Plant Availability of Cadmium in Soils: I. Extractable Cadmium in Newly and Long-Term Cultivated Soils

Abstract

Effects of long-term use of phosphate fertilizers on extractable soil Cd in relation to its concentrations in plants were investigated. “Paired” soil samples were collected from newly and long-term cultivated fields and analyzed for Cd by extraction with NH₄OAc, DTPA, NH₄OAc-EDTA, NH₄NO₃, HCl and CaCl₂. Plant samples were also collected and analyzed for Cd. Significant differences in extractable Cd by all the extractants except NH₄NO₃ were observed between the newly and long-term cultivated soils. The Cd concentrations in plants were not increased by the elevated extractable Cd. Although significant relationships were observed between plant Cd and extractable soil Cd, none of the extractants used alone gave a good assessment of plant-available Cd for all the samples used in this study.     

Plant Availability of Cadmium in Soils: II. Factors Related to the Extractability and Plant Uptake of Cadmium in Cultivated Soils

Abstract

Eighty four soil samples collected from southeastern Norway were analyzed for Cd by extraction with NH₄OAc, DTPA, NH₄OAc-EDTA, NH₄NO₃, HCl and CaCl₂. The total Cd, pH, exchangeable K and Ca, dithionite-extractable Mn, available P and fine sand (0.2–0.02 mm) contents were the principal factors related to the extractable Cd, with some inter-extractant variations. Cadmium extracted by NH₄NO₃, NH₄OAc, HCl and CaCl₂ decreased with increasing soil pH, but the Cd extracted by all the extractants increased with increasing total Cd, exchangeable K and Ca, available P, and Mn-oxide contents in the soils. The Cd concentrations in plants were significantly related to the extractable Cd, exchangeable Ca and Mg, pH, Mn-oxides and organic matter content.     

不同氮磷钾肥对土壤pH和镉有效性的影响

采用土壤培养方法研究了不同氮、磷、钾肥对土壤pH和镉有效性的影响。结果表明,在培养60 d时,所有氮肥处理均降低了土壤pH,增加了Cd的提取量;但高量尿素和氯化铵处理土壤pH降低最多,提取的Cd也最多;硫酸铵提取的Cd较对照增加最小。所有磷肥处理均引起土壤pH小幅降低,但对土壤Cd提取量的影响以普钙稍大。3种钾肥处理均降低了土壤pH,其中氯化钾在0 d时提取的Cd在所有钾肥处理中为最高,其提取能力15 d后逐渐消失,试验结束时所有钾肥处理对Cd提取量均低于对照。本研究进一步表明,在土壤Cd含量处于污染临界值附近或已受Cd污染的土壤上,应避免施用高量的酸性肥料如尿素、氯化铵、普钙,以及其他酸性物料。在常用磷、钾肥中,磷酸二铵和硫酸钾在Cd污染土壤上施用更为适合。     

石灰性土壤上三种磷肥不同用量的磷肥利用率及其缓效率

供试三种磷肥虽其性质不同,但其磷肥利用率和缓效率表现趋势一致。磷肥利用率均随磷肥用量的增高而降低,呈负相关,且随种植年限的延长而减小。磷肥缓效率随磷肥用量的增加而提高,呈正相关。磷肥利用率和缓效率,过磷酸钙的利用率最大,缓效率最小;磷酸二铵磷肥利用率最低,缓效率最高;钙镁磷肥利用率及缓效率均居上述两种磷肥之间,但其P2O5用量小于142.5kghm-2时,利用率大于缓效率,P2O5用量大于285kghm-2时,利用率小于缓效率。由此可知:P2O5含量以水溶性为主要成分的磷酸二铵和P2O5为枸溶性的钙镁磷肥对石灰性土壤均不是理想的磷肥品种。本试验结果提示:石灰性土壤施用磷肥最好的方法是水溶性磷与枸溶性磷合理搭配,其最佳的配合比例应据当地试验结果而定。     

Potassium Availability as Related to Clay Mineralogy and Rates of Potassium Application

Availability of applied potassium (K) as a fertilizer to plants is influenced by soil mineralogy, environmental factors, and rates of K application. The objective of this research was to study the effects of clay minerals and K application on K supply characteristics of calcareous soils in Iran. Surface and subsurface horizons of six sites with different ranges of clay content and exchangeable K were selected. The soils were treated with potassium chloride (KCl) solution with different K concentrations. Four wet–dry cycles were sequentially applied. Illites, vermiculites, and chlorites were present in all soils. Smectites were present in larger amounts in one soil. The increase in soluble and exchangeable K was expressed by linear equations in which the slops influenced by the dominant clay minerals. Potassium fixation was much higher in soils with more illites and vermiculites. Results revealed that different forms of K were affected by the dominant clay minerals but were independent from the rate of applied K.     

