双氰胺单次配施和连续配施的土壤氮素形态和蔬菜硝酸盐累积变化

采用田间试验研究了双氰胺(dicyandiamide,缩写DCD)单次配施和连续配施的土壤氮素形态和蔬菜硝酸盐累积变化。结果表明,与单施化肥相比,DCD单次配施的长期叶菜甘蓝生长过程中土壤铵态氮含量增幅为21.3%—339.4%,土壤硝态氮和菜体硝酸盐含量降幅分别为5.4%—80.2%和4.4%—58.3%;短期叶菜空心菜收获时土壤铵态氮含量增加了299.4%,土壤硝态氮和菜体硝酸盐含量分别降低了26.2%和31.7%。DCD连续配施的"甘蓝-菠菜-空心菜-萝卜-大白菜"种植体系中,土壤铵态氮、硝态氮和菜体硝酸盐含量均呈累积的趋势,配施DCD的土壤铵态氮含量从略高于化肥处理(44.0%)发展到极显著高于化肥处理(392.5%,P<0.01),土壤硝态氮含量从极显著低于化肥处理(-68.2%,P<0.01)发展到显著高于化肥处理(146.6%,P<0.05),菜体硝酸盐含量从显著低于化肥处理(-30.2%,P<0.05)发展到极显著高于化肥处理(40.4%,P<0.01)。由此可见,DCD单次配施可显著降低菜体硝酸盐含量,而连续配施DCD的土壤能维持一定量的铵态氮水平,这些盈余的铵态氮会进一步转化为硝态氮残留在土壤中,并可能产生蔬菜硝酸盐累积的风险。     

采用浇灌方式施用甲酸钙消减当季叶菜镉污染

中轻度Cd污染农业土壤修复多采用以pH调控为主的钝化措施,但在pH中性土壤中综合效果不佳。本研究提出在蔬菜生长时期用甲酸钙溶液强化根际土壤交换态Cd向碳酸盐结态Cd的转化,在基本不改变土壤pH的情况下,阻控蔬菜对pH中性土壤Cd的吸收。室内模拟实验表明,低浓度甲酸钙(1.3~3.9 g·L~(-1))可以促使土壤交换态Cd转化为水溶态Cd;24 h内甲酸根生物降解后,土壤碱度有所提高,但pH变化不大,利于土壤交换态Cd转化为水溶态Cd后继续转化为生物有效性相对更低的碳酸盐结合态Cd。采用全Cd含量为1.03 mg·kg~(-1)、pH为7.3的棕壤开展油菜盆栽实验,结果显示,在油菜生长中后期2次以灌溉形式按76、152或228 mg·kg~(-1)土的剂量添加甲酸钙后,收获时油菜Cd含量分别比对照组减少31.6%、32.7%和39.4%,但不同处理土壤pH变化很小;在第二茬盆栽实验中,不同处理油菜Cd含量差异消失。甲酸钙阻控当季蔬菜吸收土壤Cd的作用主要归因于其对土壤交换态Cd向碳酸盐结合态Cd转化的促进作用,但土壤交换性阳离子组成与含量、铁锰氧化物和有机质等因素对甲酸钙阻控效果的影响还有待进一步研究。     

蕹菜不同镉积累品种的根际土壤化学特征

采用根箱试验研究蕹菜低Cd品种(QLQ)和高Cd品种(T308)的根际土壤化学特征.结果表明:QLQ根际DTPA-Cd含量显著高于T308(P0.05).QLQ根际pH、氧化还原电位(Eh)显著高于T308(P0.05),电导率(EC)的表现相反.QLQ根际有机质和水溶性有机质(DOM)含量均高于T308.在污染土壤上,根际低分子量有机酸的组成与含量均存在品种间差异.T308根际检测到乙酸、丙酸、柠檬酸和延胡索酸,而QLQ根际仅检测到柠檬酸和延胡索酸.QLQ根际低分子量有机酸总量(1.93 nmol·g-1DM)极显著低于T308(15.11 nmol·g-1DM,P0.01).与T308比较,QLQ表现出明显不同的根际土壤化学特征.推测QLQ低Cd积累的根际化学机理可能在于:较高含量的有机质、较低含量的低分子量有机酸及其特异性组成,以及较低的酸化和还原能力,导致其对土壤Cd的活化能力较低,从而降低植株对Cd的吸收积累.     

