立式环模秸秆压块成型机作业参数优化

针对秸秆压块成型燃料生产过程中,由于作业参数不当造成的生产率低、成型燃料质量低、能耗高等问题,以玉米秸秆为原料,通过正交试验,研究原料的粒度、含水率及压块机模辊间隙3种工艺参数的不同组合对生产率、压块燃料成型率、密度、机械耐久性及吨产品能耗的影响。结果表明：从提高成型燃料质量方面来考虑,最佳工艺参数组合为原料粒度为10～20 mm、含水率为15%、模辊间隙为2 mm,从提高生产率和降低吨产品能耗方面来考虑,最佳工艺参数组合为原料粒度为10～20 mm、含水率为20%、模辊间隙为2 mm。在实际使用中,根据压块燃料成型率、密度和机械耐久性的质量要求,在能达到使用要求的情况下,可在含水率10%～20%的范围内,适当提高含水率,有利于提高生产率和降低吨产品能耗。     

环模式成型机压缩水稻秆成型工艺参数优化

为了确定环模式成型机压缩水稻秸秆最佳的成型工艺参数,该文以水稻秸秆为原料,利用9JYK-2000A型环模式成型机进行压缩成型,寻求工作参数对环模式成型机压缩水稻秸秆成型影响规律和优化工艺参数组合。采用四因素五水平二次回归正交旋转中心组合设计试验方法,以含水率、成型温度、模辊间隙和主轴转速为影响因子,以成型压块的松弛密度和抗破碎性为评价指标。利用Design-expert8.0.6的回归分析法及响应面分析法,建立并分析了4个因子对评价指标影响的数学模型。结果表明：当参数组合含水率为17.5%～27.1%、成型温度为81.9℃～88.1℃、磨辊间隙为2.49～3.78 mm、主轴转速为157.6～186.5 r/min条件下,成型压块的松弛密度大于1.0 g/cm3,成型压块的抗破碎性大于65%;各因素对松弛密度贡献率的主次关系为：主轴转速>磨辊间隙>含水率>成型温度,各因素对抗破碎性贡献率的主次关系为：含水率>主轴转速>磨辊间隙>成型温度。研究可为环模式成型机压缩水稻秸秆成型提供一定理论依据和技术支撑。     

柱塞式平模生物质成型机设计与试验

针对传统生物质平模成型机能耗高、磨损严重的问题,该文提出了一种柱塞式平模生物质致密成型方法,并设计了成型机样机。该样机通过斜盘与弹簧配合,驱动柱塞往复运动,柱塞在运动过程中仅对模具内的原料进行挤压。成型模具部分采用组合式设计,主要由平模与成型套筒组成,可在成型套筒外部加热,同时可更换长径比不同的成型套筒以适应不同生物质原料。该文以不同颗粒度玉米秸秆和刺槐为原料,在原料含水率为15%,斜盘转速为50 r/min,原料温度为室温的条件下进行了试验,结果表明柱塞式平模生物质成型方法可行,颗粒成型率均高于98%,成型密度大于1.1 g/cm3,机械耐久性高于97%。该文提出的成型方法为生物质致密成型技术提供参考。     

基于响应面法的玉米秸秆成型工艺优化

为了研究玉米秸秆成型过程中各参数之间的交互作用,获得最佳的工艺参数,该文采用5因素的响应面试验设计研究了原料水分(8%~24%)、温度(50~150℃)、压缩速度(10~50 mm/min)、压力(51.0~127.4 MPa)、保压时间(10~50 s)5个成型参数对玉米秸秆成型颗粒的松弛密度、Meyer强度以及压缩比能耗3个成型技术指标的影响,建立了响应面模型,结合成型燃料标准,获得了最佳的工艺参数,并对优化后的试验参数进行了试验验证。试验结果表明:在选取试验参数范围内,温度、原料水分、压力均会对技术指标产生较大影响;而压缩速度和保压时间所产生的影响相对较小。最优化的工艺参数(压力、温度、水分)为:4 k N(51.0 MPa)、110.8℃、17%,在该参数组合下的验证试验结果为:松弛密度为1031 kg/m~3,Meyer强度为27.1 N/m~2,比能耗为10.03 k J/kg。该研究可为秸秆生物质成型燃料制备产业提供参考。     

