Effects of long-term 2,4-D application on microbial populations and biochemical processes in cultivated soil

Summary The effects of 15 years of field applications of 2,4-dichlorophenoxy acetate (2,4-D) on soil microbial population and biochemical processes were studied in a field cropped with maize followed by potatoes. Amine or ester formulations at the rate of 0.95 kg 2,4-D per hectare applied in May and October every year. Fungal, bacterial, and actinomycete populations, and microbial biomass C and N were reduced by the 2,4-D treatment, the reduction being more marked where the ester was used. N mineralization, nitrification, and potentially mineralizable N were reduced by the 2,4-D ester only, while urease activity was depressed by both formulations. Dehydrogenase activity and soil microbial respiration tended to be temporarily increased by the amine, but were reduced substantially by the ester, indicating that the ester probably interfered with nutrient cycling.     

Influence of heavy metals on the functional diversity of soil microbial communities

Three soil types-Calcaric Phaeozem, Eutric Cambisol and Dystric Lithosol-in large container pots were experimentally contaminated with heavy metals at four different levels (light pollution: 300 ppm Zn, 100 ppm Cu, 50 ppm Ni, 50 ppm V and 3 ppm Cd; medium pollution: twofold concentrations; heavy pollution: threefold concentrations; uncontaminated control). We investigated the prognostic potential of 16 soil microbial properties (microbial biomass, respiration, N-mineralization, 13 soil enzymes involved in cycling of C, N, P and S) with regard to their ability to differentiate the four contamination levels. Microbial biomass and enzyme activities decreased with increasing heavy metal pollution, but the amount of decrease differed among the enzymes. Enzymes involved in the C-cycling were least affected, whereas vartous enzyme activities related to the cycling of N, P and S showed a considerable decrease in activity. In particular, arylsulfatase and phosphatase activities were dramatically affected. Their activity decreased to a level of a few percent of their activities in the corresponding unpolluted controls. The data suggest that aside from the loss of rare biochemical capabilities-such as the growth of organisms at the expense of aromatics (Reber 1992)-heavy metal contaminated soils lose very common biochemical propertities which are necessary for the functioning of the ecosystem. Cluster analysis as well as discriminant analysis underline the similarity of the enzyme activity pattern among the controls and among the polluted soils. The trend toward a significant functional diversity loss becomes obvious already at the lowest pollution level. This implies that concentrations of heavy metals in soils near the current EC limits will most probably lead to a considerable reduction in decomposition and nutrient cycling rates. We conclude that heavy metal pollution severely decreases the functional diversity of the soil microbial community and impairs specific pathways of nutrient cycling.Dedicated to Professor J. C. G. Ottow on the occasion of his 60th birthday     

Effects of acidic precipitation and acidity on soil microbial processes

Effects of soil acidity on microbial decomposition of organic matter and transformation of N in an acid forest soil were investigated. In the oak-leaf-amended pH-adjusted acid soils, CO₂ production in 14-and 150-day preincubated samples decreased by about 6 and 37%, respectively. In the control (unamended) acidified soils, reductions in CO₂ production of 14% in 14-day preincubated samples and of 52% in 150-day samples were observed. Ammonia formation in the pH-adjusted acid soil was about 50% less than in the naturally acid soil. Increased rates of ammonification and nitrification were observed in the pH-adjusted neutral soil. Little autotrophic and heterotrophic nitrifying activity was detected in naturally acid and acidified forest soils. The rate of denitrification was rather slow in acid soils, and at greater acidities N₂O was the predominant end product. The abundance of N-fixing free-living bacteria was very low in acidic and acidified forest soils, and N gains by asymbiotic bacterial fixation in an acid forest ecosystem may be insignificant. These results suggest that further acidification of acid forest soils by addition of H₂SO₄ or by acid precipitation may lead to significant reductions in the leaf litter decomposition, ammonification, nitrification, and denitrification and thus reduce nutrient recycling in the forest ecosystem.     