豫南夏花生种植区土壤pH时空演变及其驱动力

研究花生种植区土壤酸化特征及其驱动因素,为花生种植土壤酸化改良提供理论依据。采用数据收集和田间取样分析方法,获取豫南典型花生种植区正阳县土壤pH及其驱动因子数据,利用GIS技术分析土壤pH时空变化特征,并基于增强回归树(BRT)模型识别影响土壤酸化的关键驱动因素。结果表明：正阳县花生种植土壤pH普遍呈下降趋势;土壤pH平均值由1980年的7.06降至2020年的5.15;弱碱性与中性土壤面积大幅度下降,酸性土壤面积增加;1980—2020年土壤pH面积变化最大的区间为-2.0～-1.5,占全县面积的51.9%;砂姜黑土区土壤酸化最严重,而水稻土和潮土区土壤pH下降幅度较小;BRT模型分析表明,阳离子交换量是造成2020年土壤pH空间变异的最主要因素,其贡献率达到26.2%;其次是速效钾含量、年均降雨量、有机质含量和全氮含量,贡献率分别为11.0%、7.7%、7.2%和6.9%;土壤pH随土壤阳离子交换量、年均降雨量、速效钾含量、有机质含量的增加而增加,但随土壤全氮含量的增加而降低。综上,正阳县夏花生种植土壤酸化形势严峻,尤其是砂姜黑土区,具有普遍性和局部严重性的特征;所研究因素中,阳离...     

长期种植蔬菜后土壤中氮、磷有效养分和重金属含量变化

利用区域采样调查的方法,研究了长沙市郊3类不同种植年限菜地土壤有效态氮、磷和重金属的累积差异,并对其累积风险和可能来源作了初步分析.结果表明:随着种植年限的增加,长沙市郊菜地土壤硝态氮(NO3--N)、有效磷(Olsen-P)和重金属累积量均显著增加,蔬菜种植1～2年(宁乡县)、10～15年(长沙县)和30年(开福区)后土壤NO3 - -N平均含量分别达21.1、42.0和49.5 mg·kg-1;Olsen-P平均含量分别达31.9、146.9和219.9 mg·kg-1;重金属以Cd累积最明显,平均含量分别达0.33、0.52和1.40 mg·kg-1.重金属按累积指数排序为:Cd＞＞Cu＞Pb＞Ni＞Zn.主成分分析和聚类分析表明,速效钾、pH、有机质、铵态氮等主要由自然因素决定,而Olsen-P和NO3-N等与重金属具有相似的累积特征,主要受人为施肥影响.长沙市郊菜地土壤环境和健康质量不容乐观,种植年限越长,氮、磷和重金属累积量越高,菜地土壤重金属与氮、磷来源相似,可能与菜地长期施肥活动有关.     

贵州喀斯特地区辣椒镉的累积特性及土壤风险阈值研究

辣椒(Capsicum annuum L.)是我国西南喀斯特重金属地质高背景区的特色农作物, 属茄科, 具有一定镉(Cadmium)富集能力。为明确贵州主产区辣椒Cd的富集状况, 探究辣椒质量安全的土壤风险阈值, 保障农产品安全生产。采集贵州省9个县区辣椒及土壤样品105组, 利用ICP-MS检测Cd含量, 研究土壤Cd分布, 辣椒富集系数, 并根据土壤pH建立回归模型推导基于辣椒质量安全的土壤Cd风险阈值。结果表明, 有4个研究区土壤均值超过贵州省土壤Cd背景值(0.659 mg·kg-1), 超出率分别是: WS(25.8%)、HZ(42.2%)、LD(146%)、WN(439%); 18.1%的辣椒超过国家食品安全Cd限值标准(GB2762—2017, 0.05 mg·kg-1), 点位超标率为LD(20.8%)、TZ(7.69%)和WN(100%); 辣椒Cd的生物富集系数(BCF)范围是0.019至0.108, 均值0.046。由回归模型推算出酸性土(pH<6.5)、中性土(6.5≤pH≤7.5)、石灰性土(pH>7.5)中Cd风险阈值分别为1.00、1.26和2.50 mg·kg-1, 分别为国家农用地土壤污染风险筛选值(GB15618—2018)的3.33、4.21、4.17倍, 阈值准确率达91.1%; 有效态Cd风险阈值为0.071、0.017和0.005 mg·kg-1, 阈值准确率达86.1%。地质高背景区农田土壤Cd超标严重, 但辣椒的生物富集系数相对较低, 基于辣椒质量安全的土壤Cd风险阈值远高于国家标准。     

