Soil texture effect on nitrate leaching in soil percolates

Abstract

Nitrate nitrogen (NO₃‐N), which is an essential source of nitrogen (N) for plant growth, is now also considered a potential pollutant by the Environmental Protection Agency (EPA). This is because excess applied amounts of NO₃‐N can move into streams by run‐off and into ground water by leaching, thereby becoming an environmental hazard. Soils have varied retentive properties depending on their texture, organic matter content, and cation exchange capacity (CEC). The purpose of this study was to determine the effect of soil texture on NO₃‐N retention to reduce NO₃‐N contamination in the environment. A sand, 85:15 sand:peat Greensmix, a loamy sand, and sandy clay loam soils were placed in 2×3 inch metal cylinders and soaked in a 240 ppm solution of NO₃‐N for seven days to saturate the soil with NO₃ ions. The columns were leached with water to collect 10 soil percolate samples of 50 mL each until a total volume of 500 mL was collected. Nitrate‐N was measured in each 50‐mL aliquot by automated colorimetry. The results showed that soil texture affected the retention of N03‐N in the sand, which adsorbed the least amount of NO₃‐N at 119 ppm, followed by the Greensmix at 125 ppm, loamy sand at 149 ppm, and sandy clay loam at 173 ppm. More NO₃‐N was released in the first 50 mL of the sand percolate at 63% followed by the Greensmix, loamy sand, and sandy clay loam at 58,46, and 37% NO₃‐N released, respectively. Soils with more silt, clay, and organic matter retained more NO₃‐N than the straight sand. Therefore, a straight sand would be the poorest of soil types since NO₃‐N retention was low.     

Clay Addition to Sandy Soil Reduces Nutrient Leaching—Effect of Clay Concentration and Ped Size

Leaching of nutrients, particularly in sandy soil with low nutrient and water holding capacity (WHC), is a major threat to marine and fresh water pollution. Addition of clay soil to sandy soil could be an option to increase water and nutrient holding capacity of sandy soils, but the effect of clay soil addition may depend on the form in which the clay soil is added and the addition rate. Clay soil was added to sandy soil at rate of 10 or 20% (w/w) finely ground (<2 mm) or 2 and 5 mm peds with and without nitrogen (N) and phosphorus (P) fertilizer equivalent to 60 kg N ha^?1 and 15 kg P ha^?1. The clay sand mixture for each treatment was weighed (30 g) in cores with nylon mesh at the bottom. The soils were incubated at 80% WHC for 7 weeks. To obtain leachate, 20 mL reverse osmosis (RO) water was added every week to each core. Leachate was analysed for inorganic N, P, and pH. Soil was analyzed for N, P, and pH before and after the leaching. Clay addition significantly reduced the leaching of N and P compared to sandy soil alone, with greatest reduction by finely ground clay soil and least with 5 mm peds. Compared to sandy soil alone, 83% more N was retained in clay-amended soil and P retention was doubled. This study showed that addition of finely ground clay soil can substantially reduce N and P leaching and thereby increase fertilizer retention compared to sandy soil alone.     

不同淋洗模式下干旱区盐渍土改良效果分析

通过开展室外土柱模拟淋洗试验,选择间歇淋洗与连续淋洗两种淋洗模式,分析不同灌水总量和不同灌水频次相互组合条件下盐渍土的改良效果。结果表明:(1)灌水量相同情况下,间歇淋洗滤出液的电导率高于连续淋洗,间歇淋洗的淋洗效果好于连续淋洗;(2)灌水量不同条件下,滤出液体积随灌水量的增加而增多,灌水量越大盐分淋洗效果越好;(3)淋洗结束后,土壤pH值较淋洗前均有下降,与土壤盐分有相同的变化趋势;灌水量越大,脱碱效果越好。在本试验条件下,盐分淋洗效果以15L分3次给水的间歇淋洗方式最优,淋洗效率为11.58g/L,土壤pH值均由8.42下降到8.07。可供制定维持农田水盐平衡的节水灌溉制度以及盐碱地改良参考。     

