The ability of indigenous micro-organisms to degrade isoproturon, atrazine and mecoprop within aerobic UK aquifer systems

The potential for the herbicides isoproturon, atrazine and mecoprop to degrade in the major UK aquifers of chalk, sandstone and limestone was studied using laboratory microcosms spiked at 100 microg litre(-1). Significant mecoprop degradation was only observed in sandstone groundwater samples. Atrazine transformation, based on the formation of metabolites, did occur in most groundwater samples, but only at a rate of 1-3% per year. A potential to degrade isoproturon was observed in groundwater samples from each of the aquifer types, with the most rapid and consistent degradation occurring at the sandstone field site. Biodegradation was confirmed by the formation of monodesmethyl- and didesmethyl-isoproturon. Isoproturon degradation potential rates obtained from the groundwater microcosms could not be correlated with either dissolved organic carbon or numbers of bacteria in the groundwater. It was noted that the ability of the groundwater at a field site to degrade a pesticide was not related to performance of the soil above.     

鄂尔多斯苏里格南区块浅层地下水水化学分布特征及污染源分析

利用2008年8月所采集的29件浅层地下水样品的水质资料分析了鄂尔多斯盆地苏里格南区块浅层地下水的水化学空间分布特征及地下水污染源,结果表明:苏里格南区块浅层地下水中总溶解固体(TDS)含量较低,且基本上未受到有机物污染,但存在大肠菌群、亚硝酸盐氮、氨氮、硝酸盐氮以及金属锰、铁超标的现象;浅层地下水中大肠菌群的主要污染源为生活污水、生活垃圾及人蓄粪便,而地下水中"三氮"的污染源主要为农药化肥及人蓄粪便;浅层地下水中铁、锰超标的主要原因是该地区土壤中铁、锰的本底值较高。     

Impact of maize mucilage on atrazine mineralization and atzC abundance

Soil was amended with maize mucilage, a major rhizodeposit, to study its role on the number of culturable soil micro-organisms, the structure of the bacterial community, atrazine mineralization and atzC abundance. The maximal percentage of atrazine mineralization was lower for mucilage-amended than for water-amended soil. Total culturable soil bacteria and 16S rDNA copy number, measured by RT-PCR, presented similar values and were not significantly (P < 0.05) different among treatments. Mucilage applied at a rate of 70 microg C g(-1) dry soil day(-1) over two weeks did not modify the abundance of the total soil microflora. Global structure of soil bacterial communities revealed by RISA analysis was not modified by maize mucilage amendment. Abundance of atzC sequence was only augmented by mucilage addition at the beginning of the experiment. However, this increase was not sustainable in time, as atzC copy number increased in water-amended soil which, in turn, corresponded with the higher percentage of atrazine mineralization observed in this soil. Maize mucilage amendment alone contributed only to minor changes in the atrazine-degrading community in the studied soil.     

Herbicide loss following application to a railway

Railways have been identified as a potential source of herbicides detected in surface and groundwaters, but there are few data to support this theory. Two studies were undertaken to investigate the fate of herbicides applied to railway trackbeds: a pilot study in a section of a disused, but intact, cutting where runoff and throughflow were sampled from trenches adjacent to the treated area, and a larger scale study on 0.75 km of embankment where surface water from the drainage ditch at the base of the embankment and groundwater were sampled. In the pilot study, peak concentrations of atrazine, diuron and glyphosate (1280, 210 and 15 microg litre(-1) respectively) were detected 6days after treatment (DAT). Oxadiazon, oryzalin and isoxaben were not detected above their limits of quantification. Lower concentrations were detected 81 DAT (10 and 0.8 microg litre(-1) of atrazine and glyphosate respectively). In the larger scale study, herbicides were not detected, in either the surface water or groundwater, at concentrations above the limit of detection that could be attributed to application to the railway. Rainfall volume and depth to sampling point may partly explain the different results obtained from the two studies. The findings are compared with herbicide losses from other 'hard surfaces'.     

