海洋细菌生物膜用于生物修复陆地生态系统中土壤多环芳烃污染

Polycyclic aromatic hydrocarbons(PAHs) in soil retain for a quite long period due to their hydrophobicity and aggregation properties. Biofilm-forming marine bacterial consortium(named as NCPR), composed of Stenotrophomonas acidaminiphila NCW702,Alcaligenes faecalis NCW402, Pseudomonas mendocina NR802, Pseudomonas aeruginosa N6P6, and Pseudomonas pseudoalcaligenes NP103, was used for the bioremediation of PAHs in a soil microcosm. Phenanthrene and pyrene were used as reference PAHs. Parameters that can affect PAH degradation, such as chemotaxis, solubility of PAHs in extracellular polymeric substances(EPS), and catechol2,3-dioxygenase(C23O) activity, were evaluated. P. aeruginosa N6P6 and P. pseudoalcaligenes NP103 showed chemotactic movement towards both the reference PAHs. The solubility of both the PAHs was increased with an increase in EPS concentration(extracted from all the 5 selected isolates). Significantly(P 0.001) high phenanthrene(70.29%) and pyrene(55.54%) degradation was observed in the bioaugmented soil microcosm. The C23O enzyme activity was significantly(P 0.05) higher in the bioaugmented soil microcosm with phenanthrene added at 173.26 ± 2.06 nmol min~(-1) mg~(-1) protein than with pyrene added at 61.80 ± 2.20 nmol min~(-1) mg~(-1) protein. The C23O activity and gas chromatography-mass spectrometer analyses indicated catechol pathway of phenanthrene metabolism. However, the metabolites obtained from the soil microcosm added with pyrene revealed both the catechol and phthalate pathways for pyrene degradation.     

不同细菌饲喂细菌线虫对土壤细菌数量、活性以及群落结构的影响

The effects of bacterial-feeding nematodes on bacterial number, activity, and community composition were studied through a microcosm experiment using sterilized soil inoculated with soil bacteria （soil suspension） and with bacteria and three species of bacterial-feeding nematodes （ Cephalobus persegnis, Protorhabditis filiformis, and Caenorhabditis elegans）. Catalyzed reporter deposition-fluorescence in situ hybridization, CO2 evolution, and denaturing gradient gel electrophoresis （DGGE） of PCR ampli- fied 16S rRNA gene fragments were used to investigate bacterial numbers, antivity, and community composition, respectively. Our results showed that bacterial numbers and activity significantly increased in the presence of bacterial-feeding nematodes, which indicated that bacterial-feeding nematodes had a significant positive effect on soil bacteria. The different nematode species had different effects on bacterial numbers and activity. C. persegnis and P. filiformis, isolated from native soil, increased the bacterial number and activity more than C. elegans. The DGGE analysis results showed that dominant bacterial species significantly differed among the treatments, which suggested that bacterial-feeding nematode species modified the bacterial community composition in soil. Further gene sequence analysis results showed that the dominant bacterial species in this study were gram-negative bacteria. Given the completely same conditions except nematode species, the varied selective feeding behavior of different nematode species was the most likely reason for the altered bacterial community composition. Overall, the alteration of bacterial numbers, activity and community composition resulting from the bacterial-feeding nematodes may ult!mately affect soil ecological functioning and processes.     

Soil organic carbon and phosphorus availability regulate abundance of culturable phosphate-solubilizing bacteria in paddy fields

Low availability of phosphorus(P) is a major constraint for optimal crop production, as P is mostly present in its insoluble form in soil. Therefore,phosphate-solubilizing bacteria(PSB) from paddy field soils of the Indo-Gangetic Plain, India were isolated, and their abundance was attempted to be correlated with the physicochemical characteristics of the soils. Ninety-four PSB were isolated on Pikovskaya's agar medium, and quantitative phosphate solubilization was evaluated using NBRIP medium. The isolates solubilized P up to a concentration of 1 006 μg mL~(-1) from tricalcium phosphate with the secretion of organic acids. These isolates were identified by 16 S rRNA gene sequence comparison, and they belonged to Gammaproteobacteria(56 isolates),Firmicutes(28 isolates), Actinobacteria(8 isolates), and Alphaproteobacteria(2 isolates). Phylogenetic analysis confirmed the identification by clustering the isolates in the clade of the respective reference organisms. The correlation analysis between PSB abundance and physicochemical characteristics revealed that the PSB population increased with increasing levels of soil organic carbon, insoluble P, K~+, and Mg~(2+). The promising PSB explored in this study can be further evaluated for their biofertilizer potential in the field and for their use as potent bio-inoculants.     