The use of microbial parameters in monitoring soil pollution by heavy metals

Microbial parameters appear very useful in monitoring soil pollution by heavy metals, but no single microbial parameter can be used universally. Microbial activities such as respiration, C and N mineralization, biological N₂ fixation, and some soil enzymes can be measured, as can the total soil microbial biomass. Combining microbial activity and population measurements (e.g., biomass specific respiration) appears to provide more sensitive indications of soil pollution by heavy metals than either activity or population measurements alone. Parameters that have some form of internal control, e.g., biomass as a percentage of soil organic matter, are also advantageous. By using such approaches it might be possible to determine whether the natural ecosystem is being altered by pollutants without recource to expensive and long-running field experiments. However, more data are needed before this will be possible. Finally, new applications of molecular biology to soil pollution studies (e.g., genetic fingerprinting) which may also have value in the future are considered.     

Bioavailability and effects of heavy metals on soil microbial biomass survival during laboratory incubation

In this work we studied the influence of Pb, Zn, and Tl on microbial biomass survival and activity during a laboratory incubation of soil. In comparison to uncontaminated soil, the microbial biomass C decreased sharply in soil contaminated with Zn and Tl, whereas the addition of Pb did not have any significant inhibitory effect on the level of microbial biomass C. Zn displayed the greatest biocidal effect, confirmed by the measurement of the death rate quotient (q D). The microbial activity, measured as CO₂ evolution, increased significantly in contaminated soils, emphasizing the need of living organisms to expend more energy to survive. The greater demand for energy by microorganisms in order to cope with the toxicity of pollutants was also confirmed by measurement of the metabolic quotient (q CO₂). In order to determine whether soil microorganisms affect the bioavailability of these metals through their mobilization and release, we studied the relationships between available Pb, Zn, and Tl, and microbial biomass C. The water-soluble fraction of Tl, available Tl, and Zn, and microbial biomass C were related significantly, but not Pb.     

Effects of genetically modified plants on microbial communities and processes in soil

The development and use of genetically modified plants (GMPs) has been a topic of considerable public debate in recent years. GMPs hold great promise for improving agricultural output, but the potential for unwanted effects of GMP use is still not fully understood. The majority of studies addressing potential risks of GMP cultivation have addressed only aboveground effects. However, recent methodological advances in soil microbial ecology have allowed research focus to move underground to try to gain knowledge of GMP-driven effects on the microbial communities and processes in soil that are essential to key terrestrial ecosystem functions. This review gives an overview of the research performed to date on this timely topic, highlighting a number of case studies. Although such research has advanced our understanding of this topic, a number of knowledge gaps still prevent full interpretation of results, as highlighted by the failure of most studies to assign a definitively negative, positive or neutral effect to GMP introduction. Based upon our accumulating, yet incomplete, understanding of soil microbes and processes, we propose a synthesis for the case-by-case study of GMP effects, incorporating assessment of the potential plant/ecosystem interactions, accessible and relevant indicators, and tests for unforeseen effects.     

Effects of 15-year application of municipal wastewater on microbial biomass,fecal pollution indicators,and heavy metals in a Tunisian calcareous soil

Purpose

Water shortage in most countries of the southern Mediterranean basin has led to the reuse of municipal wastewater for irrigation. Despite numerous advantages for soil fertility and crop productivity, recycling wastewater in the soil also has several ecotoxicological and sanitary problems. To evaluate the chronic soil contamination and the cumulative impact of wastewater, we compared seven plot sites irrigated with treated wastewater 1, 2, 7, 9, 13, and 15 years and one nonirrigated taken as control, and these were sampled for soil analysis.

Materials and methods

Soil samples were analyzed for pH, electrical conductivity (EC), total Kjeldahl nitrogen (TKN), total organic matter, and total concentrations of Cu, Zn, Fe, Ni, Pb, and Cd. Microbial biomass and enteric bacteria (fecal coliforms and fecal streptococci) were determined in all soil samples.