华西雨屏区植被恢复对紫色土酸化的影响

退耕还林是控制水土流失、恢复森林植被和改善生态环境的重要举措。植被恢复进程中会发生土壤酸化,进而影响植物生长,厘清不同还林树种对土壤酸化的影响程度及作用机制有助于更好地评估退耕还林工程的整体生态效益。以华西雨屏区紫色土坡耕地(玉米)为对照,分析了退耕20a后柳杉纯林、柳杉-光皮桦混交林、慈竹林和茶园土壤pH的变化及其垂直分异。结果表明：玉米地、慈竹林、茶园、混交林和柳杉纯林0—50 cm土壤pH平均值分别为5.66、5.55、5.12、5.03和5.00,相较于玉米地,柳杉纯林、混交林和茶园土壤pH值显著下降(P<0.01),土壤酸化严重。相较于玉米地土壤pH值的均匀分布,植被恢复不同类型人工林pH值随土壤深度的增加而增加,10—50 cm土壤pH值(4.89—5.90)显著高于0—10 cm土壤pH值(4.72—5.21)(P<0.01),表层土壤酸化最明显。土壤pH值与土壤有机质含量呈极显著负相关,而与风化指数Na/K比值呈显著正相关,有机质含量和Na/K比值共同解释了土壤pH值变化的53.9%,表明植被恢复引起的土壤有机质积累与长石类矿物风化是驱动紫色土酸化的重要过程...     

湘南某矿区蔬菜中Pb、Cd污染状况及健康风险评估

通过采集湘南某矿区周边农田中种植的蔬菜和对应耕作层土壤样品,探究了农田蔬菜Pb、Cd的污染状况,蔬菜和土壤重金属含量之间的相关性,蔬菜中重金属对人体的健康风险。结果表明:(1)污染区土壤重金属污染非常严重,叶菜类蔬菜重金属超出了食品卫生标准限值,但是果菜类蔬菜中重金属没有超出食品卫生标准限值。(2)不同蔬菜不同器官所含Pb、Cd有明显差异;不同种类蔬菜Pb、Cd含量不同,一般是叶菜类果菜类;同种蔬菜不同部位重金属含量的顺序也不同,一般为根茎叶果,或根叶茎果。(3)二类蔬菜中重金属Pb、Cd的富集系数顺序为叶菜类果菜类。(4)蔬菜中Pb、Cd含量与土壤中相对应元素含量均无显著相关性。(5)就农产品安全性而言,果菜类蔬菜比叶菜类蔬菜更适合在此矿区栽培。     

镉胁迫下深色有隔内生真菌对玉米生长和根际土壤养分的影响

为探究重金属胁迫下深色有隔内生真菌(Dark septate endophytes, DSE)对植物和土壤微环境的影响,设置3个水平土壤Cd质量浓度(0,5,10 mg/kg),选择6种DSE菌种(链格孢菌Alternaria terricola、尖镰孢菌Fusarium acuminatum、穗状弯孢菌Curvularia spicifera、高粱细基格孢菌Ulocladium sorghi、丛赤壳菌Neonectria radicicola和根异茎点霉菌Phoma radicina)进行玉米回接试验,以阐明Cd胁迫下DSE真菌对玉米生长、光合作用、抗氧化性,以及土壤pH、养分及酶活性的重要意义。结果表明:Cd胁迫下,6种DSE菌种均能定殖玉米根系。双因素方差分析结果表明,DSE与Cd胁迫及其交互作用显著影响玉米地径、叶绿素含量、丙二醛(MDA)、过氧化物酶(POD)、土壤pH和养分及酶活性(P0.05)。当植物未受到Cd污染时,菌株U. sorghi(US)显著增加地下干质量和土壤有机质含量,降低MDA的积累和土壤速效磷含量。在5,10 mg/kg Cd质量浓度下,A. terricola(AT)显著增加玉米叶片数和地径;菌株P. radicina(PR)和N. radicicola(NR)显著提高植物地上干质量;AT、C. spicifera(CS)、F. acuminatum(FA)和US显著提高叶片中叶绿素含量和降低MDA在植物中的积累;AT降低了土壤速效磷含量;US显著提高土壤有机质含量并对土壤酸性磷酸酶和脲酶活性具有抑制作用(P0.05)。Pearson相关性分析表明:土壤pH与地径和叶绿素含量显著正相关,有机质与植物谷胱甘肽显著负相关,速效磷与叶片数显著负相关(P0.05)。方差分解表明:DSE菌种、Cd浓度和土壤因子共解释玉米生长和生理指标的42.1%和14.5%,其中DSE单独解释了27.4%和8.4%。随着Cd浓度升高,DSE显著促进植物生长,增加植物抵抗重金属能力,提高土壤养分可利用性,证明了在土壤Cd胁迫下DSE对植物生长和抗逆性有重要影响。     