营养和病虫害管理对双季杂交水稻种植系统的土壤生态质量的共同影响

The mineral fertilizers(NPK) and pesticide,including herbicides,insecticides and fungicides,were applied alone or in combination and the soil sampling was done at different growth stages during the crop cycle to study the changes in soil organic matter,microbial biomass and their activity parameters in a paddy soil with different nutrient and pest management practices in a hybrid rice double-cropping system.A consistent increase in the electron transport system(ETS) activity was measured during the different crop growth stages of rice.The use of fertilizers(NPK) alone or with pesticides increased ETS activity,while a decline of ETS activity was noticed with pesticides alone as compared with the control.Nearly an increasing trend in soil phenol content was observed with the progression of crop growthstages,while the usage of pesticides alone caused maximum increments in the soil phenol content.The soil protein content was found nearly stable with fertilizers and/or pesticides application at various growth stages in both crops taken.But notable changes were noticed at different growth stages probable because of fluctuations in moisture and temperature at particular stages,which might have their effects on N mineralization.Marked depletions in the phospholipid content were found with the advancement of crop growth stages,while the incorporation of fertilizers and/or pesticides also produced slight changes,in which a higher decline was noticed with pesticide application alone compared with the control.     

砷污染土壤中接种丛枝菌根真菌和蚯蚓对土壤线虫群落和植物砷吸收的影响

The influences of arbuscular mycorrhizal fungi (MF, Acaulospora spp. and Glomus spp.), rice straw and earthworms (RE, Eisenia foetida) on nematode communities and arsenic (As) uptake by maize (Zea mays L.) in As-contaminated soils were examined in a field experiment conducted in Wujiang, Jiangsu Province, China. The experiment was designed as a 2 × 2 factorial with the factors of MF (inoculated or uninoculated) and RE (added or not added). The results demonstrated that MF inoculation led to significantly higher root colonization of MF and root dry weight. Plants inoculated with both MF and RE had the highest As concentrations in root. The number of total nematodes increased with MF inoculation when RE was absent, and decreased with RE addition when MF was inoculated. The improved abundance of nematodes with the MF treatment implied that the tested MF acted as food sources for fungivores. The abundances of omnivores-predators and plant parasites were reduced by earthworm activity. Twenty-seven genera of nematodes were identified, with Filenchus dominant in all treatments. Trophic diversity (TD), Shannon-Weaver diversity (H′), Simpson dominance index (λ), and species richness (SR) indicated higher species diversity, more proportionate species composition, evenly distributed species, and more food sources in the MF, RE, and their interaction treatments. Maturity index (MI) showed a moderately disturbed environment due to As pollution. Besides enhancing plant uptake of contaminants, MF and RE amendments could also improve soil health by restoring the structure of soil communities, as reflected by more stable nematode community structure.     

蚯蚓与黑麦草相互作用对土壤中荧蒽去除的影响

Earthworms can promote the bioremediation of contaminated soils through enhancing plant growth and microorganism development. The individual and combined effects of earthworms and ryegrass (Lolium multiflorum Lam.) on the removal of fluoranthene from a sandy-loam alluvial soil were investigated in a 70-d microcosm experiment. The experiment was set up in a complete factorial design with treatments in four replicates: without earthworms or ryegrass (control, CK), with earthworms only (E), with ryegrass only (P), and with both earthworms and ryegrass (EP). The residual fluoranthene, microbial biomass C, and polyphenol oxidase activity in the soil changed significantly (P<0.01) with time. In general, the residual concentration of fluoranthene in the soil decreased sharply from 71.8-88.7 to 31.7-37.4 mg kg 1 in 14 d, and then decreased gradually to 19.7-30.5 mg kg 1 on the 70th d. The fluoranthene concentration left in the soil was the least with both earthworms and ryegrass, compared to the other treatments at the end of the experiment. Half-life times of fluoranthene in the E, P, and EP treatments were 17.8%-36.3% smaller than that of CK. More fluoranthene was absorbed by earthworms than ryegrass. However, the total amounts of fluoranthene accumulated in both the ryegrass and earthworms were small, only accounting for 0.01%-1.20% of the lost fluoranthene. Therefore, we assumed that microbial degradation would play a dominant functional role in fluoranthene removal from soil. We found that earthworms significantly increased microbial biomass C and polyphenol oxidase activity (P<0.01) in the presence of ryegrass at the end of the experiment. Furthermore, microbial biomass C and polyphenol oxidase activity were significantly (P<0.05) and negatively related to the residual fluoranthene concentration. This implied that earthworms might promote the removal of fluoranthene from soil via stimulating microbial biomass C and polyphenol oxidase activity.     

