变化特征一直以来都是土壤水文学界的热点问题和难点问题。该文从优先流的定义、表现类型、形成和影响因素、模型理论与观测技术等5个方面综述了土壤优先流的研究进展,指出该领域今后的主要研究方向为建立土壤优先流的统一判别标准、提升优先流模型理论的有效性、发展优先流的专用观测技术设备。文章对深入研究土壤优先流具有参考价值。     

土壤优先流运动的活动流场模型模拟和敏感性分析

在壤土和砂土条件下分别采用碘-淀粉染色示踪方法和亮蓝染色示踪方法各开展了2个染色示踪试验,分别采用活动流场模型和二域模型模拟计算了各试验入渗后染色区内的土壤含水率和溶质浓度分布,通过相对均方根误差分析评价了两个模型模拟预测优先流发展的有效性;此外,通过敏感性分析研究了不同入渗条件（土壤质地、入渗水量和土壤初始含水率）下活动流场模型模拟预测结果（入渗深度）对活动流场模型分形特征参数变化的敏感度。模型检验分析结果显示活动流场模型对土壤水入渗深度、入渗后染色区内土壤含水率和溶质浓度分布的预测精度要明显高于二域模型的模拟预测精度;活动流场模型较好的捕捉到了优先流运动整体的非均匀特征。敏感性分析结果显示,当降雨入渗水量和土壤初始含水率相同时,入渗深度对活动流场模型分形特征参数（γ）的敏感度随着γ的增大而增大;相同活动流场模型分形特征参数（γ）值条件下（即流动非均匀程度相同）,入渗深度对活动流场模型分形特征参数（γ）的敏感度随着入渗水量的增大和土壤初始含水率的升高而减小。     

土壤水流模式染色剂示踪及优先流程度评估

优先流是结构性土壤水分入渗的主要方式。为了直接利用土壤水流模式分析其优先流程度,该文采用亮蓝染色剂示踪原状与扰动土柱的土壤水运移,定量评估优先流的相对发育程度。研究结果表明原状土柱优先流发育,空间变异性强,受优先流通道的特征控制,其优先流水流模式以蚯蚓大孔隙流和土块裂隙优先流为主。扰动土柱的水流模式为活塞流,无优先流发育。该文提出了随深度变化的染色面积比的变异系数指示原状土柱优先流相对程度的评价准则:变异系数越低,优先流程度越高。变异系数≥0.5%为优先流程度一般发育,0.25%~0.5%之间为发育,≤0.25%为非常发育。并通过原状和扰动土柱灌溉模拟试验验证了评价准则的可靠性。     

The effect of preferential flow on the short and long-term spatial distribution of surface applied solutes in a structured soil

We measured the lateral distribution of Br in a well-structured soil (clayey, kaolinitic, thermic Typic Kanhapludults) over a period of 40 days following a leaching event with Br-tagged water (Event-1) and the subsequent vertical transport of solute that occurred following a second leaching event (Event-2). The objectives were to determine the time required for solutes to become uniformly distributed laterally after Event-1, and the effect that degree of spatial distribution had on the subsequent vertical movement induced by Event-2. Six field plots, 1.2 by 1.2 m, were exposed to Event-1. Three of the plots were sampled 1, 5, or 40 days later. The three remaining plots were exposed to Event-2 either 1, 5, or 40 days following Event-1 and they were sampled the day following Event-2. A total of 319 samples were taken from a 38.5 by 38.5 cm area in the center of each plot at each of five depths ranging from 5 to 60 cm. Bromide was practically uniformly distributed in the surface layer immediately following Event-1, as indicated by a coefficient of variation (CV) of 25% among the 319 samples, but became progressively more non-uniformly distributed as depth increased. At 60 cm, the CV had increased to 240%. The longer we delayed sampling following Event-1, the more uniform was the lateral distribution of Br. During the 40 days following Event-1, Br became practically uniformly distributed across all layers, and the center of mass moved from 8 to 16 cm below the surface. Event-2 caused Br to move deeper within the soil profile when it occurred the day following Event-1 than when it occurred later, but because drainage occurred during the 40-day delay, the center of Br mass was located 26–27 cm below the surface after Event-2 regardless of the delay time.     