Potential Nitrification Rates of Semiarid Cropland Soils from the Central Ebro Valley,Spain

The nitrification capacity of agricultural soils has received little attention in semi arid Mediterranean environments in spite of the importance of the NO3 - in the ecosystem. In this study, a laboratory experiment was carried out to evaluate potential nitrification in six agricultural soils from semiarid Central Ebro Valley (northeastern Spain). Triplicate topsoil samples (0¨15 cm) were collected seasonally over a 2-year period. The nitrification capacity was evaluated by fresh soil incubation at 25°C and field capacity with a NH +- N source. Verhulst s equation was used to express the accumulation of NO3 - 4 - N with time. The potential nitrification rate (K max) was derived from the equation and used to quantitatively characterize the nitrification process. The K max , obtained by fitting a sigmoidal curve, allowed us to distinguish the nitrification capacity of each semiarid agricultural soil. The K max varied between seasons which indicate that time-variable soil properties (such as salinity and climate) influenced the nitriflcation rate even in an incubation experiment. On an annual average, K max for saline or highly gypsiferous soils ranged from 8.5 to 9.4 mg NO3 -N kg -1 d -1. For the rest of the soils. K max ranged from 36.3 to 43.7 mg NO3 -N kg -1 d -1. Potential nitrification rate was negatively correlated (P < 0.01) with soil salinity and positively correlated with total organic C, microbial activity, and microbial biomass. Highly gypsiferous soil showed a low K max (8.9 mg NO3 -N kg -1 soil d -1) with the lowest NO3 -N content in field samplings (7 kg NO3 3- N ha -1 as an annual average). This fact demonstrated certain inhibition of the nitrification that can be attributed to unbalanced properties in the highly gypsiferous soil.     

Characterisation of new strains of atrazine-degrading Nocardioides sp. isolated from Japanese riverbed sediment using naturally derived river ecosystem

A Gram-positive bacterial strain able to degrade the herbicide atrazine was isolated using a simple model ecosystem constituted with Japanese riverbed sediment and its associated water (microcosm). Treatment of the water phase of the microcosm with 1 mg litre-1 [ring-14C]atrazine resulted in the rapid degradation of atrazine after a 10 day lag phase period. The [ring-14C]cyanuric acid formed was transiently accumulated as an intermediary metabolite in the water phase and was subsequently mineralised through triazine ring cleavage. Possible atrazine-degrading microbes suspended in the water phase of the microcosm were isolated by the plating method while rapid degradation of atrazine was in progress. Among the 48 strains that were isolated, 47 exhibited atrazine-degrading activity. From these 47 isolates, 12 strains that were randomly selected were found to identically convert atrazine to cyanuric acid via hydroxyatrazine. Polymerase chain reaction (PCR) amplification of the genes corresponding to atrazine degradation revealed that these strains at least carried the genes trzN (atrazine chlorohydrolase from Nocardioides C190) and atzC (N-isopropylammelide isopropyl amidohydrolase from Pseudomonas ADP). Physiological characteristics and 16S rDNA partial sequences of six strains that were further selected strongly suggested that all these isolates originated from the same Nocardioides sp. strain. Additionally, only one isolate could mineralise the triazine ring of cyanuric acid. Based on microscopic observations, this strain appears to be a two-membered microbial consortium consisting of Nocardioides sp. and a Gram-negative bacterium. In conclusion, atrazine biodegradation in the microcosm appeared to occur predominantly by Nocardioides sp. to yield cyanuric acid, which could be mineralised by the other relatively ubiquitous microbes.     

Spatial variability in herbicide degradation in the subsurface environment of a groundwater protection zone

The aim of this study was to investigate the spatial variability in degradation and mineralization of atrazine and isoproturon in subsurface samples taken from sandy loam soils overlying gravel terraces which form part of a groundwater protection zone. Percussion drilling was used to obtain samples from 11 boreholes (maximum depth 3 m). Unlabelled atrazine or isoproturon, and ring-14C-labelled atrazine or isoproturon were added to samples, incubated at 25 degrees C for up to 16 weeks, and analyzed for the residual herbicide or [14C]carbon dioxide. All samples showed the potential to degrade these herbicides, although the percentage degradation decreased by a factor of 2-3 from the surface soil to a depth of 3 m. This was associated with a decrease in organic matter content, but there was no change in the potential to mineralize acetate, indicating that specific changes in the catabolic ability of the microbial population occurred with depth. The capacity of samples to mineralize atrazine and isoproturon to carbon dioxide decreased markedly with depth, with no mineralization potential observed at a depth of 80 cm.     