Results and discussion

The soil pH values were not consistently affected. Soil salinity, measured as EC, appeared significantly high and proportional to the duration of wastewater irrigation. Also, concentrations of total Ni, Zn, Cu, Pb, and Cd increased significantly (P?≤?0.05) according to the number of irrigation years but are usually under Tunisian standards. The concentration of heavy metals (Ni, Zn, Cu, Pb, and Cd) showed a significant decrease in the soil profile. The microbial biomass carbon (MB_C) is 1.5 times larger in the soil irrigated for 15 years with treated wastewater as compared to the one taken as control. The growth of microorganisms might be explained by the ready source of easily degradable compounds in the oligotrophic soil environment brought about by wastewater irrigation. Soil bacteriological analysis showed that the number of fecal coliforms (FC) and that of fecal streptococci (FS) were affected appreciably (P?≤?0.05) by the duration of wastewater application (number of years) and by the soil depth (0–20, 20–40, and 40–60 cm).

Conclusions

Treated wastewater irrigation led to changes in physicochemical and microbiological soil properties. The magnitude and specificity of these changes significantly correlated with the duration of such practice. It can be concluded, based on these results, that the proper management of wastewater irrigation and periodic monitoring of soil fertility and quality parameters are required to ensure successful, safe, and long-term reuse of wastewater irrigation.     

Macrokinetics of microbial populations in soil

The model of macrokinetics of microbial populations in soil was deducted based on the assumption that the resulting indicator of productivity is the consequence of simultaneously occurring but oppositely directed processes of growth and death, which are proportional to biomass and inversely proportional to time (for growth to the time raised to the power whose indicator is greater than one). The adequacy of the model for the experiment has been confirmed by comparison with reliable data in the literature on the study of microbial growth in soil in laboratory conditions. The model’s application indicated eight intervals on the curve of microbial growth, each of which characterizes the unique combination of the status of three kinetic characteristics: the biomass index, and the rate of its growth and acceleration. The intervals are strictly defined and conform to the notion of qualitatively different growth phases.     

种植利用紫云英对稻田土壤微生物及酶活性的影响

为研究种植利用紫云英对稻田土壤的培肥作用,设不种紫云英不施化肥(对照)、种植紫云英不施化肥(紫云英)、不种紫云英施化肥(单施化肥)、种植紫云英同时施化肥(紫云英+化肥)4个处理,分析了各处理在紫云英苗期、紫云英盛花期、早稻收获期、晚稻收获期的土壤养分、酶活性、微生物数量。结果表明,与对照相比,施化肥及种植翻压紫云英均能改善土壤养分状况,紫云英与化肥配施效果最好,能够明显提高土壤速效磷、速效钾、全氮含量。施用化肥或种植翻压紫云英均能促进微生物生长,4个处理中以紫云英+化肥处理对土壤微生物生长有较好的促进作用,其次为单施化肥处理。     

化肥施用对土壤中重金属生物有效性的影响研究

本研究综述了化学肥料施用对土壤中重金属生物有效性的影响,土壤施肥能改变土壤理化性质如pH、溶液中的离子组成、阳离子交换量等,或直接与重金属离子发生反应,影响重金属的生物有效性。故施用化肥时要考虑到所施化肥的种类和施用量可能会导致重金属的植物可利用性发生改变,从而对人体健康产生影响。     

Effects of heavy metals in soil on microbial diversity and activity as shown by the sensitivity-resistance index,an ecologically relevant parameter