化肥对黑土不同粒级碳水化合物的影响

本文采集公主岭市长期定位监测基地不施肥和施用不同化肥的黑土,通过超声波分散-离心分离得到细黏粒(<0.2μm)、粗黏粒(0.2～2μm)、粉粒(2～53μm)、细砂粒(53～250μm)、粗砂粒(250～2000μm)5个颗粒级别,分析全土及不同粒级中土壤碳水化合物并进行含量与分布的比较。结果表明,黑土中不同粒级碳水化合物库的性质差异显著,碳水化合物多集中在粉+黏粒中;长期施用化肥后,黑土全土及各粒级碳水化合物库大小和浓度基本上没有变化;粗砂粒级(Gal+Man)(:Ara+Xyl)下降,表明该粒级中植物来源碳水化合物所占比重有所增加,暗示出粗砂粒级对施肥措施更为敏感。     

Heavy Metals in Soil, Crops and Grass as a Source of Human Exposure in the Former Mining Areas (6 pp)

Background, Aim and Scope   In the former mining area of Northern France, the number of diseases (cancers, malformations, backwardness...) is about ten times above the national average. Environmental surveys conducted by the Ministry of Health have showed that more than 10% of children living around a lead smelter located in this area had lead levels higher than 100 μg L-1 of blood (25% in the closest city). Two main factors contribute to the population exposure: the ingestion of plants produced in the contaminated area, and also the ingestion or the inhalation of contaminated dust or soil. It is usually known that these particles are key routes of exposure to lead for younger children, in particular via hand-to-mouth transfer. For a better understanding of this problem, researchers investigated different exposure sources like soil and vegetable contamination. Materials and Methods: All these parameters that contribute to the assessment of environmental and health risks of metal contamination have been measured. About 170 composite samples of soils around the smelter or from reference areas have been analysed, mainly from fields (70), kitchen gardens (60) and lawns (38). Total contents of Cd and Pd were measured in the organo-mineral layer (0-25cm) of soils. Composite grass samples were also taken on the lawns. Crops and vegetables were sampled from fields and kitchen gardens and then, parts of the plant intended only for consumption were washed and analysed. Results: For the organo-mineral horizon of the studied sites and located in various environmental contexts (contaminated or reference), we found a very broad range of metal concentrations. Generally, the data showed a strong variability of the physicochemical parameters of the urban soils (kitchen gardens, lawns), in particular with regard to lead. This work showed that cultivated plants (agricultural or kitchen garden) or lawn grass are also significantly contaminated by heavy metals, especially lawn grasses, cereal grains (wheat, barley) and lettuces sampled around the smelter. Moreover, the proportion of lead present on lawn grasses could reach 50% of the total contamination of the plants because of the deposition of contaminated dust. Discussion: The values of pollution in agricultural field and kitchen garden soils located near the smelter were respectively about 20 and 30 times above the agricultural regional reference values, probably due to the atmospheric emissions from a smelter that significantly increases the concentrations of metals in the upper horizons of the neighbouring soils. This pollution results in a high level of contamination of plants grown of these soils. Conclusions: The results showed that heavy metals in soils, particles (dust and soil) or plants of kitchen gardens, lawns or playgrounds could be potentially transferable to the users, and in particular to young children, and could then contribute, to a considerable share, to the exposure of the population living in a contaminated area around a smelter brought about through inhalation, direct ingestion of particles or consumption of plants. Recommendations and Perspectives: These results highlight that the consumption of home-grown vegetables can constitute a risk of exposure, in particular for cadmium, and especially in children. This work also underlined the role of the contaminated soil particles in the Pb contamination of the human food chains and their environment. Because of the complexity of the various methods of population exposure, it will be necessary to complete the data base, in partucular in urban areas. The main objective of this future work will be to relate the degree of environmental contamination with the lead level in child bood, and to integrate other environmental compartments like outdoor and indoor dust of the dwellings into the model of pathway exposure.     