Sulcotrione versus atrazine transport and degradation in soil columns

A soil column experiment under outdoor conditions was performed to monitor the fate of 14C-ring-labelled sulcotrione, 2-(2-chloro-4-mesylbenzoyl)cyclohexane-1,3-dione and atrazine, 6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine, in water leachates and in the ploughed horizon of a sandy loam soil. Two months after treatment, the cumulative amounts of herbicide residues leached from the soil were 14.5% and 7% of the applied radioactivity for sulcotrione and atrazine, respectively. Maximum leachate concentrations for each herbicide were observed during the first month following application: 120 and 95 microg litre(-1) for sulcotrione and atrazine respectively. After 2 weeks, 78% of the sulcotrione and atrazine was extractable from the soil, whereas after two months only 10 and 4%, respectively, could be extracted. The maximum sulcotrione content in the first 10 cm of soil was identical with that of atrazine. For both molecules, the content of non-extractable residues was low, being around 15%. Sulcotrione seems to be more mobile than atrazine but the consequences for water contamination are similar since lower doses are used.     

Field experiments for the evaluation of pesticide spray-drift on arable crops

Two distinct approaches were used to characterise spray-drift during the application of atrazine and alachlor to a maize crop. The first consisted in determining the quantities which did not reach their target. A first experiment was carried in 2001 to improve the sampling method. A second experiment in 2002 showed that losses represented 46 and 38% for atrazine and alachlor, respectively. The second approach was to follow the spatiotemporal evolution of the cloud formed during application. The concentrations observed near the application zone during spraying reached 4.5 microg m(-3) for atrazine and 8.5 microg m(-3) for alachlor. With alachlor these concentrations decreased rapidly when increasing distance from the plot or time following treatment, whereas in the case of atrazine they stabilised rapidly (between 0.5 and 0.3 microg m(-3)) both in space and in time. Deposits around the plot were light and slightly higher for alachlor (from 20 to 130 microg m(-2)). Alachlor was more rapidly diluted in space than atrazine, reflecting a differentiated evolution of physical form during the process. Alachlor, being more volatile than atrazine, is quickly transferred to the gaseous phase which was more rapidly dispersed than aerosols.     

Pesticide adsorption in the vadose zone: a case study on Eocene and Quaternary materials in Northern France

We present a set of adsorption coefficients measured on various Eocene and Quaternary materials sampled from the vadose zone of a catchment in Northern France for three herbicides, atrazine, isoproturon and metamitron. Some vadose zone materials were found to have higher adsorption coefficients than the topsoil. The adsorption coefficients were strongly dependent on the clay content of the material. From 83% to 97% of the variability in the adsorption coefficients could be explained by a linear relationship to the clay content. Adsorption coefficients normalized for clay content, Kclay, ranged between 1.6 and 17.6 litre kg(-1) for atrazine. Neglecting the adsorption properties of the vadose zone and relying exclusively on Koc values to predict mobility may bias regional or local risk assessment of groundwater contamination by pesticides. More information on the adsorption properties of geological materials should be collected to improve our ability to predict pesticide concentrations in groundwaters.     

Degradation and sorption of atrazine, hexazinone and procymidone in coastal sand aquifer media