Determination of water-soluble arsenic compounds in commercial edible seaweed by LC-ICPMS

This paper reports arsenic speciation in edible seaweed (from the Galician coast, northwestern Spain) produced for human consumption. Chondrus crispus , Porphyra purpurea , Ulva rigida , Laminaria ochroleuca , Laminaria saccharina , and Undaria pinnatifida were analyzed. The study focused on arsenosugars, the most frequently occurring arsenic species in algae. As(III) and As(V) were also determined in aqueous extracts. Total arsenic in the samples was determined by microwave digestion and inductively coupled plasma mass spectrometry (ICPMS). For arsenic speciation, a water extraction especially suitable for arsenosugars was used, and the arsenic species were analyzed by liquid chromatography with both anionic and cationic exchange and ICPMS detection (LC-ICPMS). The total arsenic content of the alga samples ranged from 5.8 to 56.8 mg As kg(-1). The mass budgets obtained in the extracts (column recovery × extraction efficiency) ranged from 38 to 92% except for U. pinnatifida (4%). The following compounds were detected in the extracts: arsenite (As(III)), arsenate (As(V)), methylarsonate (MA), dimethylarsinate (DMA), sulfonate sugar (SO(3)-sug), phosphate sugar (PO(4)-sug), arsenobetaine (AB), and glycerol sugar (Gly-sug). The highest concentrations corresponded to the arsenosugars.     

Dissipation of monosodium methane arsonate (MSMA) on peanuts

Monosodium methane arsonate (MSMA), the salt of methylarsonic acid (MAA), is a herbicide commonly used to control weeds along roadsides, in cotton, in turf, and on noncrop sites. In recent years questions have arisen regarding the source and nature of arsenic residues in raw agricultural commodities relative to misuse, inadvertent exposure, or rotational crop residues. Field experiments were conducted to determine the fate of MSMA that is applied to peanut foliage. Persistence, dissipation, recovery, and detection of MAA from leaf rinsate were characterized as well as resulting total arsenic and MAA concentrations in peanut kernels. MSMA was applied to peanut foliage at 105, 210, 315, and 420 g of active ingredient (ai)/ha. Peanut leaves were sampled before and after irrigation events over the next 7 days. Peanuts were harvested at maturity and analyzed for MAA and total arsenic. A confined rotational crop experiment was conducted to determine the potential for MAA residues in soil to be taken up by peanuts in fields rotated from cotton that was treated with MSMA. MAA was not detected in any peanut samples from the rotational crop experiment, even when peanuts were planted only 30 days after MSMA application to the soil at 2.24 kg of ai/ha. Field experiments showed that MSMA recovery from leaves with an aqueous rinse declined quickly but was not greatly affected by irrigation. However, quantifiable amounts of MAA were present 1 week after application and after two irrigation events, and MAA and total arsenic were measured in mature peanut kernels from all plots that received MSMA. MAA was not detected in untreated checks. Total arsenic was below the limit of quantification in untreated controls.     

Effects of nonylphenols on soil microbial activity and water retention

The main aim of this study is to analyze the influence of 4-nonylphenol (NP) on soil water retention and biological activity. Two doses of 4-nonylphenol (25 and 50 mg kg⁻¹) were tested in a loam soil with and without peat amendment. In general, one week after the start of the experiment, the soil water content retained at −0.75 MPa of soil suction was 18% higher in the soil amended and its basal respiration (BR) was 15% higher than soil without peat. In contrast, the microbial activity indices (CM: coefficient of mineralization or BR:total organic carbon (TOC) ratio; Cmic:Corg: microbial biomass carbon (MBC):TOC ratio; qCO₂: metabolic quotient or BR:MBC ratio) were higher in the soil without peat, compared to the soil amended with peat. On the other hand, the addition of NP to soil was able to modify soil biological but not physical (water retention, desorption) properties. When soil was amended with peat, MBC was reduced one week after applying NP. In contrast, no effects of NP on MBC were observed in the soil without peat. BR was reduced by 16% one week after applying 50 mg kg⁻¹ of NP to soil with peat, and was increased by 46% one week after applying 25 mg kg⁻¹ of NP to soil without peat. The effects of NP on MBC and BR could be associated more with the adsorption of NP by soil organic matter, while changes in CM or Cmic:Corg ratio were more closely related to changes in soil water retention. The potential toxic effects of NP (high qCO₂ values) were only observed in the absence of peat amendments. Peat addition reduced NP toxic effects on microorganisms.     