济南地区日光温室土壤养分的分布状况和累积规律

对济南地区不同种植年限日光温室内外及不同蔬菜作物的土壤养分差异和频数分布进行比较,分析了日光温室土壤养分随种植年限的累积特征,并对不同蔬菜温室土壤养分与种植年限进行曲线拟合,研究了引起土壤盐渍化和酸化的原因.结果表明:济南地区日光温室土壤碱解氮、速效磷、速效钾、有机质、电导率显著高于棚外土壤,增幅分别为135.3％、475.2％、290.1％、97.7％、188.7％,pH值较棚外土壤降低0.31;各土壤养分的频数分布曲线均呈正态分布.不同蔬菜温室土壤养分含量各有差别,其中,碱解氮和电导率为番茄＞甜椒＞黄瓜,有机质和pH为黄瓜＞甜椒＞番茄,速效磷为黄瓜＞番茄＞甜椒,速效钾为番茄＞黄瓜＞甜椒.日光温室土壤有酸化的趋势,但不明显.温室土壤盐渍化程度加重主要由碱解氮和速效钾的累积所致.pH的降低与碱解氮的累积关系密切.日光温室各土壤养分随种植年限的累积规律基本一致,1～2年为养分的快速累积期,3～4年的累积速率变缓,以后处于一个稳定水平,整体上表现为土壤系统的动态平衡.除pH的累积为负向外,其他均为正向累积.不同蔬菜温室土壤养分与种植年限呈极显著相关,且可用2次或3次曲线进行拟合.     

利用富镉基质栽培快速比较不同叶菜镉累积能力的差异

利用穴盘育苗技术,通过设置不同基质镉浓度水平和育苗时间,获得不同叶菜对镉累积能力数据,并与田间条件下相关结果进行比较、验证,探讨通过富镉基质栽培比较叶菜镉累积能力的可行性。结果表明: 供试叶菜品种地上部镉含量和生物富集系数(BCFs)值随基质镉浓度和培养时间变化显著。多因素方差分析表明,叶菜品种、育苗天数和基质镉含量以及因素间交互作用对叶菜BCFs均有显著影响,而叶菜品种对总变差的贡献最多,是叶菜BCFs值差异的主导因素,控制着叶菜BCFs值的绝对水平。育苗10 d且基质镉含量1.0 mg·kg^-1条件下,穴盘基质栽培和田间试验中叶菜BCFs的相关系数最大(R²=0.90),且该条件下的聚类分析以及单因素方差分析结果也与田间结果最为吻合,试验结果具有较好的稳定性。表明育苗10 d且基质镉含量1.0 mg·kg^-1条件下富镉基质栽培得到的结果能够相对真实地反映不同叶类蔬菜田间种植时的镉累积能力,可用于今后镉低累积型蔬菜的快速筛选。     

Cadmium Pollution and its Transfer in Agricultural Systems in the Suburbs of Tianjin,China

Cadmium (Cd) pollution in the suburbs of Tianjin, China, was monitored over a period of several years. During this period, 193 soil, 120 vegetable, and 71 rice samples from four suburban districts were analyzed. The Cd content of the soil samples was considerably higher than the background level. Some soil samples from the Dongli and Xiqing districts were even in the S5 (Heavy) and S6 (Severe) pollution levels, with a maximum concentration of 5.20 mg kg^?1. The Cd contents of vegetable and rice samples were found to be in the F3 (Intermediate) and F4 (Heavy) pollution levels. Vegetables showed differing abilities to accumulate Cd and can be ranked in the following order: leaf vegetables > rootstalk vegetables > fruit vegetables. The average Cd level in different vegetables ranged from 0.0064 to 0.132 mg kg^?1, and a positive relationship (p < 0.05) was found between soil and plant Cd contents. Chinese cabbage and celery were more likely to pose risks to human health, as shown by their higher hazardous quotient (HQ) values. The soil Cd pollution in Tianjin seemed primarily to be caused by wastewater irrigation and the application of sewage sludge to farmlands. Other possible sources include the use of phosphate fertilizer and atmospheric deposition.     