Extracts from Marine Macroalgae and Opuntia ficus-indica Cladodes Enhance Halotolerance and Enzymatic Potential of Diazotrophic Rhizobacteria and Their Impact on Wheat Germination Under Salt Stress

Soil salinity, which affects more than 6% of the earth’s land surface and more than 20% of its irrigated areas, is a major threat to agriculture. Diazotrophic bacteria are among the functional groups of soil microbiota that are threatened by this abiotic stress, as their activity is mostly inhibited by salt stress. Seventy bacterial strains with distinct characteristics were isolated from soils by using N-free Jensen’s selective medium. Based on their ability to produce metabolites of agricultural interest, four strains were selected and identified as Flavobacterium johnsoniae, Pseudomonas putida, Achromobacter xylosoxidans, and Azotobacter chroococcum. The selected strains were grown at different NaCl concentrations (0–600 mmol L~(-1) in N-free broth and 0–2 000 mmol L~(-1) in Luria-Bertani medium) in the presence and absence of glycine betaine (GB), aqueous and hydro-alcoholic extracts from marine macroalgae, Ulva lactuca and Enteromorpha intestinalis, and Opuntia ficus-indica cladodes. The selected bacterial strains, GB, and the aforementioned extracts were tested for their ability to promote the germination of wheat (Triticum durum) seeds at 0–300 mmol L~(-1) NaCl. Compared with the results obtained with the synthetic osmoprotectant GB, the extracts from O. ficus-indica, U. lactuca, and E. intestinalis significantly promoted bacterial growth and seed germination under salt stress.     

中国小麦田土壤线虫对生物炭添加的响应

While studies have focused on the use of biochar as soil amendment, little attention has been paid to its effect on soil fauna. The biochar was produced from slow pyrolysis of wheat straw in the present study. Four treatments, no addition (CK) and three rates of biochar addition at 2 400 (B1), 12 000 (B5) and 48 000 kg ha-1 (B20), were investigated to assess the effect of biochar addition to soil on nematode abundance and diversity in a microcosm trial in China. The B5 and B20 application significantly increased the total organic carbon and the C/N ratio. No significant difference in total nematode abundance was found among the treatments. The biochar addition to the soil significantly increased the abundance of fungivores, and decreased that of plant parasites. The diversity of soil nematodes was significantly increased by B1 compared to CK. Nematode trophic groups were more effectively indicative to biochar addition than total abundance.     

水分含量对秸秆还田土壤碳矿化和微生物特性的影响

An 80-d incubation experiment was conducted to investigate straw decomposition,the priming effect and microbial characteristics in a non-fertilized soil(soil 1) and a long-term organic manure-fertilized soil(soil 2) with and without13 C-labeled maize straw amendment under different moisture levels. The soil 2 showed a markedly higher priming effect,microbial biomass C(Cmic),and β-glucosidase activity,and more abundant populations of bacteria and fungi than the soil 1. Also,soil CO2 emission,Cmic,β-glucosidase activity,and bacterial and fungal population sizes were substantially enhanced by straw amendment. In the presence of straw,the amount of straw mineralization and assimilation by microbes in the soil at 55% of water holding capacity(WHC) were significantly higher by 31% and 17%,respectively,compared to those at 25% of WHC. In contrast,β-glucosidase activity and fungal population size were both enhanced as the moisture content decreased. Cmicdecreased as straw availability decreased,which was mainly attributed to the reduction of straw-derived Cmic. Amended soils,except the amended soil 2 at 25% of WHC,had a more abundant fungal population as straw availability decreased,indicating that fungal decomposability of added straw was independent of straw availability. Non-metric multidimensional scaling analysis based on fungal denatured gradient gel electrophoresis band patterns showed that shifts in the fungal community structure occurred as water and straw availability varied. The results indirectly suggest that soil fungi are able to adjust their degradation activity to water and straw availability by regulating their community structure.     