Bioavailability of Silicon from Different Sources and Its Effect on the Yield of Rice in Acidic,Neutral, and Alkaline soils of Karnataka,South India

Several silicon (Si) sources have been reported to be effective in terms of their effectiveness on rice growth and yield. Apart from that, it is crucial to understand the bioavailability of silicon from different silicon sources for adequate plant uptake and its performances in varying types of soils. In this point of view, a pot experiment was conducted to assess the bioavailability of silicon from three Si sources and its effect on yield of rice crop in three contrasting soils. Acidic (pH 5.86), neutral (pH 7.10), and alkaline (pH 9.38) soils collected from different locations in Karnataka were amended with calcium silicate, diatomite, and rice husk biochar (RHB) as Si sources. Silica was applied at 0, 250, and 500 kg Si ha^?1, and the pots were maintained under submerged condition. There was a significant increase in the yield parameters such as panicle number pot^?1, panicle length pot^?1, straw dry weight pot^?1, and grain weight pot^?1 in acidic and neutral soils with the application of Si over no Si treatment, whereas only straw dry weight pot^?1 increased significantly with the application of Si sources over control in alkaline soil. Higher Si content and uptake was noticed in neutral soil followed by acidic and alkaline soils. The bioavailability of Si increased with the application of Si sources but varied based on the types of soil. Application of calcium silicate followed by diatomite performed better in acidic and neutral soils whereas RHB was a better source of Si in alkaline soil. A significant difference in plant-available silicon status of the soil was noticed with the application of Si sources over control in all three studied soils.     

Availability of Cadmium and Zinc as Affected by the Use of Reactive Phosphate Rock,Lime, and Chicken Manure on an Indonesian Acidic Upland Soil under Field Conditions

We assessed cadmium (Cd) and zinc (Zn) availability when applying reactive phosphate rock (RPR) in combination with lime and chicken manure on Indonesian acidic upland soils. Maize plants were grown on unamended soil and soils treated with several combinations of 2 tons dolomite ha^–1, 2 tons of chicken manure ha^–1, 1 ton ha^–1 of RPRL (reactive phosphate rock containing 4 mg Cd kg^–1 and 224 mg Zn kg^–1), and 1 ton ha^–1 of RPRH (RPR containing 69 mg Cd kg^–1 and 745 mg Zn kg^–1). In addition to its positive effect on plant yield, application of RPR in combination with chicken manure did not result in toxic Cd concentrations. Although liming is effective to reduce plant Cd concentrations, it results in more soil Cd accumulation and more plant Zn deficiency. Cadmium and Zn concentrations in shoots and grains can be predicted well from amounts extracted from the soil by 0.5 M ammonium (NH₄) acetate + 0.02 M ethylenediaminetetraacetic acid (EDTA) at pH 4.65.     

改良剂对滴灌棉田镉分布及迁移特征的影响

通过田间桶栽试验研究了高浓度镉(40mg/kg)条件下,4种改良剂(有机—无机复合稳定剂、无机高分子复配材料、聚丙烯酸盐复配材料、有机高分子复配材料)对棉田土壤剖面中土壤pH、阳离子交换量(CEC)、镉含量及其形态分布迁移的影响。结果表明:(1)4种改良剂均显著提高了棉田土壤剖面各层的pH和CEC,无机高分子复配材料在0—20cm土层效果最好,分别增加了0.43个单位和4.43cmol/kg;(2)改良剂促进土壤可交换态镉向其余4种形态的转化,相关分析表明碳酸盐结合态与可交换态镉含量呈极显著负相关(P0.01),在0—20cm和20—40cm土层中效果尤为突出,以无机高分子复配材料对土壤可交换态镉的降低效果最好,降低了3.61mg/kg;(3)各土层中的pH和CEC均与可交换态镉呈负相关,与其他形态呈正相关。即改良剂通过改变土壤pH和CEC,影响土壤镉的分布及迁移,从而降低镉的有效性,达到改善土壤环境的目的。     

草甸草原土壤不同组分有机碳含量及化学结构对长期氮输入的响应

为探究长期氮输入对草甸草原土壤不同组分有机碳含量及化学结构影响，以内蒙古东北草甸草原为研究对象，于2010年设置0(CK)、30(N30)、50(N50)、100(N100)、150(N150)、200(N200)kg/(hm~2·a) 6个不同施氮水平处理，测定土壤不同组分有机碳含量及红外光谱特征。结果表明：(1)相比CK，长期氮输入条件下可提高土壤总有机碳(SOC)含量(增幅0.3%～13.6%)，且主要表现为颗粒有机碳(POC)含量的增加(9.22%～16.39%)，但降低土壤轻组有机碳(LOC)含量。(2)红外光谱主成分分析(PCA)结果表明，土壤LOC主要来源于脂肪碳、芳香碳、酚醇化合物，POC主要来源于芳香碳和酚醇化合物，矿物结合有机碳(MOC)主要来源于烷基碳和多糖。(3)相比CK，施氮处理凋落物和LOC官能团中烷氧碳(单糖+多糖)的相对强度降低，烷基碳、芳香碳相对强度增加；土壤POC和MOC官能团中烷氧碳、烷基碳及芳香碳相对强度增加，酚醇化合物相对强度降低；且施氮处理下凋落物及其不同土壤碳组分有机碳结构稳定性(芳香碳/脂肪碳)均高于CK。(4)结构方程模型(SEM)结果...     