水稻秸秆与木屑混合原料热压成型试验

为提高水稻秸秆原料成型燃料的质量,提出通过添加木质素含量较高的木屑原料即形成混合原料并通过试验的方法进行研究。首先进行对比试验,以成型燃料的物理性能为指标,在相同实验条件下对木屑、水稻秸秆和水稻秸秆与木屑1∶1混合的原料进行热压成型试验。对比试验的结果表明小压力下1∶1混合原料成型燃料的综合物理性能优于水稻秸秆成型燃料,证明混合原料可提高单一水稻秸秆原理成型燃料的质量。但由于木屑原料本身的结构特性使得1∶1混合原料成型燃料的抗渗水性较弱。因此为进一步提高混合原料成型燃料质量,我们利用正交试验方法研究对于混合原料成型燃料各物性指标而言最显著影响因素及最佳参数组合并通过试验加以论证。试验结果表明:针对不同性能指标,各工艺参数的影响力不一致,对于松弛密度、抗碎强度及抗压强度而言,压力对其影响较大;而对于抗渗水性而言,原料混合比影响较大。对于松弛密度,最佳成型参数(混合比、温度、压力)组合为0.5∶1,70℃、31.11 MPa,抗碎强度的成型参数最优组合为2∶1,110℃、31.11 MPa,抗渗水性和抗压强度的最佳成型参数组合为1.5∶1,90℃、31.11 MPa。     

秸秆厌氧发酵改性制备成型燃料试验

针对秸秆压缩成型存在设备磨损严重和厌氧发酵面临的抗生物降解屏障,提出了一种秸秆厌氧发酵改性制备成型燃料的思路并进行了试验研究。以玉米秸秆为原料,在总固体质量分数12%和中温38℃下对原料分别进行为期10和20 d的厌氧发酵,发酵后的沼渣经粉碎和干燥后于100℃、6 MPa条件下压缩成直径10 mm的颗粒燃料。试验结果表明：经过10和20 d的发酵,玉米秸秆所含能量仅有14.49%和32.01%转移到沼气中;沼渣弹筒发热量分别达到18.01和18.05 MJ/kg,高于原料的17.02 MJ/kg;沼渣挥发物质质量分数为74.08%和72.63%,比原料的81.02%分别下降了8.6%和10.4%;沼渣木质素质量分数为16.98%和17.92%,比原料的13.65%分别增加了24.4%和31.3%;沼渣苯醇抽提物质量分数为5.66%和4.86%,比原料的7.52%分别降低了24.7%和35.4%。成型试验结果表明：沼渣制成的成型燃料松弛密度分别为1.041 g/cm3和1.150 g/cm3,比未发酵秸秆的1.019 g/cm3分别提高了2.2%和12.86%。厌氧发酵改性有助于提高秸秆成型性能、制粒达到相同松弛密度所需的压力较小,有助于降低成型过程设备磨损,且基于上述思路形成的气固二元燃料生产工艺,使秸秆的转化利用不再受制于抗降解屏障,值得深入研究。     

含水率对草坪草常温开模成型的影响

为研究含水率对草坪草常温成型的影响规律,对草坪草修剪剩余物进行了不同含水率不同锥度模具的成型试验。结果在同一锥度模具下,随着含水率的增加,成型块密度都是由小变大再变小;含水率不变的情况下,随模具锥度增大长度减小成型块密度降低;在原料含水率11%、模具锥度25°时,成型块密度达到最大值1.10 g/cm3;原料含水率在20%及以上时,挤出的压块密度都在0.9 g/cm3以下,成型效果差或不成型。研究表明：随着生物质原料含水率增加,常温成型块密度和压缩力的变化趋势一致,都是由小变大再变小。     

秸秆压块燃料炉设计与试验

农村地区采用秸秆压块燃料供暖可有效降低温室气体和大气污染物的排放。针对秸秆压块燃料炉存在自动化程度低、低效、高排放等问题,该研究基于秸秆压块燃料燃烧特性,设计了一台具有自动供料装置的秸秆压块燃料炉,采用分区燃烧、鼓风与引风耦合配风等方式,提高燃烧性能。对秸秆压块燃料炉进行性能测试,试验结果表明：通过自动供料机可以实现连续10 h小流量输送燃料,燃料输送量为2.57～4.12 kg/h;其额定热功率8 kW,热效率71.20%,耗电量3.83 (kW·h)/d;最佳燃烧工况下,粉尘、CO、NOx排放量分别为35.78 mg/m3、0.104 %、191.1 mg/m3,符合国家标准的排放限值。对燃料炉的测试与评价验证了锅炉结构合理,可为后续高自动化秸秆压块燃料炉的研发提供参考依据。     