A sensitivity-resistance index was developed, and proved to be a very sensitive biomonitor of soil pollution with heavy metals. The index was developed by a step-by-step approach. Ultimately, the bacterial soil microflora was divided into three groups, senstivive, tolerant, and resistant microflora. Zn and Cd sensitivity was defined as no growth occurring in the presence of 5 and 0.5 mg l^-1 of these metals, respectively, while resistance was defined as distinct growth in the presence of 50 and 16 mg l^-1, respectively. The sensitivity: resistance ratio of a referent clay soil (0.57 mg Cd kg^-1 and 140 mg Zn kg^-1) was 0.53, but for polluted (6 mg Cd kg^-1 + 670 mg Zn kg^-1) clay soil, the ratio was 0.24. For a referent (0.06 mg Cd kg^-1 + 12 mg Zn kg^-1) sandy soil the sensitivity: resistance ratio was 1.50 whereas polluted (2.3 mg Cd kg^-1 + 252 mg Zn kg^-1) sandy soil had a ratio 0.19. The ecological value of the sensitivity-resistance lies in its capacity to reflect potential deradation of aromatic compounds. It has been shown repeatedly that sensitive bacteria grow significantly better on a range of selected aromatic compounds. It has been speculated that resistance fo heavy metals may reduce the bioremediation capacity of soil towards chlorinated aromatics and polyaromatic hydrocarbons.     

Solubility of heavy metals in a contaminated soil: Effects of redox potential and pH

To assess the mobilities of Pb, Cd, and Zn from a contaminated soil, the effects of redox potential and pH value on metal solubilities were investigated. Both redox potential and pH were found to greatly affect heavy metal solubility in the soil. Results showed that the soil suspension under continuous oxygen aeration for 21 days resulted in increases of redox potential from 290 to 440 mV and pH value from 6.9 to 7.0, respectively. Soluble concentrations of Pb, Cd, and Zn varied with time, and were all lower than 1 mg kg^?1. When the soil suspension was aerated with nitrogen, final redox potential was ?140 mV and pH value of 7.1. The soluble metal concentrations were slightly higher than those aerated with oxygen. The equilibrium solubility experiments were conducted under three different pH values (3.3, 5.0, 8.0) and three redox potential (325, 0, ?100 mV). Results showed that metals were sparingly soluble under alkaline conditions (pH = 8.0). Metal solubilities were higher when under slightly acidic conditions (pH = 5.0), and increased drastically when pH was kept at 3.3. When solubilities were compared under same pH values, it was observed that metal solubilities increased as redox potential decreased. Generally speaking, acidic and reducing conditions were most favorable for metal solubilization, and the effect of pH was more significant than that of redox potential. It was proposed that heavy metals were mostly adsorbed onto Fe-Mn oxyhydroxides. The pH-dependent metal adsorption reaction and the dissolution of Fe-Mn oxyhydroxides under reducing conditions was the mechanism controlling the release of heavy metals from soils.     

Effects of air drying on the respiration of forest soil microbial populations

Microbial populations from dried, remoistened and undried forest organic horizons were allowed to thoroughly recolonize sterilized forest organic horizons; oxygen uptake was followed during recolonization. Microbial damage due to air drying was in the order litter (L) > fermentation (F) > humus (H). Species diversity, as indicated by diminished O₂ uptake, was reduced in all horizons which had been air dried, with greatest damage in the litter horizon. Samples of these forest organic soil materials, designated for the study of microbially driven processes, should not be air dried.     

Effects of Folsomia candida (Collembola) on the microbial biomass in a grassland soil

Summary The effects of the presence of Folsomia candida on substrate-induced respiration, CO₂-C evolution, bacterial count and NH ₄ ⁺ -N were investigated in a grassland soil. Differences in these parameters, with the exception of NH ₄ ⁺ , were correlated with the age of the collembolan Folsomia candida. In the presence of juvenile animals total CO₂-C evolution was enhanced, but substrate-induced respiration and the bacterial count were unchanged. In fumigated soil with imagos, substrate-induced respiration and the number of bacteria were increased, but total CO₂-C evolution was unaltered. Different food selection strategies between adults and juvenile animals may explain the results.     