若尔盖湿地土壤酶活性和理化性质与微生物关系的研究

本文旨在研究若尔盖湿地土壤退化过程中四种典型土壤(泥炭土、沼泽土、草甸土、风沙土)中脲酶、过氧化氢酶、转化酶、多酚氧化酶活性及其与理化性质、微生物数量的相关性分析。结果表明:随着土壤深度增加,脲酶、转化酶活性逐渐降低,过氧化氢酶、多酚氧化酶活性逐渐升高;随着土壤退化程度的加剧,脲酶和转化酶活性总体呈下降趋势;泥炭土、沼泽土、草甸土中过氧化氢酶活性显著高于风沙土;多酚氧化酶活性随退化程度加剧呈上升趋势,但在沼泽土中活性最低。泥炭土、沼泽土、草甸土中转化酶及风沙土中过氧化氢酶与土壤理化性质、微生物数量的相关性较显著。因此土壤酶活性(脲酶、转化酶)、理化性质(含水量、有机质、全氮、全磷、全钾)、微生物数量能较好地指示当地土壤退化状况。     

吉林西部土壤砷的形态分布及其与土壤性质的关系研究

按照生态地球化学土壤样品元素形态分析方法,将土壤无机砷分成水溶态、离子交换态、碳酸盐态、腐植酸结合态、铁锰氧化物结合态、强有机结合态和残渣态。通过对吉林西部36个表层土壤样品的测试,分析了土壤不同形态砷的分布和不同形态砷与土壤性质的关系。研究表明,洮南市不同形态砷的分布为：残渣态（65．30％）〉腐植酸结合态（17．39％）〉铁锰氧化物结合态（10．70％）〉碳酸盐态（2．23％）〉水溶态（2．17％）〉强有机结合态（1．17％）〉离子交换态（1．04％）。通榆县不同形态砷的分布为：残渣态（56．66％）〉腐植酸结合态（23．82％）〉铁锰氧化物结合态（11．08％）〉碳酸盐态（2．81％）〉水溶态（2．18％）〉离子交换态（1．98％）〉强有机结合态（1．46％）。残渣态砷是吉林西部土壤砷的主要形态。土壤水溶态砷和铁锰氧化物结合态砷与土壤pH值皆呈极显著正相关;残渣态砷与土壤有机质皆呈极显著负相关,而与土壤阳离子交换量呈极显著正相关。离子交换态砷、碳酸盐态砷、铁锰氧化物结合态砷和强有机结合态砷与土壤矿质元素的关系不大,而水溶态砷、腐植酸结合态砷和残渣态砷与土壤矿质元素的关系密切。     

Sediment flow behavior in agro-watersheds of the purple soil region in China under different storm types and spatial scales