生物质颗粒燃料成型的黏弹性本构模型

为研究生物质颗粒成型燃料压缩成型机理,该文用玉米秸秆、花生壳、小麦秸秆、大豆秸秆、棉花秸秆、木屑等6种生物质原料,采用生物质颗粒燃料成型机进行压缩成型,研究生物质颗粒燃料压缩成型过程,采用黏弹性理论,建立生物质颗粒成型燃料的本构模型,从力学角度提出生物质颗粒成型燃料的压缩成型机理,并研究对比不同种类生物质原料压缩的最大应力与能耗.结果表明,6种生物质原料中棉杆和木屑的最大应力较高,其余4种原料略低;木屑的压缩能耗最高,其次为棉秆、花生壳和豆秸,小麦秸秆和玉米秸秆较小.该研究结论为解决生物质颗粒成型燃料成型加工能耗高,关键部件受力磨损导致寿命低等问题提供一定参考.     

水稻秸秆压块热值模型构建及其影响因子相关性分析

为建立水稻秸秆压块热值预测模型和高品质燃料压块挤压成型提供理论依据,该文以9JYK-2000A型环模式成型机生产的水稻秸秆压块作为试验样本,并以密度、含水率、挥发分为影响因素,以压块的低位热值和高位热值为试验指标进行试验研究。试验采用三因素五水平二次回归正交旋转中心组合设计试验方法,利用Design-Expert8.0.6软件回归分析法和响应面分析法,建立了3个因素对热值影响的数学模型,对所建立的数学模型进行了试验性验证,其误差范围小于5%。试验分析表明：各因素对低位热值的影响顺序依次是：密度＞含水率＞挥发分,对高位热值的影响顺序依次是：密度＞挥发分＞含水率;密度是1.39 g/cm3,含水率为13.68%,挥发份为0.76,可达最佳低位热值2849.45 kcal;最佳高位热值的模型指标为：密度为0.92 g/cm3,含水率为23.21%,挥发分为0.75,可达最佳高位热值4313.54 kcal。最佳综合热值模型的指标为：密度为1.18 g/cm3,含水率为11.78%,挥发分为0.78,水稻秸秆压块的低位热值为2830.48 kcal,高位热值为4488.05 kcal;密度为1.43 g/cm3,含水率为14.84%,挥发分为0.80,水稻秸秆压块的低位热值为2851.08 kcal,高位热值为4480.91 kcal。     

低氮和干旱胁迫对富士和秦冠生长及氮素利用的影响

【目的】以富士（Fuji）、 秦冠（Qinguan）嫁接在平邑甜茶（Malus hupehensis Rehd.）上的当年生盆栽苗为试验材料,采用砂培方法,研究了缺氮胁迫和干旱对富士和秦冠生长情况、 光合参数、 植株各部位氮磷钾含量及氮素利用效率的影响,分析比较了低氮干旱条件下富士和秦冠生长及氮素利用的差异,以期为果树生产高效肥水利用提供理论指导。【方法】试验共设四个处理: 正常氮正常水（ZZ）、 低氮正常水（DZ）、 正常氮干旱（ZG）、 低氮干旱（DG）。氮素和水分均设置两个水平,分别为正常氮（6 mmol/L NO-3-N）、 低氮（0.3 mmol/LNO-3-N）、 正常供水（保持盆中砂子相对含水量为饱和含水量的80%~85%）、 干旱处理（保持盆中砂子相对含水量为饱和含水量的60%~65%）。【结果】富士和秦冠的生物量（茎和叶）、 株高茎粗等生长指标以及光合速率、 气孔导度、 蒸腾速率均为正常氮正常水（ZZ）＞低氮正常水（DZ）＞正常氮干旱（ZG）＞低氮干旱（DG）,并且相对应处理下秦冠的以上指标均高于富士;正常供水下,缺氮处理使富士、 秦冠的根冠比比正常氮处理均有所增加,富士提高了2.05%,秦冠提高了22.40%。富士和秦冠的氮、 磷、 钾含量均表现出正常氮正常水（ZZ）＞低氮正常水（DZ）＞正常氮干旱（ZG）＞低氮干旱（DG）; 氮、 钾元素含量在植株各部位的分布顺序依次是叶＞根＞茎,磷元素则是根＞叶＞茎;光合氮素利用效率（PNUE）和氮素利用效率表现为秦冠处理之间差异极显著,富士处理之间差异不显著;秦冠的PNUE和NUE明显高于富士,在低氮正常水（DZ）处理下,秦冠氮肥利用率比富士高42.07%,在低氮干旱（DG）处理下高64.14%;低氮胁迫下富士和秦冠的NUE显著提高,并且秦冠提高的幅度高于富士。【结论】施用氮肥能够显著提高富士与秦冠的干物质量,同等水肥条件下,秦冠生长优于富士;水分亏缺会减少叶片对氮的吸收,干旱条件下适度增施氮肥,可提高果树的抗旱能力;低氮干旱胁迫下秦冠的生长指标、 光合指标及氮素利用效率指标均优于富士,表现出较强的抗低氮干旱胁迫的能力。     