Soil tests to inventory the initially available levels and to assess the fates of added P and K as bases for improved fertilizer recommendations

Abstract

Preliminary studies indicate that “quicktest”; methodology can be used to measure the individual tendencies of soils to fix (or not to fix) added P and K into plant‐unavailable forms. It appears that the fraction of added P recovered after two hours in Bray 1 or of K in 1N NH₄OAc extracts can be used for adjusting fertilizer recommendations to maximize yields. It is suggested that the reciprocals of the recovery fractions be used as multiples of the differences between sufficiency and existing levels of available P and K to obtain amounts of fertilizer P and K to apply. This method offers promise for both improving fertilizer recommendations and identifying soils of unique fixation tendencies where choice of fertilizer materials or application methodology may be most suitable. The possibility of using the new extracting solution of Mehlich to assess‐ the fate of both applied P and K in one extractant looks promising.     

长期肥料定位试验栗钙土中磷肥在莜麦上的产量效应及行为研究

研究了高寒半干旱区8年肥料定位试验中,磷肥和有机肥在莜麦上的产量效应、土壤磷素的平衡、土壤Olsen-P及各形态无机磷的变化。结果表明,单施磷肥(N0P1)莜麦增产30.8%、单施氮肥(N1P0)增产109.4%、氮肥和磷肥配合(N1P1)施用莜麦增产314.0%;NP间表现出显著正交互作用,NP(N1和P1)交互作用增产86.9%;施用22.5和45.0.t/hm2有机肥分别比N0P0处理增产115.1%和220.1%;施用有机肥基础上增施磷肥无明显增产效应。不同施肥处理土壤Olsen-P和各形态无机磷的增减取决于土壤磷素的积累与消耗量,7年不施磷肥土壤Olsen-P降低3.3mg/kg。施用磷肥和有机肥土壤各形态磷库均有不同程度的积累;土壤磷素积累以无机磷为主,其中Ca2-P和Ca8-P的积累量分别占土壤无机磷变化总量的19.3%和25.4%,Al-P和Fe-P分别占23.8%和14.8%,O-P和Ca10-P共占13.0%。依据土壤磷素收支平衡状况计算出维持土壤磷素平衡的P2O5用量为45.0.kg/hm2。根据肥料效应函数计算出有机肥用量为0、22.5.t/hm2时,P2O5的最高产量用量分别为98.4.kg/hm2和87.4.kg/hm2。     

The Effect of Liming on the Availability of Cadmium in Soils and Its Uptake in Cacao (Theobroma c acao L.) In Trinidad & Tobago

ABSTRACT

Cadmium (Cd) is absorbed and bio-accumulated by cacao (Theobroma cacao L.) trees, resulting in unacceptably elevated levels in cocoa beans, necessitating measures to reduce its uptake from soils. A field experiment, lasting 18 months, was carried out to assess the effectiveness of liming on pH, bioavailability of Cd in soils and its uptake in cacao tissues. The treatments were: (a) control (untreated) and (b) lime-treated trees. Results demonstrated a significant (P < 0.05) increase in the soil pH (lime treated) and a natural fluctuation in pH for the control. For the lime-treated trees, bioavailable Cd levels generally stabilized with no significant change (P > 0.05) compared to the significant (P < 0.05) increase showed by control trees. The Cd levels in the leaves of both treatments decreased, however, the rate of decline in leaf Cd concentrations for lime-treated trees (?0.1378) was 3x faster than control (?0.0497) trees demonstrating the effectiveness of liming.     