Biogeochemistry of heavy metals in contaminated excessively moistened soils (Analytical review)

The biogeochemical behavior of heavy metals in contaminated excessively moistened soils depends on the development of reducing conditions (either moderate or strong). Upon the moderate biogenic reduction, Cr as the metal with variable valence forms low-soluble compounds, which decreases its availability to plants and prevents its penetration into surface- and groundwater. Creation of artificial barriers for Cr fixation on contaminated sites is based on the stimulation of natural metal-reducing bacteria. Arsenic, being a metalloid with a variable valence, is mobilized upon the moderate biogenic reduction. The mobility of siderophilic heavy metals with a constant valence grows under the moderate reducing conditions at the expense of dissolution of iron (hydr)oxides as carriers of these metals. Zinc, which can enter the newly formed goethite lattice, is an exception. Strong reduction processes in organic excessively moist and flooded soils (usually enriched in S) lead to the formation of low-soluble sulfides of heavy elements with both variable (As) and constant (Cu, Ni, Zn, and Pb) valence. On changing aquatic regime in overmoistened soils and their drying, sulfides of heavy metals are oxidized, and previously fixed metals are mobilized.     

Long‐term effects on soil microbial properties of heavy metals from industrial exhaust deposition

Soil samples were taken at 0—10 cm and 10—20 cm depth from 7 clay‐marsh sites used as grassland close to Nordenham in the north of Lower Saxony, Germany. The sites had been contaminated by deposition of heavy metals from industrial exhausts, the level of contamination varying according to their distances from a lead factory. The soils were analyzed to assess the depth‐specific effects of NH₄NO₃ extractable and total amounts of Zn, Pb, and Cu on basal respiration, adenylates, ergosterol, and biomass C estimated by fumigation extraction (FE) and substrate‐induced respiration (SIR). Most of the chemical and biological properties studied decreased with depth, but depth‐specific differences in the relationships between these properties rarely occurred. The biomass C/soil organic C ratio was at a relatively high level, but most consistently reflected pollution as a decrease with increasing heavy metal load, independently of the method used for biomass C estimation. However, the SIR estimates were on average 44 % lower than those of FE, mainly due to pH effects. The metabolic quotient SIR‐qCO₂ increased with increasing NH₄NO₃ extractable and total heavy metal contents, but also with decreasing pH, whereas the FE‐qCO₂ remained unaffected by heavy metals and pH. The ATP/FE‐biomass C ratio was on average 8.2 μmol g^—1 and negatively affected by soil pH, but also by total Zn, NH₄NO₃ extractable Zn and Cu. The ergosterol/FE‐biomass C ratio was on average 0.29 %, i.e. at a very low level, and increased with increasing heavy metal content. This indicates a change in the community structure towards fungi.     

水分特征对伴矿景天生长和重金属吸收性的影响

利用盆栽试验研究了水分特征对伴矿景天生长和重金属吸收性的影响.结果表明:在70% 土壤最大田间持水量(70%WHC)处理下,伴矿景天生长最好,其地上部鲜重显著高于其他处理;70%WHC处理伴矿景天对重金属吸收能力最强,其茎中Zn的浓度显著高于其他处理,茎中Cd的浓度分别比35%WHC、100%WHC、淹水处理高27.1%、29.0%、63.1%;叶中的Zn浓度表现出与茎中相同趋势,叶中Cd的浓度与茎中不同,以100%WHC处理最高.70%WHC处理下,植物提取Zn、Cd的效率最高,其修复效率均显著高于其他处理.这些结果表明,土壤水分状况在土壤重金属污染伴矿景天植物吸取修复中起着重要作用.     

扬州市土壤重金属含量的调查

对全市385个土样分析结果表明,扬州市耕地土壤总体是清洁的,全市89 35%的土壤属于清洁地范围(综合污染指数≤0 7),7 53%的土壤属尚清洁范围(综合污染指数0 7～1 0之间),3 12%的土壤属轻度污染(综合污染指数1 0～2 0之间)。全市土壤重金属污染的顺序是Hg>Cr>As>Cu>Pb>Cd,各地区中,以邗江区土壤重金属含量相对较高。