生物碳可以防止土壤活性有机质矿化吗？

Biochar could help to stabilize soil organic(SOM) matter, thus sequestering carbon(C) into the soil. The aim of this work was to determine an easy method i) to estimate the effects of the addition of biochar and nutrients on the organic matter(SOM)mineralization in an artificial soil, proposed by the Organization for Economic Co-operation and Development(OECD), amended with glucose and ii) to measure the amount of labile organic matter(glucose) that can be sorbed and thus be partially protected in the same soil, amended or not amended with biochar. A factorial experiment was designed to check the effects of three single factors(biochar, nutrients, and glucose) and their interactions on whole SOM mineralization. Soil samples were inoculated with a microbial inoculum and preincubated to ensure that their biological activities were not limited by a small amount of microbial biomass, and then they were incubated in the dark at 21℃ for 619 d. Periodical measurements of C mineralized to carbon dioxide(CO_2) were carried out throughout the 619-d incubation to allow the mineralization of both active and slow organic matter pools. The amount of sorbed glucose was calculated as the difference between the total and remaining amounts of glucose added in a soil extract. Two different models, the Freundlich and Langmuir models, were selected to assess the equilibrium isotherms of glucose sorption. The CO_2-C release strongly depended on the presence of nutrients only when no biochar was added to the soil. The mineralization of organic matter in the soil amended with both biochar and glucose was equal to the sum of the mineralization of the two C sources separately. Furthermore, a significant amount of glucose can be sorbed on the biochar-amended soil, suggesting the involvement of physico-chemical mechanisms in labile organic matter protection.     

木糖氧化无色杆菌反硝化亚种细菌的分离鉴定及其菲降解特性研究

利用选择性富集培养及升华法,从石油污染的土壤中分离到2株菲降解细菌,它们在以菲为唯一碳源的培养基上生长良好。应用BIOLOG细菌鉴定系统和分子生物学方法对两株细菌进行鉴定,两株菌分别为坚强芽孢杆菌(Bacillus.firmus)和木糖氧化无色杆菌反硝化亚种(Achromobacter.xylosoxidanssub.sp.denitrificans),两株菌均具有邻苯二酚氧化酶活性。两株细菌在液体培养条件下都表现较强降解菲的能力,液体培养60.h约90%的加入菲被降解。通过测定液体培养基中菲浓度和菌体密度变化发现,菌株降解菲的量与其生长密度相关;随着菌体浓度(吸光度)的增加,代谢底物菲的浓度明显降低,两株菌混合使用能够大幅度提高降解菲的能力。     

Influence of bacterial-feeding nematodes (Cephalobidae) on soil microbial communities during maize growth

The effect of several bacterial-feeding nematodes of the Cephalobidae family (Zeldia punctata, Acrobeloides nanus and Cephalobus pseudoparvus) on the microbial community of a Sahelian soil (Senegal) was investigated in microcosm. The consequences of the activity of these nematodes on the growth and nitrogen nutrition of young maize plants (aerial biomass, root biomass and nitrogen content) were also estimated. Laboratory-cultured nematodes were inoculated into soil containing maize seedlings where the natural nematofauna had been previously eliminated by alternately freezing and defrosting (five cycles). The microbial compartment of the soil community was characterised through total microbial biomass (using fumigation-extraction), density of bacteria (using colony forming units counts), microbial activity (using alkaline phosphatase) and genetic structure of soil microbial community (using denaturing gradient gel electrophoresis) at sowing and at 12, 26 and 47 days after planting. Final nematode densities in the different treatments (between 4 and 20 Ind g⁻¹ dry soil) demonstrated a high level of reproduction. The different types of nematodes tested induced similar trends in changes in the microbial pool of the soil and in maize seedling growth. Compared to control soils, the presence of nematodes led to an increase (+12%) in plant biomass and reduced concentrations of soil ammonium but had no effect on concentrations of nitrate by the end of the experiment. Sixty-three percent of the inorganic nitrogen initially present in the soil was incorporated into the maize plants with nematodes whereas only 47% was incorporated without nematodes. Nematode activity led to a significant decrease in microbial biomass (−28%) and density of cultivable bacteria (−55%), however, nematodes stimulated bacterial activity (+18%). The effects of Z. punctata were weakest compared to A. nanus and C. pseudoparvus. The presence of nematodes modified the genetic structure of the microbial community essentially by changing the relative abundance of dominant bacterial populations. Among nematode species tested, A. nanus modified the structure of the microbial communities the most compared with control soils without nematodes. Overall, results from this study provide evidence for the ability of microbial feeding nematodes to alter microbial activity, microbial community structure, nitrogen mineralisation and growth of maize seedlings in a Sahelian soil from Senegal, West Africa.     