The equivalence of the Calcium‐Acetate‐Lactate and Double‐Lactate extraction methods to assess soil phosphorus fertility

A large variety of extraction methods are used worldwide for the estimation of “plant‐available P” in soils. In Germany, the standard extractants are Calcium‐Acetate‐Lactate (CAL) and Double‐Lactate (DL). Until now there is no validated transformation procedure available and studies on the comparability of both methods have reported conflicting evidence. The uncertainty about the equivalence of CAL‐P and DL‐P hinders a direct comparison of the P fertility status and P fertilizer recommendations across Germany. Based on 136 datasets for soil samples from an interlaboratory comparison program and three P fertilization field trial sites, for which plant‐available P had been determined by both the CAL and DL method, we assessed the comparability of both extraction methods and derived simple and multiple regression equations to transform DL‐P into CAL‐P values. On average, DL extracted 30% more P than CAL. However, this strongly depended on soil pH and carbonate content. A simple linear regression model explained 70% of the variance. However, if simple linear regression models were fitted to pH‐specific samples (pH range 4.5 to 7.0) the R² increased to 0.96. Based on an independent validation dataset (n = 48) we demonstrated that such pH‐specific models were more accurate than models that did not consider pH when transforming DL‐P to CAL‐P values. Multiple regression results showed that out of soil pH, C_org, N_t, and C : N ratio, only soil pH improved the model. The transformation equations in this study provide a step towards an improved comparability of P fertility status assessments of soils across Germany.     

Relationship of soil phosphorus with uranium in grassland mineral soils in Ireland using soils from a long‐term phosphorus experiment and a National Soil Database

Phosphorus fertilizer contains contaminants that may increase the content in the soil and in plants. The relationship between soil P and soil uranium (U) was investigated to determine potential effects of P‐fertilizer use. This study is based on a long‐term experiment (38 years with 0, 15, and 30 kg fertilizer P ha^–1 y^–1) for beef production on grassland at Teagasc, Johnstown Castle, Wexford, Ireland and also on soils from a National Soil Database (NSD). The NSD soils were taken at fixed locations on a predetermined grid system at the density of one sample every 50 km². Of the 1310 samples in the NSD, the 760 grassland mineral soils were selected for this study. The aim was to determine to what extent P fertilizer increases the content of U in the soil. The results showed that there was a small but significant increase in soil U in the high‐P treatments, which contained high levels of soil P, in the long‐term field experiment. The results from the NSD showed that there was not a significant relationship between extractable (Morgan's) soil test P (STP) and U. It is concluded that the use of chemical P fertilizer at normal rates used in agriculture in Ireland is not a major threat to U content of soil based on the results of this study. There was a significant relationship between total P and STP, in the NSD, with the latter making up approx. 1% of the former. Soil available P increased with soil pH, probably reflecting the use of chemical P fertilizer and lime on agricultural soils.     

干旱区绿洲农业生态系统土壤优先流量化方法可行性综述

归因于多孔介质的空间变异性和各向异性,非饱和带中土壤水流运动与溶质运移对优先流响应机制的研究十分具有挑战性。绿洲农业生态系统作为新疆主要经济支柱,如果忽略土壤优先流现象,可能会降低灌溉用水利用效率,减少土壤吸附污染物时间,进而导致地下水污染,最终制约农业发展。因此,该研究以新疆干旱绿洲区121团场的棉花种植区作为研究对象,旨在探明与探究在干旱绿洲区中可能导致优先流发生的潜在因素以及适合该地区观测优先流现象的量化技术。总体而言,在121团场,冻融循环,残膜现象,深耕棉秆还田的耕作方式以及土壤盐渍化会影响并改变土壤耕层孔隙度与结构,形成大孔隙式的优先入渗路径,进而引发大孔隙流。与此同时,因为该地区的主要土壤类型为壤土,其斥水性驱动着水流不稳定运动,导致指流或漏斗流的发生。通过对量化技术的总结与评估,染色示踪成像方法由于其能直观表述土壤水的非均匀运动,在121团场具有能很好地评价土壤优先流的潜力与实用性。其他的量化技术虽然具有各自的优势,但相较于染色示踪成像试验成本高,前期准备工作量大,评价尺度不统一,因此不适用于干旱绿洲区优先流的量化评价。在未来的研究中,需要适当地采用将微尺度观察与大尺度调查相结合的方法来对优先流现象进行全面评估。该研究有望为新疆及世界其他类似地区干旱绿洲生态系统优先流的研究提供理论参考。     

Deformation damages in forest topsoils—An assessment based on Level‐I soil monitoring data from Baden‐Württemberg (SW Germany)

Acidification and eutrophication of soils had been the main activators for the implementation of forest soil monitoring in Central Europe. Thus, field and lab studies focused on gathering information that is essential for the evaluation of the chemical status and its trend. A systematic assessment of soil physical threats caused by machine use in forests has not been integrated yet into the soil‐monitoring systems. In this study, a first approach to get a deeper insight into structure damages of forest topsoils was derived for 302 systematically distributed grid points in the Federal State of Baden‐Württemberg (SW Germany) during the nation‐wide soil survey performed from 2006 to 2008. We derived an approach to assess structure damage based on a key system using field information on structural and hydromorphic topsoil properties. It covers eight satellites surrounding the central monitoring soil pit at each grid point. Our survey focused on the mere stand area excluding visible damage and systematic skid trails. Analysis of structure‐damage intensity and spatial distribution leads to the conclusion that damage caused by vehicle traffic off the skid trails is a wide‐spread phenomena in Baden‐Württemberg forests, where wheeling is not restricted by steepness of terrain. Although regulations to control machine use recommending vehicle traffic to skid trails and fortified roads have been in place since the early 1980s, soil‐structure damages off these trails have reached significant levels. In the future, it will thus be indispensable to put more emphasis on the importance of soil‐protection aims in the ranking of the economic objectives of forest organizations and forest owners.     