Derivation of Critical Soil Cadmium Concentrations for the State of São Paulo,Brazil, Based on Human Health Risks

The main cadmium exposure pathway for humans is through diet. A database on Cd concentration in soils and accumulation in edible vegetables from tropical and temperate regions was organized, soil–plant relationships were derived, and then critical soil Cd concentrations were calculated based on human exposure parameters for the State of São Paulo, Brazil. Cadmium accumulation in leafy and root vegetables could be predicted by multiple regression analysis and most of the variance was explained when total Cd concentration and pH in soil were included as predictors. The calculated Cd bioconcentration factors (BCFs) for the tropical dataset were higher as compared to the temperate dataset. Consequently, critical soil Cd concentrations were from 1.7- to 3.2-fold lower for tropical conditions. Higher humidity and temperature at the tropics, as well as more weathered soils with lower retention capacity of Cd, may explain the higher Cd uptake and accumulation in tropical than in temperate regions. To protect human health, exclusive data regarding Cd in soils and edible vegetables from tropical regions should be used for the State of São Paulo to derive critical soil Cd concentrations, instead of (additional) data from temperate regions.     

Phytoavailability of Cadmium (Cd) to Pak Choi (Brassica chinensis L.) Grown in Chinese Soils: A Model to Evaluate the Impact of Soil Cd Pollution on Potential Dietary Toxicity

Food chain contamination by soil cadmium (Cd) through vegetable consumption poses a threat to human health. Therefore, an understanding is needed on the relationship between the phytoavailability of Cd in soils and its uptake in edible tissues of vegetables. The purpose of this study was to establish soil Cd thresholds of representative Chinese soils based on dietary toxicity to humans and develop a model to evaluate the phytoavailability of Cd to Pak choi (Brassica chinensis L.) based on soil properties. Mehlich-3 extractable Cd thresholds were more suitable for Stagnic Anthrosols, Calcareous, Ustic Cambosols, Typic Haplustalfs, Udic Ferrisols and Periudic Argosols with values of 0.30, 0.25, 0.18, 0.16, 0.15 and 0.03 mg kg⁻¹, respectively, while total Cd is adequate threshold for Mollisols with a value of 0.86 mg kg⁻¹. A stepwise regression model indicated that Cd phytoavailability to Pak choi was significantly influenced by soil pH, organic matter, total Zinc and Cd concentrations in soil. Therefore, since Cd accumulation in Pak choi varied with soil characteristics, they should be considered while assessing the environmental quality of soils to ensure the hygienically safe food production.     

Heavy metal phyto-accumulation in leafy vegetables irrigated with municipal wastewater and human health risk repercussions

In this study, heavy metal phyto-accumulation potential of selected cultivars of two leafy vegetables on irrigation with municipal wastewater and human health risks were investigated. Municipal wastewater chemistry was recorded significantly different from groundwater control and led to the two-fold high enrichment of soil heavy metal contents (Ni, 19.46; Pb, 23.94; Co, 4.68; Cd, 1.4 in mg/kg, respectively). Interactive effects for phyto-accumulation of most heavy metals were also recorded significant at p?相似文献   

Health Risk Assessment of Heavy Metals in Soils and Vegetables from a Typical Greenhouse Vegetable Production System in China

There is growing concern about food safety and environmental contamination due to potential health risks to consumers. The aims of this study are to elucidate the accumulation status, influencing factors, and potential risk of selected heavy metals in soils and vegetables from a typical greenhouse vegetable production system in China. The potential health risks of heavy metals through soil contact and vegetable consumption were evaluated for greenhouse and corresponding open field production. The results indicated that the mean concentrations of Hg, Pb, Cu, and Zn in greenhouse vegetable soils were greater than those in open field soils. Leaf vegetables had relatively higher concentrations and transfer factors of heavy metals than root and fruit vegetables, especially for Cd. The target hazard quotient of heavy metals through vegetable consumption was greater than 1 for leaf vegetables, and was also higher for greenhouse vegetables than those from open field. The risk of exposure to heavy metals through direct contact with soil and vegetable consumption was higher for children than for adults and seniors. Planting patterns with different vegetable types and soil properties can affect bioavailability and crop accumulation of heavy metals.     