Soil microbial community dynamics and phenanthrene degradation as affected by rape oil application

The effects of rape oil application on soil microbial communities and phenanthrene degradation were characterized by examining phenanthrene concentrations, changes in microbial composition and incorporation of [¹³C] phenanthrene-derived carbon into phospholipid fatty acids (PLFAs). A Haplic Chernozem was incubated with and without rape oil in combination with and without phenanthrene over 60 days. High-performance liquid chromatography (HPLC) analysis showed a net reduction in extractable phenanthrene in the soils treated with rape oil but no net reduction in the soils without rape oil. Rape oil application increased the total PLFA content and changed microbial community composition predominantly due to growth of fungal groups and Gram-positive bacterial groups. Under rape oil and phenanthrene amendment all detected microbial groups grew until day 24 of incubation. The ¹³C PLFA profiles showed ¹³C enrichment for the PLFAs i14:0, 15:0, 18:0, 18:1ω5 and the fungal biomarker 18:2ω6,9 under rape oil application. Fungal PLFA growth was highest among detected all PLFAs, but its ¹³C incorporation was lower compared to the Gram-positive and Gram-negative bacteria PLFAs. Our results demonstrate the effect of rape oil application on the abundance of microbial groups in soil treated with phenanthrene and its impact on phenanthrene degradation.     

连作番茄根区病土对番茄生长及土壤线虫与微生物的影响

探索连作番茄根区病土对番茄根结线虫病的诱导效果及引起连作障碍的微生态机制,可为深入了解番茄连作障碍发生机理及探究番茄连作障碍防治方法提供科学依据。本研究利用盆栽试验,测定了番茄在健康土壤及接种病土土壤中生物学特性变化及根结线虫侵染状况,并分析鉴定了土壤中微生物及线虫的种类与数量。结果表明,接种连作番茄根结线虫病株根区病土会对番茄生长及根结线虫侵染产生影响:1)番茄苗期根系根结数达9个?株~(-1),健康土壤无根结;土壤线虫数量较健康土壤增加390.4%;收获期番茄根结线虫侵染率达62.7%,病情指数为80.0%。2)番茄生长受到抑制,叶片防御酶活性降低,收获期茎叶及根系鲜质量较健康土壤分别减少50.2%及33.1%,苗期番茄叶片PPO活性较健康土壤降低15.8%,POD活性较健康土壤增加24.0%,差异均达显著水平(P0.05)。3)番茄根系更易感染有害菌,根系内病原菌甘蓝假单胞菌数量较健康土壤增加463倍,根区土壤细菌、真菌及放线菌总数分别增加46.3%、94.5%及134.0%。4)食细菌线虫、食真菌线虫及植物寄生性线虫数量分别为健康根区土壤的3.3倍、1.6倍及7.3倍,其中的植物寄生线虫95.6%为根结线虫。综上所述,接入连作番茄根结线虫病株根区病土不仅导致番茄遭受根结线虫侵染,而且会导致土壤线虫总量及植物寄生线虫所占比例大幅增加,并使番茄根系内有害细菌数量显著增加,对番茄生长造成显著抑制作用,同时影响番茄的生理生化特性,受线虫侵染番茄防御性酶活性降低,使其更易被根结线虫及病原菌侵染,番茄根区土壤线虫、微生物及根系内优势细菌的种类与数量及其之间的作用发生改变。     