Long‐term changes of aggregate‐associated and labile soil organic carbon and nitrogen after conversion from forest to grassland and cropland in northern Turkey

Soil management systems can have great effect on soil chemical, physical and biological properties. Conversion of forest to grassland and cropland can alter C and N dynamics. The objective of this study was to evaluate the changes in aggregate‐associated and labile soil organic C and N fractions after conversion of a natural forest to grassland and cropland in northern Turkey. This experiment was conducted on plots subject to three different adjacent land uses (forest, grassland and cropland). Soil samples were taken from 0–5, 5–15 and 15–30 cm depths from each land use. Some soil physical (soil texture, bulk density), chemical (soil pH, soil organic matter, lime content, total organic C and N, inorganic N, free and protected organic C) and biological (microbial biomass C and N, mineralizable C and N) properties were measured. The highest and lowest bulk densities were observed in grassland (1.41 g cm⁻³) and cropland (1.14 g cm⁻³), respectively. Microbial biomass C and total organic C in forest were almost twice greater than grassland and four‐times greater than cropland. Cultivation of forest reduced total organic N, mineralizable N and microbial biomass N by half. The great portion of organic C was stored in macroaggregates (>250 µm) in all the three land uses. Free organic C comprised smaller portion of soil organic C in all the three land uses. Thus, this study indicated that long‐term conversion of forest to grassland and cropland significantly decreased microbial biomass C, mineralizable C and physically protected organic C and the decreases were the greatest in cropland. Copyright © 2011 John Wiley & Sons, Ltd.     

A systems analysis of soil and forest degradation in a mid‐hill watershed of Nepal using a bio‐economic model

Forest degradation, manifested through decline in forest cover, and the resulting soil erosion and organic carbon losses, is a serious problem caused by a complex coupling of bio‐physical, socio‐economic and technological factors in the Himalayan watersheds. Greater understanding of the linkages between these factors requires a systems approach. We have proposed such an approach using a bio‐economic model to explore the system behaviour of forest degradation, soil erosion, and soil C losses in the forest areas. The outcome of the model simulation over a 20‐year period indicates that soil erosion and C loss rates may increase more than four‐fold by the year 2020 under the existing socio‐economic and biophysical regime (the base scenario). Reductions in the population growth rate, introduction of improved agricultural technology and increase in the prices of major agricultural crops can help slow down the rates of forest decline, soil erosion and C loss or even stabilize or reverse them. The results suggest that economic incentives may be highly effective in the reduction of soil loss, as well as C release to the atmosphere. Copyright © 2005 John Wiley & Sons, Ltd.     

Short‐term transport of cadmium during a heavy‐rain event simulated by a dual‐continuum approach

The transport of solutes in soils, and its intensification due to preferential flow, plays crucial role when problems related to the groundwater pollution are dealt with. The objective of this study was to examine transport of cadmium (Cd) in response to an extreme rainfall event for three different soils using numerical modeling. The ^115mCd²⁺ concentration profile had been measured in the Bodiky reference site (Danubian Lowland, Slovakia) by the radioactive‐tracer technique and used for the calibration of the dual‐continuum model S1D. The Cd transport during a single rain event was predicted with the S1D model for light, medium‐heavy, and heavy soil in the same region. The Cd transport through the soil profile was simulated by the one‐dimensional dual‐permeability model, which assumes the existence of two pore domains: the soil‐matrix domain and the preferential‐flow domain. The model is based on Richards' equation for water flow and advection‐dispersion equation for solute transport. A modified batch technique enables to distinguish process of adsorption in the matrix domain and the preferential pathways. Modeling with classical single‐permeability approach and dual‐continuum approach without considering the particle‐facilitated transport led to negligible Cd penetration. The rainfall event with extremely high rainfall intensity induced deep penetration of Cd in the medium‐heavy and heavy soil, which may indicate increased vulnerability to shallow groundwater pollution for the respective sites in Danubian Lowland region. The highest Cd leaching was predicted for heavy clay soil, where the preferential particle‐facilitated transport of Cd through the soil profile was significant due to the contrasting properties of the soil‐matrix domain and the preferential‐flow domain. The results of the sensitivity analysis suggested only slight effect of the transfer rate coefficients on simulated Cd leaching.     