扬州市耕地土壤pH值30年演变及其驱动因子

【目的】土壤pH值是衡量耕地质量的重要指标,开展江苏省扬州市30年来种植制度、耕作制度、施肥、降雨等对耕地土壤酸化影响的研究,为预测和控制土壤酸化提供科学依据。【方法】调查了1984年、1994年、2005年、2014年四个时期耕地土壤pH、成土母质、土壤类型、土壤有机质含量,以及各时期的耕作制度、种植制度、降雨量和施用化肥品种和数量。数据来源于1984年第二次土壤普查的农化样点（4107个）、1994年的土壤普查点（2862个）、2005年土壤养分调查点（4018个）、2014年土壤养分调查点（6009个）,共16996个。参照《中国土壤》对我国土壤酸碱度分级指标将扬州市耕地土壤pH分为5级,分别为Ⅰ级（ 7.5）、Ⅱ级（6.5~7.5）、Ⅲ级（5.5~6.5）、Ⅳ级（4.5~5.5）、Ⅴ级（ 4.5）。应用地统计学中克里格法（Kriging）和相关的统计学方法,用ArcGIS10.1、SPSS19等软件进行了数据统计分析。【结果】扬州市1984年、1994年、2005年、2014年土壤pH平均分别为7.51（4107个）、7.07（2862个）、6.83（4018个）、6.74（6009个）;1984~2014年四个时期土壤pH空间分布格局基本不变,即里下河地区 沿江圩区 通南高沙土区 丘陵地区。1984、1994年和2005年,土壤pH以Ⅰ级、Ⅱ级为主,1984年占总面积的90%以上,1994年和2005年占总面积的75%以上;2014年土壤pH空间分布以Ⅱ级、Ⅲ级水平为主,占总面积的65.7%。30年间,土壤pH值下降0~1个单位的面积占总面积的47.2%,下降大于1个单位的面积占总面积的39.3%。前20年土壤pH值下降严重,下降了0~2个单位的面积占80%以上。30年间不同成土母质、土壤类型的整体土壤pH值呈下降趋势,分别下降0.9、0.8个单位;土壤有机质含量的变化与土壤pH变化呈负相关关系,30年间土壤有机质含量平均上升了6.01 g/kg,是土壤pH整体呈下降趋势的原因之一;30年间扬州市降水pH值整体呈下降趋势,其中丘陵、沿江地区下降最快,与丘陵、沿江地区土壤pH下降较快是一致的;30年间化肥投入量与土壤pH变化呈高度的负相关,2005年化肥投入量约505300吨,比1984年化肥投入量增加了2.42倍;2005~2014年化肥投入量呈稳定趋势,与30年土壤pH变化趋势是一致的;种植大棚蔬菜的田块土壤pH平均值比周边种植稻麦田块下降1.5~2个单位,表明土地利用类型改变也会影响土壤pH值。【结论】扬州市耕地土壤pH值30年间持续下降,前20年下降幅度较大,后10年渐趋稳定。影响土壤pH值空间分布因子主要有成土母质、土壤类型、土壤有机质含量;影响土壤pH时间分布因子主要有酸雨、施肥及土地利用类型,其中酸雨、施肥是导致土壤酸化的主要驱动因子。     

丛枝菌根真菌对紫花苜蓿吸收土壤中镉和锌的影响

采用温室盆栽试验方法,研究了镉（Cd）、锌（Zn）污染土壤中,8种不同丛枝菌根真菌（AMF）Glomus lamellosum（G.la）、Acaulospora mellea（A.m）、Glomus mosseae（G.m）、Glomus intraradices（G.i）、Glomus etunicatum（G.e）、Glomus constrictum（G.c）、Diversispora spurcum（D.s）、Glomus aggregatum（G.a）对紫花苜蓿（Medicagosativa L.）吸收Cd、Zn的影响。结果表明,Cd、Zn污染下AMF仍然明显侵染紫花苜蓿,并促进紫花苜蓿对Cd、Zn的吸收积累,但不同AMF影响的效应和植株不同部位对重金属的吸收积累规律存在差异。AMF处理下紫花苜蓿根部Cd、Zn含量和积累量明显增加,但地上部Cd、Zn的含量则降低,地上部Zn的积累量也减小,这表明AMF处理减弱了Cd、Zn由根部向地上部的运移,减轻了植物地上部毒害。接种AMF条件下,植株尤其是根部生物量增加是Cd、Zn在其体内含量和积累量增加的重要因素,不同种类AMF促进植株生物量增加的幅度不同,导致植株对Cd、Zn的积累和抗性存在差异。     