Conservation tillage has proved to be an effective option to reduce erosion and particulate phosphorus (PP) losses from fields to watercourses. However, it usually leads to the higher losses of P in dissolved reactive (DRP) form if P accumulates into the uppermost soil layer.In this study we investigated the influence of CaCO₃-rich boardmill sludge (BMS, applied 7 years ago) on soil properties and percolate quality with soil samples as well as undisturbed soil columns taken from a long-term conservation-tilled clay field experiment. A short-term laboratory experiment was undertaken to evaluate whether newly added BMS (FS) can be used to improve the quality of percolate waters by reducing turbidity, PP and DRP.The hypothesis was that BMS reduces the risk of dissolved reactive phosphorus (DRP) leaching from conservation-tilled soil. BMS served as a liming agent and counteracted the no-till-induced acidification and accumulation of DRP in the surface soil layer. Surface soil pH was on average 0.3 units higher and soil Ca 500 mg kg⁻¹ higher in the BMS-treated plots than in the plots without BMS. The ability of BMS to enhance the availability of P for plants may prevent the surface accumulation of P in NT cultivation. At the same time, BMS-induced increase in Ca and EC of the surface soil suppress the diffuse double layer by reducing P desorption and leaching risk.In the laboratory experiment, FS reduced percolate turbidity by 80% and PP and DRP concentrations by 62% and 50%, respectively. FS probably improved stability of soil aggregates in the laboratory experiment, obviously through enhanced microbial activity. The covering effect and infiltration of water through the 0–5 cm soil layer seemed to be crucial factors in reducing P losses.BMS proved to be a promising material for the applications to the conservation-tilled fields. The effect of BMS on microbes and immobilization of nutrients, should, however, be studied further in fields under a wider diversity of conditions.     

Hydrophysical Properties of the High-Ash Lowmoor Peat Soils

The water retention curve (WRC), density, botanical composition, and ash contents were determined for high-ash lowmoor peat soils (Rheic Sapric Histosols) developing on the floodplain of the Yakhroma River (Moscow oblast) from the herb–hypnum and hypnum peat enriched in carbonates, agromineral peat soils (Rheic Drainic Sapric Histosols (Mineralic)), and peat soils developed from woody peat underlain by herb, sedge, and woody peat layers (Rheic Sapric Histosols (Lignic)). The WRC was determined by capillarimetric method in the range of water pressure from 0 to 80–90 кPa. For the studied peat soils, the WRC represents a close to linear dependence of the water content on the water pressure in semilogarithmic scale. In contrast to mineral soils, a characteristic point of the air-entry pressure is virtually absent on the WRC of peat soils. The WRC of peat largely depended on their density: denser peat samples were characterized by a higher water content at the same water pressure, which attests to the increased water retention capacity. An increase in the degree of decomposition of peat and its ash content also leads to the rise in the water retention capacity, but the effect of these factors is considerably smaller than the effect of peat density.     

Effect of Palygorskite Clay,Fertilizers, and Lime on the Degradation of Oil Products in Oligotrophic Peat Soil under Laboratory Experimental Conditions

The effect of native palygorskite clay and that modified with dodecyltrimethylammonium chloride on the degradation of oil products in an oligotrophic peat soil under complete flooding at the application of lime and mineral fertilizers has been studied under laboratory conditions. It has been shown that the incubation of oil-contaminated soil with unmodified clay and fertilizers at the application of lime under complete flooding with water affects the dynamics of pH and Eh and slows the development of reducing conditions compared to the use of clay without fertilizers. The addition of organoclay under similar conditions favors the formation of potential-determining system with a high redox capacity, which is capable of retaining the potential on a level of 100–200 mV at pH ～ 7 for two months. It has been found that, under the experimental conditions, unmodified and modified clay, which has no toxic effect on the bacterial complex, does not increase the biodegradation efficiency of oil products in the oligotrophic peat soil compared to the experimental treatments without clay addition. Possible reasons for no positive effect of palygorskite clay on the biodegradation rate of oil products under experimental conditions have been analyzed.     

土壤中砷的形态和生物有效性研究现状与趋势

砷在自然界中分布极为广泛,与人类生活密切相关,在工农业生产活动中得到了广泛应用。但砷是高毒元素,在微量水平上就能对动植物体产生一定危害。砷的危害通过生物有效性予以表达,其生物有效性与其在土壤中的累积量及形态密切相关。本文就砷在土壤中的存在价态、与土壤胶体的结合形态、砷生物有效性及不同形态砷的测定技术与方法等方面进行了简要综述,并对未来相关领域的研究发展趋势作了展望,以期为深入开展土壤中砷形态及其生物有效性研究提供科学参考。     