Pasture Vegetation Elemental Analysis by Laser-Induced Breakdown Spectroscopy

Laser-induced breakdown spectroscopy (LIBS) is a new technique for the analysis of plant material. This study investigates the application of LIBS to pasture-based plant samples. The LIBS measurements were obtained from pelletized pasture samples (100 samples) that had been also analyzed by inductively coupled plasma–optical emission spectroscopy (ICP-OES) following microwave digestion for calibration and comparison purposes. Comparisons for elements sodium (Na), potassium (K), magnesium (Mg), calcium (Ca), manganese (Mn), iron (Fe), copper (Cu), zinc (Zn), boron (B), phosphorus (P), and sulfur (S) showed that LIBS could be used for almost all the standard profile total elements with concentrations down to low mg/kg levels (observed error of Na: 0.024 percent, K: 0.18 percent, Mg: 0.016 percent, Ca: 0.073 percent, P: 0.017 percent, Mn: 31 mg/kg, Fe: 150 mg/kg, Zn: 6.6 mg/kg, and B: 1.1 mg/kg). Elemental analysis at less than mg/kg levels was not possible using LIBS. The elements S and Cu were particularly difficult to analyze with reliability using LIBS at the concentration levels found in the plant samples. Replacing microwave digestion and subsequent ICP analysis with a direct analysis of dried plant samples using LIBS has the potential to improve the productivity and reduce the cost of testing.     

Faculty positions:Center for Agricultural Resources Research,Chinese Academy of Sciences

正The Center for Agricultural Resources Research(CARR),the Institute of Genetics and Developmental Biology(IGDB),Chinese Academy of Sciences,invites applicants for several research group leader positions.CARR is one of the research organizations in Chinese Academy of Sciences(CAS).We seek nominations and applications from individuals who have expertise and a record of accomplishment in research areas related to ecology,agro-hydrology,     

Macroinvertebrates in North American tallgrass prairie soils: effects of fire, mowing, and fertilization on density and biomass

The responses of tallgrass prairie plant communities and ecosystem processes to fire and grazing are well characterized. However, responses of invertebrate consumer groups, and particularly soil-dwelling organisms, to these disturbances are not well known. At Konza Prairie Biological Station, we sampled soil macroinvertebrates in 1994 and 1999 as part of a long-term experiment designed to examine the effects and interactions of annual fire, mowing, and fertilization (N and P) on prairie soil communities and processes. For nearly all taxa, in both years, responses were characterized by significant treatment interactions, but some general patterns were evident. Introduced European earthworms (Aporrectodea spp. and Octolasion spp.) were most abundant in plots where fire was excluded, and the proportion of the total earthworm community consisting of introduced earthworms was greater in unburned, unmowed, and fertilized plots. Nymphs of two Cicada genera were collected (Cicadetta spp. and Tibicen spp.). Cicadetta nymphs were more abundant in burned plots, but mowing reduced their abundance. Tibicen nymphs were collected almost exclusively from unburned plots. Treatment effects on herbivorous beetle larvae (Scarabaeidae, Elateridae, and Curculionidae) were variable, but nutrient additions (N or P) usually resulted in greater densities, whereas mowing usually resulted in lower densities. Our results suggest that departures from historical disturbance regimes (i.e. frequent fire and grazing) may render soils more susceptible to increased numbers of European earthworms, and that interactions between fire, aboveground biomass removal, and vegetation responses affect the structure and composition of invertebrate communities in tallgrass prairie soils.     