石油污染土壤中芳烃降解菌及邻苯二酚2,3双加氧酶的克隆

石油污染土壤中的芳烃降解菌是进行土壤修复的主要生物资源,本研究对某炼油厂附近土壤中的芳烃降解菌及邻苯二酚2,3双加氧酶基因进行了研究。结果表明,部分石油烃污染土壤中存在着大量的芳烃降解菌;对其中一个土壤样本中的邻苯二酚2,3双加氧酶基因进行克隆,获得了7个不同的邻苯二酚双加氧酶基因序列,序列分析表明这些基因可能来源于土壤中的假单胞菌,且该基因在土壤中的丰度与污染水平及芳烃降解菌的数量相关。可见,土壤中芳烃降解菌数量及降解基因的丰度和多样性,可以对石油污染土壤的生物修复进行监控并为生物修复提供丰富的微生物资源。     

Nutrition on bacteria by bacterial-feeding nematodes and consequences on the structure of soil bacterial community

Bacterial-feeding nematodes are, with protozoa, the main grazers of soil bacteria. Interactions between bacteria and nematodes have important repercussions on soil functioning and particularly on nutrient availability. We assessed the influence of the bacterial strains ingested on bacterial-feeding nematodes population development and also the consequences of nematode feeding behaviour on the structure of the soil microbial community with a special attention to different soil micro-habitats for nematode and bacteria. In vivo studies conducted in the presence of single bacterial strains showed that the type of ingested bacteria conditioned the development of the different bacterial-feeding Cephalobidae nematode species tested and that the effect of bacteria differed between nematode species. The spatial distribution of soil nematodes between three soil habitats (fresh organic matter, inter-aggregates pores and aggregates) depended of the trophic behaviour of nematodes. Bacterial-feeding nematodes and fungal-feeding nematodes showed comparable distribution: being preferentially located in the fresh organic matter and in the inter-aggregate pores. Besides, the activity of inoculated bacterial-feeding nematodes modified the genetic structure of the soil microbial community. Bacterial community of the macroporosity was significantly influenced by the nematodes. On the contrary, no modification of the structure of the bacterial community linked with nematode activity was measured in the bulk soil.     

苏丹草对土壤中菲芘的修复作用

采用盆栽试验法,研究了苏丹草对土壤中菲、芘的去除效果,及植物和微生物在去除土壤菲、芘中的交互效应。结果显示,在试验浓度范围（0-322．06mg·kg^-1内,土壤～苏丹草系统（TR,）对菲、芘的去除效果明显。种植苏丹草60d后,土壤菲、芘去除率分别为73．07％-83．92％、63％～77．62％;平均去除率分别比对照1（无植物,不加0．1％NaN3）高55．58％、50．71％,比对照2（无植物,加0．1％NaN3）高72．71％、66．57％,说明种植苏丹草可以促进微生物对土壤中芘、菲降解。土壤酶活性测定结果也显示,酶活性越高,污染物降解率越高,反之亦然。因此,植物一微生物间的交互效应是土壤中多环芳烃降解的主要途径。     

In‐situ remediation of contaminated farmland by horizontal transfer of degradative plasmids among rhizosphere bacteria

Horizontal transfer of catabolic genes for pollutant degradation among rhizobacteria plays an important role in environmental bioremediation, but lacks support from field trial data. To address this problem, Pseudomonas fluorescens strain TP13 was inoculated into the soils of a phenol‐contaminated farmland on which tomato seedlings were growing in April 2009, 2010, 2011 and 2012. Results consistently showed that introduction of TP13 strain significantly reduced phenol content and increased plant biomass after 20 days, compared with controls. Strain TP13 was able to colonize the plant rhizosphere and the number of rhizosphere bacteria which were grown on phenol and contained the plasmids containing the gene encoding for catechol 2, 3‐dioxygenase (C23O) increased gradually in the later stages of the experiment. The increase in magnitude of the plasmid‐containing rhizosphere bacteria correlated well with plant biomass, while the number of plasmid‐containing rhizosphere bacteria and phenol content was strongly negatively correlated. Furthermore, six strains (T1‐T6) of rhizosphere bacteria were isolated and found to possess large plasmids containing identical C23O genes and similar HindIII restriction patterns. Sequence alignment showed that the C23O genes from strains T1‐T6 contained almost identical sequences and the sequence of the C23O of strain T1 was the same as that of strain TP13. These data indicated that the plasmids were transferred from strain TP13 to these rhizosphere bacteria and that horizontal gene transfer stimulated phenol degradation and plant growth in the contaminated farmland. This is an important finding for in situ remediation of contaminated farmland.