Distribution of applied cadmium in different size fractions of soils after incubation

The size fraction of soils is one of the important factors that influence the retention of heavy metals. The sorptive properties of soils for heavy metals are principally associated with clay and silt-size fractions. Phosphate fertilizers that are applied to highly weathered tropical soils contain a wide concentration range of cadmium (Cd) as an impurity. Tropical soils contain kaolinite and oxides of Al, Fe, and Mn, which have the ability to sorb Cd. However, the distribution of Cd in different size fractions and the chemical speciation of particulate-bound Cd in the clay size fractions when introduced to soil and allowed to incubate at field moisture capacity merits attention. Cadmium was, therefore, applied to selected surface Kenyan soils varying widely in physicochemical properties to investigate its distribution in different soil particle size fractions and the speciation of particulate-bound species in clay size fractions after incubation. The Cd content in different particle fractions was analyzed by graphite furnace atomic absorption technique after HF-HClO₄ digestion. The particulate-bound Cd species were investigated using chemical sequential extraction method. The study showed that clay size fraction of the natural and the Idaho monoammonium phosphate (MAP)-fertilizer or the Cd perchlorate-added MAP chemical reagent-treated soils contained the highest amount of the total Cd. However, silt and sand fractions of the treated soils also retained appreciable amounts of Cd. Speciation studies revealed that metal-organic complex-bound Cd was the most predominant compared to other particulate-bound Cd species in the clay size fractions of the soils treated with Idaho MAP fertilizer or the Cd perchlorate-added MAP chemical reagent. The distribution of total Cd in the different soil particle size fractions and the speciation of particulate-bound Cd in the clay size fractions varied with the soil type. The results indicate that clay size fractions can retain Cd making it less available; however, the influence of farming practices, which affect Cd mobility, should not be overlooked.     

Soil Water Retention at Varying Matric Potentials following Repeated Wetting with Modestly Saline‐Sodic Water and Subsequent Air Drying

Abstract

Coal bed natural gas (CBNG) development in the Powder River (PR) Basin produces modestly saline, highly sodic wastewater. This study assessed impacts of wetting four textural groups [0–11%, 12–22%, 23–33%, and >33% clay [(g clay/100 g soil)×100%)] with simulated PR or CBNG water on water retention. Soils received the following treatments with each water quality: a single wetting event, five wetting and drying events, or five wetting and drying events followed by leaching with salt‐free water. Treated samples were then resaturated with the final treatment water and equilibrated to ?10, ?33, ?100, ?500, or ?1,500 kPa. At all potentials, soil water retention increased significantly with increasing clay content. Drought‐prone soils lost water‐holding capacity between saturation and field capacity with repeated wetting and drying, whereas finer textured soils withstood this treatment better and had increased water‐retention capacity at lower matric potentials.     

添加外源有机物和黏粒材料对沙黄土有机碳和菠菜生长的影响

快速提升贫瘠土壤的有机碳含量是改良土壤、增加土地生产力的重要途径。本研究通过温室盆栽试验，设置了沙黄土对照(CK)、沙黄土+木本泥炭(LW)、沙黄土+褐煤1(LC1)、沙黄土+褐煤2(LC2)、沙黄土+木本泥炭+红黏土黏粒(LWR)、沙黄土+木本泥炭+砒砂岩黏粒(LWS)和沙黄土+木本泥炭+膨润土(LWB)共7个处理，每个处理5次重复，研究了不同处理下菠菜生育期内(35d)生长、生理指标差异及各处理对土壤有机碳含量提升效果。结果表明：与CK相比，各处理收获期菠菜产量和土壤有机碳含量均显著增加(P<0.05),LW、LC2、LC1、LWS、LWR和LWB处理下收获期菠菜产量分别增加了18.6%、51.3%、80.8%、127.6%、148.1%和203.8%，对应处理土壤有机碳含量分别增加了92.4%、84.3%、66.8%、84.0%、116.3%和98.3%，土壤pH均有一定程度降低。与LW处理相比，补充黏粒材料后，LWS、LWR和LWB处理下收获期菠菜叶面积分别显著增加了55.0%、86.5%和98.3%(P<0.05)，各处理土壤pH、电导率、有机碳和全氮含量有一定程...     