Variation in quality parameters between and within 14 Nordic tree fruit and berry species

Abstract

A 3-year study was carried out to investigate quality parameters in 14 tree fruit and berry species grown in southern Norway. The species were blueberry, apple, aronia, sour cherry, sweet cherry, red raspberry, strawberry, blackcurrant, gooseberry, red currant and elderberry, harvested along with wild bilberry, cloudberry and lingonberry. Significant differences between species were identified for all quality parameters. The coefficient of variation between species was lowest for pH (12.5%), dry matter (18.9%) and soluble solids (25.3%), followed by titratable acids (59.3%), total phenolics (83.8%), antioxidant capacity FRAP (85.7%) and antiradical power by the DPPH-assay (97.8%), total monomeric anthocyanins (132%) and ascorbic acid (137%). Average coefficient of variation within species were lower and ranged from 4 (pH) to 62% (ascorbic acid). Only the FRAP values were significantly affected by harvesting year with lower levels in 2004 than in 2005 and 2006. There were significant interactions between species and harvesting year for dry matter, soluble solids, pH, ascorbic acid and FRAP. The results indicate generic ranges in composition within species independent upon growing location and climate, and the composition of the tree fruits and berries is not likely to deviate from these ranges. It is concluded that desirable composition of tree fruits and berries and their products should primarily be achieved by selection among species rather than searching fors broadened variation within individual species.     

Potassium Fixation and Release Characteristics in Rhizosphere and Nonrhizosphere Soils for a Rapeseed–Rice Cropping Sequence

Potassium (K) fixation and release in soil are important factors in the long-term sustainability of a cropping system. Changes in K concentration and characteristics of K fixation and release in rhizosphere and nonrhizosphere soils in the rapeseed (Brassica napus L.)–rice (Oryza sativa L.) rotation were investigated using a rhizobox system. The concentrations of different forms of K in both rhizosphere and nonrhizosphere soils decreased with plants compared to without plants, regardless of K fertilizer application. Potassium uptake by crops mainly came from the rhizosphere soil. In the treatment without K fertilizer (–K), the main form of K supplied by the soil to the crops was 1.0 mol L^?1 nitric acid (HNO₃) nonextractable K, followed by nonexchangeable K, and then exchangeable K. In the treatment with K fertilizer (+K), the main K forms supplied by the soil to the crops were exchangeable K and nonexchangeable K. The amount and rate of K fixation after one cycle of the rapeseed–rice rotation was greater in rhizosphere soil than in nonrhizosphere soil. The amount and rate of K fixation of soil in the +K treatment were significantly less than in the –K treatment. The cumulative amounts of K released with 1.0 mol L^?1 ammonium acetate (NH₄OAc) and 1.0 mol L^?1 HNO₃ extraction increased with the increasing numbers of extractions, but the K-releasing power of soil by successive extraction decreased gradually and finally became almost constant. The release of K was less in rhizosphere soil than in nonrhizosphere soil. The release of K in the +K treatment was similar to that in the –K treatment in rhizosphere soil, but the K release in nonrhizosphere soil was greater with the +K than the –K treatment. Overall, the information obtained in this study will be helpful in formulating more precise K fertilizer recommendations for certain soils.     

Electron donor and pH relationships for biologically enhanced dissolution of chlorinated solvent DNAPL in groundwater

Biologically enhanced dissolution offers a method to speed removal of chlorinated solvent dense non-aqueous-phase liquid (DNAPL) sources such as tetrachloroethene (PCE) and trichoroethene (TCE) from aquifers. Bioremediation is accomplished by adding an electron donor to the source zone where fermentation to intermediates leading to acetic acid and hydrogen results. The hydrogen and possibly acetic acid are used by dehalogenating bacteria to convert PCE and TCE to ethene and hydrochloric acid. Reductive dehalogenation is thus an acid forming process, and sufficient alkalinity must be present to maintain a near neutral pH. The bicarbonate alkalinity required to maintain pH above 6.5 is a function of the electron donor: 800 mg/L of bicarbonate alkalinity is sufficient to achieve about 1.2 mM TCE dechlorination with glucose, 1.7 mM with lactate, and a much higher 3.3 mM with formate. Laboratory studies indicate that in mixed culture, formate can be used as an electron donor for complete conversion to ethene, contrary to pure cultures studies indicating it cannot. Various strategies can be used to add electron donor to an aquifer for DNAPL dehalogenation while minimizing pH problems and excessive electron donor usage, including use of injection-extraction wells, dual recirculation wells, and nested injection-extraction wells.     