Transfer of Cadmium from Soil to Vegetable in the Pearl River Delta area,South China

The purpose of this study was to investigate the regional Cadmium (Cd) concentration levels in soils and in leaf vegetables across the Pearl River Delta (PRD) area; and reveal the transfer characteristics of Cadmium (Cd) from soils to leaf vegetable species on a regional scale. 170 paired vegetables and corresponding surface soil samples in the study area were collected for calculating the transfer factors of Cadmium (Cd) from soils to vegetables. This investigation revealed that in the study area Cd concentration in soils was lower (mean value 0.158 mg kg⁻¹) compared with other countries or regions. The Cd-contaminated areas are mainly located in west areas of the Pearl River Delta. Cd concentrations in all vegetables were lower than the national standard of Safe vegetables (0.2 mg kg⁻¹). 88% of vegetable samples met the standard of No-Polluted vegetables (0.05 mg kg⁻¹). The Cd concentration in vegetables was mainly influenced by the interactions of total Cd concentration in soils, soil pH and vegetable species. The fit lines of soil-to-plant transfer factors and total Cd concentration in soils for various vegetable species were best described by the exponential equation (), and these fit lines can be divided into two parts, including the sharply decrease part with a large error range, and the slowly decrease part with a low error range, according to the gradual increasing of total Cd concentrations in soils.     

Effects of Sludge and pH Adjustment on Cd Speciation in Soil and Growth and Cd Accumulation in Pak Choi

The objective of this study was to assess the effects of soil pH adjustment and treatment with sludge on plant growth and cadmium (Cd) accumulation in Chinese cabbage (pak choi) grown in Cd-contaminated soils. Soil was artificially spiked to raise its final Cd concentration to 10 mg/kg, adjusted to different pH values, and then supplemented with 5% sludge: either biosolids (BS) from wastewater treatment or sludge from drinking water treatment (WTS). Pak choi seeds were sown and the seedlings potted in the treated soil, then grown in a greenhouse for 45 days. The added Cd was present mainly in the exchangeable fraction and its concentration correlated to Cd accumulation in the roots and shoots. Adjustment of pH and application of sludge affected the Cd concentration in different fractions. Addition of WTS and adjustment to pH 7.0 inhibited growth and Cd accumulation in the pak choi. Addition of BS increased the soil nutrient content and thus enhanced growth and Cd accumulation. If pak choi were the only form of vegetable intake, consumption of pak choi grown in the Cd-spiked soils used in this study would exceed the maximum acceptable intake (60 μg/day) proposed by the World Health Organization.     

Effects of Soil Type and Genotype on Cadmium Accumulation by Rootstalk Crops: Implications for Phytomanagement

The variations of Cd accumulation in three rootstalk crop species (radish, carrot and potato) were investigated by using twelve cultivars grown in acidic Ferralsols and neutral Cambisols under two Cd treatments (0.3 and 0.6 mg kg^?1) in a pot experiment. The result showed that the total Cd uptake was significantly affected by genotype, soil type and interaction between them, suggesting the importance of selecting proper cultivars for phytoextraction in a given soil type. Among the cultivars tested, potato cultivar Luyin No.1 in Ferralsols and radish cultivar Zhedachang in Cambisols exhibited the highest Cd phytoextraction efficiency in aerial parts (4.45% and 0.59%, respectively) under 0.6 mg kg^?1Cd treatment. Furthermore, the Cd concentrations in their edible parts were below the National Food Hygiene Standard of China (0.1 mg kg^?1, fresh weight). Therefore, phytomanagement of slightly Cd-contaminated soils using rootstalk crops for safe food production combined with long-term phytoextraction was feasible, and potato cultivar Luyin No.1 for Ferralsols and radish cultivar Zhedachang for Cambisols were promising candidates for this approach.