不同镁肥在中国南方三种缺镁土壤中的迁移和淋洗特征

镁淋溶是导致南方土壤缺镁的关键因素之一。通过室内土柱模拟实验探究了四种镁肥(氧化镁、改性硫酸镁、钙镁磷肥和硫酸钾镁)在中国南方三种酸性缺镁土壤(黄壤、紫色土、红壤)中的有效性、迁移和淋洗特征以及土壤对镁素的固持,以期为南方缺镁土壤科学施用镁肥提供依据。结果表明,施用镁肥(折合MgO 100 mg·kg~(-1))后,三种土壤施镁土层(0～15 cm)交换性镁含量提高了2倍～15倍。氧化镁、改性硫酸镁和硫酸钾镁在三种土壤中均表现出较强的迁移能力,经过60 d淋洗(模拟降雨量1 000 mm),未施用镁肥土层(15～50 cm)中有效镁含量均较对照增加17.9～105.4mg·kg~(-1)。黄壤上不同镁肥处理镁累积淋失量在104.9～243.8 kg·hm-2,与紫色土接近,但较红壤高2倍～3倍。实验结束后,黄壤和紫色土上施用氧化镁、改性硫酸镁和硫酸钾镁的淋失率均超过三分之一,而红壤中均低于7%;施用钙镁磷肥增加了土壤对镁的固持,减缓了镁向下淋洗,淋失率在三种土壤中均为负值。综合四种镁肥在土壤中的有效性、土壤对镁素的固持、镁在土壤中的淋洗特征以及土壤酸化阻控,钙镁磷肥在三种酸性缺镁土壤中施用效果均较好。     

Cadmium and lead affect the status of mineral nutrients in alfalfa grown on a calcareous soil

ABSTRACT

Cadmium (Cd) and lead (Pb) are toxic trace elements which are not essential for plants but can be easily taken up by roots and accumulated in various organs, and cause irreversible damages to plants. A pot experiment was carried out to investigate the individual and combined effects of Cd (0, 10, 20 mg kg^?1) and Pb (0, 500, 1000 mg kg^?1) level in a calcareous soil on the status of mineral nutrients, including K, P, Ca, Mg, S, Fe, Mn, Cu, and Zn, in alfalfa (Medicago sativa L.) plants. Soil Pb level considerably (P ≤ 0.05) affected the concentrations of more elements in plants than soil Cd level did, and there were combined effects of soil Cd level and Pb level on the concentrations of some nutrients (Ca, Mg, and Cu) in plants. The effects of soil Cd level and Pb level on plant nutrient concentrations varied among plant parts. Cd and Pb contamination did not considerably affect the exudation of carboxylates in the rhizosphere. An increase in rhizosphere pH and exudation of significant amounts of carboxylates (especially oxalate) in the rhizosphere might contribute to the exclusion and detoxification of Cd and Pb. Neither shoot dry mass nor root dry mass was significantly influenced by soil Cd level, but both of them were considerably reduced (by up to 25% and 45% on average for shoot dry mass and root dry mass, respectively) by increasing soil Pb level. The interaction between soil Cd level and Pb level was significant for root dry mass, but not significant for shoot dry mass. The results indicate that alfalfa is tolerant to Cd and Pb stress, and it is promising to grow alfalfa for phytostabilization of Cd and Pb on calcareous soils contaminated with Cd and Pb.     

Equations to describe the amount and rate of sorption

The partition of materials that react with soil between the solid and the solution phase, and how this changes with time, can often be described by a simple equation: S = a c^b1t^b2 where S is the amount sorbed, c is the solution concentration, t the time of contact, and a, b₁ and b₂ are parameters. However, when the range of values for sorption is large, it is apparent that both b₁ and b₂ increase with decreasing sorption. At low values for sorption, b₁ approaches 1, and sorption plots are nearly linear. These observations are consistent with a mechanistic model in which it is postulated that the materials react with heterogenous sites. As the amount of sorption decreases, the heterogeneity of the occupied sites decreases. This is why b₁ increases. Because there is heterogeneity of occupied sites, there is a range of rates for the subsequent reaction. This is why the rates are proportional to a fractional index of time. It is better to describe the effects of time this way than by using several first-order equations.     

黄河源头区水土流失及草业综合治理探讨

黄河源头区属高寒半干旱气候区 ,地带性植被为高寒草甸。由于自然条件恶劣和超载过牧、沙金开采等 ,致使水土流失日益严重 ,草业在该区水土流失治理中的地位和作用独特 ,具有不可替代性。具体对策是 :草原牧区禁止超载过牧 ;沙化严重的地区进行封育改良 ;陡坡地退耕 ,缓坡地实行草、粮带状间作 ;加强对工矿和城镇建设的管理