The degradation, sorption and transport of atrazine, hexazinone and procymidone in saturated coastal sand aquifer media were investigated in batch and column experiments. The pesticides were incubated with sterilised and non-sterilised groundwater or a mixture of groundwater and the aquifer material in the dark at 15 degrees C for 120 days. The estimated half-lives of the pesticides (and their ranges) in the mixture of groundwater and aquifer sand were 36 (31-40), 54 (40-77) and 84 (46-260) days for atrazine, procymidone and hexazinone, respectively. Compared with the relevant results for the groundwater-sand mixture phase, the estimated half-life of pesticides in the groundwater phase alone was shorter for procymidone (21 days) but longer for hexazinone (134 days); atrazine was not degraded in the groundwater phase. Chemical degradation appeared to have played the predominant role in the degradation of hexazinone and procymidone in the aquifer system, while both chemical and biological processes seemed to be important for the degradation of atrazine. Batch isothermal experiments were carried out at pH 4.6-4.7 to obtain sorption coefficients under equilibrium conditions. The isothermal data of the pesticides fitted well with the non-linear Freundlich function with an exponent of sorption coefficient that was greater than one. Contrary to reports in the literature, sorption of atrazine was the greatest, and procymidone was slightly more sorbed than hexazinone. A column experiment was conducted at a typical field-flow velocity of 0.5 m day(-1) over 60 days to study pesticide attenuation and transport in flow dynamic conditions. Retardation factors, R, derived from a two-site sorption/desorption model were 8.22, 1.76 and 1.63 for atrazine, procymidone and hexazinone, respectively. Atrazine displayed the lowest mobility and the mobility of procymidone was only slightly less than that of hexazinone, which is consistent with observations in the batch experiment. A possible explanation for these observations is that ionic atrazine is bound to oppositely charged ionic oxides, and ionic oxides have less effect on the sorption of the non-ionic procymidone. The significant tailing in the pesticide breakthrough curves (BTCs) in comparison with the bromide BTC, together with model-simulated results, suggests that the transport of the pesticides was under chemical non-equilibrium conditions with R values that were less than their equivalent values predicted using the batch equilibrium isothermal data. As a result of non-linear kinetic sorption, retardation factors of the pesticides in groundwater systems would not be constant and will decrease with decreasing pesticide concentrations and increasing flow velocities. Hence, the use of equilibrium isotherm data will probably over-predict the sorption of pesticides in groundwater systems. Rhodamine WT, a commonly used groundwater tracer, was significantly retarded (R = 5.48) and its BTC was much more spread out than the bromide BTC. Therefore, it would not be a good tracer for the indication of groundwater flow velocity and dispersion for the coastal sand aquifer system. In contrast to some aquifer media, the dye tracer was unsuitable as a marker of the appearance of atrazine in a coastal sand aquifer system.     

Atrazine soil metabolism in maize fields treated with organic fertilizers

The metabolism of atrazine in soil was studied in two maize fields located in regions with different soil types. Treatments were cow manure or pig slurry (50 tonnes each ha^-1) applied once either in March or in November before sowing, or green manure incorporated in March. Control plots were not treated with organic fertilizers. Atrazine at 0.75 or 1.0 kg a.i. ha^-1 was applied after sowing. In the 0-12 cm soil layer, the main atrazine metabolites found were deethylatrazine and hydroxyatrazine. Low concentrations of deisopropylatrazine and 6-chloro-1, 3, 5-triazine-2, 4-diamine were also observed. During the 4 months after sowing, the organic fertilizers decreased the rate of atrazine degradation, but subsequently there were no differences between treated and control plots. At harvest, the concentrations of atrazine and its metabolites were very low and similar in all plots. The organic fertilizers thus did not cause atrazine metabolites to accumulate in soil. In addition, atrazine and its metabolites were never detected below 12 cm in any of the plots.     

A risk assessment of atrazine use in California: human health and ecological aspects