水分非饱和条件下土壤矿物界面有毒有机物转化机制研究进展

土壤矿物作为土壤重要活性组分，可驱动土壤有毒有机物化学转化，降低污染风险。以往土壤矿物与有毒有机物界面行为研究主要集中于水环境或矿物悬浊液体系，然而实际环境中土壤及其矿物常处于干燥、湿润等水分非饱和状态。近年来，水分非饱和条件下土壤矿物界面有毒有机物转化及机制已成为研究热点，相关研究获得一系列新发现。低含水量铁锰矿物、黏土矿物和金属离子饱和黏土矿物能驱动多环芳烃、抗生素等疏水性有毒有机物化学转化。水分非饱和环境会减弱矿物界面水分子与有毒有机物竞争活性位点，并使矿物发生脱水、向高活性结构转变。此外，土壤矿物水分状态也会影响有毒有机物转化产物，水分非饱和环境更有利于持久性自由基和卤代二噁英等中间产物的形成和稳定。以往研究认为，电子转移反应是土壤矿物界面有毒有机物转化机制，随着检测技术与理论计算的发展，自由基催化和水解作用机制逐渐被发现，相关机制研究精准至矿物晶型和晶面层面。虽然水分非饱和条件下土壤矿物界面有毒有机物转化及机制已逐渐清晰，但其研究广度和深度有待进一步拓宽和加深。建议未来在实际水分非饱和土壤和矿物中开展有毒有机物转化研究，深入探究还原转化过程，研发原位反应装置及检测方法，尝试从微纳米尺度和分子水平解析有毒有机物在矿物界面转化机制。     

An integrated sampling/speciation method for inorganic arsenic in soil solution

Arsenic mobility, bioavailability, and toxicity in soil‐water systems are strongly affected by species distribution. In spite of species‐selective analytical tools, results may be biased by postsampling oxidation or reduction of compounds. The scope of this study was to delineate a speciation method for inorganic arsenic integrated with pore‐water sampling to prevent problems related to postsampling species transformation. Experiments were performed with exchange cartridges packed with quaternary amine (nitrate‐form) to separate anionic arsenate from uncharged arsenite. Standard solutions with varied As(III) : As(V) ratios as well as groundwater and soil‐solution samples were studied. Analyses were performed by ICP‐MS using either extraction cartridges directly coupled to the instrument or samples obtained by offline speciation tests. Results showed that As(III) passes the packed bed with the rinsing solution, while As(V) is retained on the column and can be released by elution with 0.25 M NH₄NO₃. Recoveries between 98% and 116% were insensitive to pH variation (pH 3–8) and competing anions. Groundwater samples from a contaminated site yielded recoveries between 92% and 125% when the sum of As(III) and As(V) was compared to total As. Integration of the speciation scheme with pore‐water sampling was tested in a soil‐column experiment. Soil material from a fen site with elevated geogenic As contents was subjected to a varying moisture regime. Reducing conditions during prolonged saturation caused a marked increase of As(III) concentrations. Conversely, when the soil was drained, total As levels decreased and were dominated by As(V). Overall, extraction results with standard solutions and natural samples indicate reliable performance of the method in experimental investigations and field studies.     