Dissolved and colloidal phosphorus fluxes in forest ecosystems—an almost blind spot in ecosystem research

Understanding and quantification of phosphorus (P) fluxes are key requirements for predictions of future forest ecosystems changes as well as for transferring lessons learned from natural ecosystems to croplands and plantations. This review summarizes and evaluates the recent knowledge on mechanisms, magnitude, and relevance by which dissolved and colloidal inorganic and organic P forms can be translocated within or exported from forest ecosystems. Attention is paid to hydrological pathways of P losses at the soil profile and landscape scales, and the subsequent influence of P on aquatic ecosystems. New (unpublished) data from the German Priority Program 1685 “Ecosystem Nutrition: Forest Strategies for limited Phosphorus Resources” were added to provide up‐to‐date flux‐based information. Nitrogen (N) additions increase the release of water‐transportable P forms. Most P found in percolates and pore waters belongs to the so‐called dissolved organic P (DOP) fractions, rich in orthophosphate‐monoesters and also containing some orthophosphate‐diesters. Total solution P concentrations range from ca. 1 to 400 µg P L^?1, with large variations among forest stands. Recent sophisticated analyses revealed that large portions of the DOP in forest stream water can comprise natural nanoparticles and fine colloids which under extreme conditions may account for 40–100% of the P losses. Their translocation within preferential flow passes may be rapid, mediated by storm events. The potential total P loss through leaching into subsoils and with streams was found to be less than 50 mg P m^?2 a^?1, suggesting effects on ecosystems at centennial to millennium scale. All current data are based on selected snapshots only. Quantitative measurements of P fluxes in temperate forest systems are nearly absent in the literature, probably due to main research focus on the C and N cycles. Therefore, we lack complete ecosystem‐based assessments of dissolved and colloidal P fluxes within and from temperate forest systems.     

草覆盖影响了沙地土壤的水文物理学参数和水流的异质性

Vegetation cover has a major effect on water flow in soils.Two sites,separated by distance of about 50 m,were selected to quantify the influence of grass cover on hydrophysical parameters and heterogeneity of water flow in a sandy soil emerging during a heavy rain following a long hot,dry period.A control soil(pure sand)with limited impact of vegetation or organic matter was obtained by sampling at 50 cm depth beneath a glade area,and a grassland soil was covered in a 10 cm thick humic layer and colonised by grasses.The persistence of water repellency was measured using the water drop penetration time test,sorptivity and unsaturated hydraulic conductivity using a mini disk infiltrometer, and saturated hydraulic conductivity using a double-ring infiltrometer.Dye tracer experiments were used to assess the heterogeneity of water flow,and both the modified method for estimating effective cross section and an original method for assessing the degree of preferential flow were used to quantify this heterogeneity from the images of dyed soil profiles.Most hydrophysical parameters were substantially different between the two surfaces.The grassland soil had an index of water repellency about 10 times that of pure sand and the persistence of water repellency almost 350 times that of pure sand. Water and ethanol sorptivities in the grassland soil were 7% and 43%,respectively,of those of the pure sand.Hydraulic conductivity and saturated hydraulic conductivities in the grassland soil were 5% and 16%of those of the pure sand, respectively.Dye tracer experiments revealed a stable flow with"air-draining"condition in pure sand and well-developed preferential flow in grassland soil,corresponding to individual grass tussocks and small micro-depressions.The grassland soil was substantially more water repellent and had 3 times the degree of preferential flow compared to pure sand.The results of this study reinforce our view that the consequences of any change in climate,which will ultimately influence hydrology,will be markedly different between grasslands and bare soils.     

北京昌平区农地土壤优先流影响硝态氮运移的试验分析

为了探讨在优先流影响下农地土壤水分与溶质的运移规律,以昌平农地土壤为研究对象,通过原状土取样和分层填充制备实验土柱,模拟存在优先流和平衡入渗2种水分下渗过程,分析优先流对农地土壤中硝态氮运移的影响。结果表明:相较于平衡入渗,存在优先流的土壤中硝态氮运移的速率更快、数量更多,且其穿透曲线表现出拖尾现象。优先流的存在会使土壤的水分出流速率达到平衡入渗过程的1.48～2.69倍,且波动程度较大;受其影响,硝态氮运移表现出快速、大量下渗的特征,原状土柱中NO3-的穿透时间为12 h,此时的孔隙体积为0.36,相较于填充土柱分别减少了57%和27%。此外,原状土柱中以NO3-标记的优先流占水流总量的43.83%,引起的NO3-累计淋出量占总量的97.60%,这表明有限的优先流流量能够引起绝大部分的硝态氮运移。土壤优先流还使得其穿透曲线表现出拖尾现象,这可能是由于优先流和基质流之间下渗速率的不平衡造成的。