A risk assessment of the triazine herbicide atrazine has been conducted by first analyzing the toxicity database and subsequently estimating exposure. Margins of safety (MOS) were then calculated. Toxicity was assessed in animal studies and exposure was estimated from occupational and dietary sources. In acute toxicity studies, atrazine caused developmental toxicity in the rabbit [no observed effect level (NOEL) 5 mg kg(-1) day(-1)] and cardiotoxicity in a dog chronic study (NOEL 0.5 mg kg(-1) day(-1)); cancer (mammary glands) resulted from lifetime exposure. The mammary tumors, which occurred specifically in female Sprague-Dawley rats, were malignant, increased in a dose-dependent manner and were also observed with other, related triazines. Evidence for a genotoxic basis for these tumors was either equivocal or negative. Triazines have been shown to be clastogenic in Chinese hamster ovary cells, in vitro, but without showing a convincing dose/response relationship. Atrazine can be converted into genotoxic N-nitrosoatrazine in the environment or the digestive system, suggesting that N-nitrosamines derived from triazines could be oncogenic. However, it was concluded that N-nitrosotriazines are unlikely to play a significant role in triazine-induced rat mammary gland tumors. An endocrine basis for the mammary tumors, involving premature aging of the female SD rat reproductive system, has been proposed. A suppression of the luteinizing hormone surge during the estrus cycle by atrazine leads to the maintenance of elevated blood levels of 17beta-estradiol (E2) and prolactin. The mechanism for tumor development may include one or more of the following: the induction of aromatase (CYP19) and/or other P450 oxygenases, an antagonist action at the estrogen feedback receptor in the hypothalamus, an agonist action at the mammary gland estrogen receptor or an effect on adrenergic neurons in the hypothalamic-pituitary pathway. None of these has been excluded as a target because there has been a lack of a rigorous attempt to address the mechanism of action for mammary tumors at the molecular level. The potential occupational exposure to atrazine was assessed during mixing, loading and application. Absorbed daily dosage values were 1.8-6.1 microg kg(-1) day(-1). The MOS values (animal NOEL/human exposure) for short-term (acute) exposure were 820-2800. Longer-term occupational exposure and risk were also calculated. Detectable crop residues are generally absent at harvest. Theoretical calculations of acute dietary exposure used tolerance levels, along with secondary residues, and water, for which there is a maximum contamination level; atrazine plus the three main chlorotriazine metabolites were combined. MOS values were above 2000 for all population subgroups. Dietary exposure to atrazine is therefore extremely unlikely to result in human health hazard. Recent publications have reported a possible feminization of frogs, measured in laboratory and field studies. This is assumed to be due to the induction of aromatase, but no measurements of enzyme activity have been reported. In field studies, the water bodies with the greatest numbers of deformed frogs sometimes had the lowest concentrations of atrazine. Other studies have also cast doubt on the feminization theory, except perhaps at very high levels of atrazine. Epidemiology studies have investigated the possibility that atrazine may result in adverse effects in humans. Although some studies have claimed that atrazine exposure results in an elevated risk of prostate cancer, the published literature is inconclusive with respect to cancer incidence.     

滴灌条件下施氮量对黄瓜-番茄种植体系中土体NO3--N淋洗的影响

为了解不同氮肥用量对土壤NO3--N淋洗的风险程度, 合理指导温棚蔬菜施肥和灌溉,2005～2006年在宁夏引黄灌区滴灌条件下,以轮作体系下的温棚黄瓜-番茄为研究对象,采用田间土壤溶液定位提取、田间试验与室内分析相结合的方法,设化肥施氮量150 kg/hm2(低氮)、300 kg/hm2(中氮)、450 kg/hm2(高氮1)、600 kg/hm2(高氮2)及有机肥和不施肥(CK)处理,研究滴灌条件下施氮量对土体中NO3--N淋洗的影响.结果表明:无论是低、中或高氮处理下,黄瓜-番茄轮作周期中,滴灌施肥对0～30 cm土壤溶液NO3--N含量变化的影响明显;在高氮处理下,由于番茄季较强的滴灌量,土体中NO3--N不断向下淋洗至90 cm土层;与CK处理相比,单施有机肥会造成的土壤NO3--N向深层淋洗.因此,提出每茬蔬菜推荐施氮量控制在300 kg/hm2左右为宜,在冬春茬后期4～6月份减少滴灌次数是减少土体NO3--N向下淋洗的措施.     

Isolation and in vitro and in vivo activity against Phytophthora capsici and Colletotrichum orbiculare of phenazine-1-carboxylic acid from Pseudomonas aeruginosa strain GC-B26

The bacterial strain GC-B26, which showed strong antifungal and anti-oomycete activity against some plant pathogens, was isolated from a grassland soil in Korea. Based on morphological, physiological and biochemical characteristics, GC-B26 was identical to Pseudomonas aeruginosa (Schroeter) Migula. The antibiotic G26A, active against Phytophthora capsici Leonian and Colletotrichum orbiculare (Berk & Mont) van Arx, was isolated from the culture filtrates of Ps aeruginosa strain GC-B26 using various chromatographic procedures. The EI mass and UV spectral results indicated that G26A is an analogue of phenazines, having molecular formula C13H8N2O2 (M+, m/z 224.0664). On the basis of NMR spectral data, G26A was confirmed as phenazine-1-carboxylic acid. C orbiculare, P capsici and Pythium ultimum Trow were most sensitive to G26A, with MIC values of approximately 5 microg ml(-1). However, no antimicrobial activity was found against yeasts and bacteria, even at a concentration of over 100 microg ml(-1). Treatment with the antibiotic gave highly significant protective activity against the development of Phytophthora disease on pepper and anthracnose on cucumber plants. The disease control efficacy was only slightly less than that of the commercial fungicides metalaxyl and chlorothalonil.     