Biodiversity and ecogeography of wild Lactuca spp. in some European countries

Staff members of the Department of Botany of Palacký University in Olomouc and Gene Bank Department – Workplace Olomouc, Research Institute of Crop Production in Prague, Czech Republic, conducted an expedition in seven European countries (Austria, Czech Republic, France, Germany, Italy, the Netherlands, Switzerland) in August/September 1999 to collect wild Lactuca spp. germplasm and study its geographic distribution, ecology and biodiversity. During the mission, more than 600 locations were visited resulting in the collection of 602 seed samples (accessions) of wild Lactuca species and 13 seed samples of related genera (Chondrilla and Mycelis). Lactuca serriola f. serriola, L. serriola f. integrifolia, L. saligna and L. viminea subsp. chondrilliflora were prevalent in southern Europe (Italy, France), however, only L. serriola was common in central and western Europe (Austria, Czech Republic, Germany, Netherlands, Switzerland). The greatest diversity of Lactuca species was found in France, where also the most seed samples (165) were collected. The most characteristic habitats with a high density of Lactuca spp. populations were observed along roads and highways, grassy ditches, ruderal communities, and dust-heaps. Natural infections by powdery mildew (Erysiphe cichoracearum) and downy mildew (Bremia lactucae) on some wild Lactuca spp. were observed. Recent observations concerning the geographic distribution, population structure, habitats, and natural occurrence of diseases of Lactuca spp. are discussed. This assemblage of genetic resources of Lactuca spp. can serve as the basis of future studies of species diversification, spatial population structure, plant microevolution, domestication processes, and genetic variability of host-parasite interactions.     

Increase in auxiliary photoprotective photosynthetic pigments in wheat seedlings induced by Azospirillum brasilense

Inoculation of wheat seedlings with the plant growth-promoting bacterium Azospirillum brasilense Cd was immobilized in alginate microbeads and, without applying any stress, significantly increased the quantity of several photosynthetic pigments, such as chlorophyll a, chlorophyll b, and the auxiliary photoprotective pigments violaxanthin, zeaxanthin, antheroxanthin, lutein, neoxanthin, and β-carotene. This resulted in greener plants with no apparent visible stress. After monitoring the quantity of photosynthetic pigments for 4 weeks, we observed that inoculated plants had higher quantities of pigments in shoot and stem. The greatest difference in the quantity of all pigments between inoculated and noninoculated plants occurred in the first week of growth. Regardless of treatment, the quantity of pigments in stems was three to four times less than the quantity of these pigments in shoots. Application of Azospirillum, either as liquid inoculant or as alginate microbeads, did not alter the positive effect of the bacteria on pigment production or the positive response of the plants towards A. brasilense Cd inoculation.     

Responses of native legume desert trees used for reforestation in the Sonoran Desert to plant growth-promoting microorganisms in screen house

Three slow-growing legume trees used for desert reforestation and urban gardening in the Sonoran Desert of Northwestern Mexico and the Southwestern USA were evaluated whether their growth can be promoted by inoculation with plant growth-promoting bacteria (Azospirillum brasilense and Bacillus pumilus), unidentified arbuscular mycorrhizal (AM) fungi (mainly Glomus sp.), and supplementation with common compost under regular screenhouse cultivation common to these trees in nurseries. Mesquite amargo (Prosopis articulata) and yellow palo verde (Parkinsonia microphylla) had different positive responses to several of the parameters tested while blue palo verde (Parkinsonia florida) did not respond. Survival of all tree species was over 80% and survival of mesquite was almost 100% after 10 months of cultivation. Inoculation with growth-promoting microorganisms induced significant effects on the leaf gas exchange of these trees, measured as transpiration and diffusive resistance, when these trees were cultivated without water restrictions.