添加外源耐砷菌与零价铁对土壤砷有效性和形态的影响

采用室内土壤培养试验,探讨了外源添加耐砷真菌棘孢木霉（TrichodermaasperellumL．）和零价铁对土壤砷的有效性和形态转化的影响。结果表明：向污染土壤中添加耐砷真菌棘孢木霉后,随着培养时间的延长,土壤中水溶态砷和NaHCO,提取态砷含量均呈稳定增加趋势,耐砷真菌促进了土壤中砷的溶出和释放;培养30d时,耐砷菌处理土壤有效砷含量比同期对照增幅达3．9％-10．7％,水溶态砷以As（V）为主,未检测到As（Ⅲ）、一甲基胂（MMA）、二甲基胂（DMA）等其他形态的砷;随着外源零价铁的加入,土壤中砷的活性大大降低,其有效砷含量降幅为76．5％-90．4％;在耐砷菌与零价铁联合作用下,相比于单纯的零价铁处理,土壤有效砷含量显著增加（p〈0．05）,因耐砷真菌棘孢木霉的加入导致零价铁对土壤砷的固定效率下降7．0％-11．1％。耐砷菌导致土壤砷活化可能主要与残渣态向非专性吸附态砷的转化等过程有关,外源零价铁对土壤砷的固定作用可能与非专性吸附态向无定形及弱晶质氧化物结合态、残渣态砷等转化过程相关;耐砷菌的加入抵消了零价铁对土壤砷的部分固定效果,但短期内（〈30d）不会构成大的影响。     

层状包气带黏土层厚度对硝态氮迁移的影响

层状包气带结构中黏土层对污染物进入地下水具有阻滞作用,黏土层的厚度对硝态氮(NO_3~--N)在包气带迁移中的淋失、累积以及反硝化作用等具有非常重要的影响,而目前关于这方面的研究还不足。该研究通过设置高度为40 cm、砂土与黏土层厚度比分别为3∶1,1∶1,1∶3的"上粗下细"型以及全黏土型的4组填充土柱,采用稳定浓度的定水头淋滤试验,研究黏土层厚度不同的土柱NO_3~--N溶液入渗过程、土壤NO_3~--N淋滤、累积和反硝化特征,进而阐明层状包气带黏土层厚度对NO_3~--N迁移的影响。结果表明:湿润锋运移深度和累积入渗量与入渗时间的关系在溶液穿越砂黏土层界面前后由非线性趋于线性,累积入渗量随黏土层厚度增加而显著减小(P0.05);当土柱内黏土层厚度达到40 cm时,其对NO_3~--N淋滤的阻滞作用明显强于黏土层厚度为10～30 cm的土柱;淋滤试验过程中在砂黏土层界面形成水分滞留层,界面处黏土层中NO_3~--N和NO_2~--N累积量均达到峰值,且随着深度的增加,NO_3~--N和NO_2~--N累积量降低;黏土层厚度差不小于20 cm的土柱内NO_3~--N累积量差异显著(P0.05),而40 cm黏土层的土柱反硝化量[(0.15±0.05) g]显著高于黏土层厚度为10～30 cm的土柱(P0.05),说明当黏土层达到一定厚度时(如40 cm),对NO_3~--N的阻滞作用和反硝化作用具有显著影响,对防止NO_3~--N淋失进入地下水产生重要作用。该研究可为层状包气带土壤条件下农田施肥管理与地下水保护提供科学依据。     

Arsenic speciation in xylem exudates of rice growing in arsenic-contaminated soil

Rice has a much enhanced arsenic (As) accumulation compared with other cereal crops. How As is transported in xylem exudates of rice growing in soil remains unknown. We quantified the chemical species of As in xylem exudates of rice growing in arsenic-contaminated soil (62.5 mg As per kg soil) under flooded and aerobic conditions. In the aerobic treatment, the predominant species of As in the xylem sap was arsenate [As(V)], accounting for 64–88%, and the remainder was arsenite [As(III)]. In the flooded treatment, dimethylarsinic acid (DMA) was detected in the xylem sap besides As(III) and As(V), and the percentages of the three species As was 11–20%, 26–77%, and 12–54%, respectively. As speciation in the soil solution was dominated by As(V) under aerobic conditions, and dominated by As(III) under flooded conditions. DMA was detected at low concentrations in some of the flooded solution samples. Our results show that As speciation in the xylem sap differs in different water management regimes and correlate with As speciation in the soil solution strongly.