NO_3~--N/NH_4~+-N不同配比下紫花苜蓿氮代谢关键酶及钼、铁元素含量研究

以西北地区广泛种植的优质植物性蛋白饲料紫花苜蓿"甘农3号"为试验材料,在室外防雨网室内,采用盆栽营养液沙培法,在适宜氮素供应水平210 mg·L-1基础上,研究NO_3~--N和NH_4~+-N混合的7种配比(NO_3~--N/NH_4~+-N分别为12.5/87.5、25/75、37.5/62.5、50/50、62.5/37.5、75/25、87.5/12.5,依次标为1/7、1/3、3/5、5/5、5/3、3/1、7/1)对紫花苜蓿全生育期各部位硝酸还原酶(NR)、固氮酶活性及钼(Mo)、铁(Fe)营养吸收的影响。结果表明:(1)从部位来看,紫花苜蓿NR活性、Mo含量趋势一致,均表现为:叶根茎,而Fe含量则表现为根叶茎;不同配比下各部位NR活性均表现为配比中NO_3~--N比例大时,其活性较高,尤其是生长前期当NO_3~--N/NH_4~+-N为7/1时,各部位NR活性均最高;不同配比对根中Mo含量的影响在苗期与NR趋势相近,在NO_3~--N比例大时达最高。现蕾期与固氮酶活性相同,1/7时最高。成熟期同固氮酶与NR活性,均为5/3时最高;不同配比对根中Fe含量的影响与固氮酶活性变化趋势相一致,苗期均为1/7处理最高,盛花期至成熟期均为5/3处理最高。(2)从生育期来看,紫花苜蓿全生育期NR和固氮酶活性变化趋势均呈单峰曲线;NR活性在盛花期最高,即盛花期氮代谢能力最强,且各生育期均为配比中NO_3~--N比例大时NR活性高,氮代谢能力较强;固氮酶活性在结荚期最高,即结荚期根瘤固氮能力最强,且苗期和现蕾期均表现为1/7处理其活性最高,盛花期至成熟期5/3处理活性最高;不同配比对茎和叶中Mo、Fe含量的影响基本相同,现蕾期至盛花期在配比中NH_4~+-N比例大时含量最高,结荚期至成熟期在配比中NO_3~--N比例大时含量最高。(3)NR和固氮酶活性相互关系在整个生育期表现不同:在紫花苜蓿生长前期(苗期至现蕾期),NO_3~--N/NH_4~+-N=7/1时NR活性最大,1/7处理NR活性最小,而固氮酶活性则相反,二者表现出拮抗关系;在紫花苜蓿生长中、后期(盛花期至成熟期)NR活性与固氮酶活性均在NO_3~--N/NH_4~+-N=5/3时达最大值,二者又相互促进。     

莠去津土壤残留生物修复菌肥作用原理与田间应用

莠去津是玉米田应用的优秀除草剂品种,然而由于其在土壤中残留时间长,常对轮作后茬敏感作物造成严重毒害。采用生物修复菌肥做种肥、结合菌肥拌种和叶面喷施方法,研究对玉米后茬旱直播水稻生长发育及药害修复机理,对土壤中莠去津残留量、水稻生长和生理指标、土壤酶活性进行测定。结果表明:颗粒菌肥做种肥+粉剂菌肥拌种+水剂菌肥叶面喷施是莠去津土壤残留毒害修复的最佳方法,土壤中莠去津含量在喷施菌肥后7 d从施用菌肥前的0.9 mg/kg下降到0.1 mg/kg,水稻叶片叶绿素含量显著增加33.74%,超氧化物歧化酶和过氧化物酶酶活性分别显著提高23.39%和92.57%,丙二醛含量则显著降低48.01%;水稻株高、地上部鲜重、干重分别比对照显著增加22.33%、67.51%和74.80%,根系鲜重和干重分别比对照显著增加33.98%和55.43%;土壤磷酸酶、脲酶及纤维素酶含量分别显著增加49.17%、528.65%和35.21%。     

Horizontal gene transfer of atrazine-degrading genes (atz) from Agrobacterium tumefaciens St96-4 pADP1::Tn5 to bacteria of maize-cultivated soil

The plasmid pADP1::Tn5 derived from pADP1[Atr+] carrying a Tn5 transposon conferring kanamycin and streptomycin resistances was constructed and introduced in Agrobacterium tumefaciens St96-4. This genetically modified strain was inoculated (approximately 10(8) cfu g(-1)) in potted soils planted with maize and treated or not with atrazine (1.5 mg kg(-1)). Bulk and maize rhizosphere soils were sampled 39 days after planting to look for soil indigenous bacteria that had acquired pADP1::Tn5. Four transconjugants were isolated from four different soil samples. The estimated transfer frequency of pADP1::Tn5 was 10(-4) per donor. Maize rhizosphere and atrazine treatment had no obvious effect on pADP1::Tn5 transfer frequency. The sequencing of the 16S rDNA sequences of the transconjugants revealed that they were almost identical and highly similar to that of Variovorax spp (97%). In addition, their characterization suggested that the atzA and atzB genes had been transferred from pADP1::Tn5 to the bacterial chromosome in two of the four transconjugants. These data suggest that the atz degrading genes are horizontally transferred in soil and possibly subjected to gene rearrangement.     

阿特拉津降解菌LY-2的分离鉴定及其对污染土壤的修复

农田土壤中残留的莠去津易对后茬敏感作物造成药害,利用功能微生物调控作物中残留莠去津降解是缓解其药害的有效方式。本研究分析了水稻根际微生物群落及土壤中莠去津降解基因对莠去津胁迫的响应,并从水稻根际土壤中分离出一株莠去津降解菌Pseudomonas sp. AT2,通过盆栽试验探究了莠去津胁迫下菌株AT2对水稻生长及莠去津降解的影响机制。结果表明：莠去津胁迫可显著改变水稻根际细菌的群落结构,并提高根际土壤中莠去津降解基因TrzN、AtzB和AtzC的丰度。莠去津胁迫下,接种降解菌AT2组水稻的株高、根长、干重和叶绿素含量均显著增加,MDA的积累减少,说明AT2可缓解水稻的氧化胁迫损伤;接种降解菌AT2处理组土壤和水稻中莠去津含量分别比对照降低了14.9% 和47.1%~57.5%,说明AT2促进了土壤及水稻中莠去津的降解。另外,莠去津胁迫下,接种AT2还可诱导水稻中莠去津降解基因的上调表达,表达量为未接菌对照组的1.3~2.7倍。研究表明,根际接种降解菌株能激活水稻体内降解基因的表达,促进“土壤-水稻”体系中莠去津的降解,缓解环境中残留莠去津对水稻的毒害作用。

     

Atrazine dissipation in irrigated sorghum cropping in southern New South Wales

Dissipation of atrazine after pre-emergence application to irrigated grain sorghum was investigated in an experiment on a Birganbigil clay loam at Yanco Agricultural Research Centre in the Murrumbidgee Irrigation Areas of New South Wales. Dissipation followed first-order kinetics with a half-life of 70 days. This rate of disappearance did not differ significantly between application rates of 2.5 and 10 kg/ha. Removal of volunteer plant growth with non-residual chemicals or by cultivation during the winter fallow periods had no significant effect on the levels of atrazine residues in the soil and dissipation rate did not differ significantly between the 2 years of the experiment. A laboratory incubation experiment demonstrated that dissipation of atrazine in Birganbigil soil was more rapid than in three other soils from the Murrumbidgee and Murray Valleys. Dissipation rate and atrazine adsorption were both correlated with the organic carbon content of the soils, which ranged from 1.43% to 0.72%. There was no correlation between either dissipation rate or adsorption and clay content, even though clay contents ranged from 37 to 78%.