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Variable charge soils make up a large percentage of agricultural soils in the world.These soils have a characteristic low pH and cation exchange capacity(CEC)and are severely affected by acidification and pollution.Plants growing in acidic soils are constantly being affected by the negative impacts of soil acidification like A1and Mn toxicity and this has resulted in soil infertility and low agricultural productivity.The relationship between plants and rhizobacteria has been exploited in several studies.While in soil,rhizobacteria promote plant growth by enhancing nutrient acquisition and protecting plants against pathogens.Of the most studied bacteria species,Pseudomonas,Bacilli,and Escherichia coli have received considerable attention.However,very little report exists in the literature regarding the interaction of variable charge soils with bacteria or their extracellular polymeric substances(EPS).
The role bacterial EPS play in bacterial adhesion in variable charge soils was studied using four bulk variable charge soils;an Oxisol from Haikou,three Ultisols(Guangzhou,Liuzhou,and Jinxian)and one constant cbarge Alfisol from Nanjing.The bacteria were cultured in beef extract-peptone and had their EPS removed following a24h magnetic stirring with cation exchange resin(CER).The effect of EPS removal on the zeta potential and adhesion behavior o f Pseudomonas fluorescens,Bacillus subtilis,and Escherichia coli were studied at varied and constant pH,with varying amounts of bacteria(0-68mg mL-1for100mg soil).It was observed that the adhesion ability of the bacteria was reduced after removal of EPS with CER at pH5.6.This suggested increased bacterial mobility in soils under EPS-deficient conditions,with E.coli demonstrating the highest mobility.Although EPS removal reduced the bacterial adhesion,their electrokinetic properties were only slightly affected.Nevertheless,bacterial adhesion increased the negative zeta potential of the soils and also enhanced Mg2+adsorption.The spectroscopic analysis demonstrated that the reduced adhesion observed for EPS-deficient cells was due to a reduction in the concentration of important functional groups needed for bonding with soils.The adsorption capacities for the untreated bacteria were significantly(p＜0.05)different,and corresponded to609.3,1113.9,2169.2mg g-1(Ultisol-Guangzhou);434.7,837.5,2067.9mg g-1(Oxisol-Haikou);and134,632.3,408.5mg g-1(Alfisol-Nanjing)for E.coli,P.fluorescens,and B.subtilis,respectively.For these soils,the content of EPS,soil pH,CEC,and Fe2O3content greatly influenced the adhesion ability of the bacteria.
EPS is ubiquitous in soils and has oxygen-containing functional groups capable of deprotonating or protonating as soil pH changes.This study investigated the role EPS from Escherichia coli plays on the adsorption behaviors of Cu(Ⅱ)and Cd(Ⅱ)on variable charge soils collected from different regions in China.The soils included Oxisol-Haikou,UltisolGuangzhou,and Alfisol-Nanjing,which were interacted with EPS suspension(40mg L-1for500mg sofl)and varying amounts of heavy metal cations(0.5-3.0mM).It was observed that EPS enhanced the negative surface charge of the soils and shifted the zeta potentials in a negative direction:from-18.6to-26.4mV for Alfisol,+5.1to-22.2mV for Oxisol,and+0.3to-28.0mV for Ultisol at pH5.0.By adsorbing EPS,the fraction of heavy metals adsorbed through the specific mechanism(formation of complexes)was increased and their bioavailability in the soils decreased.For both metals,Cd(Ⅱ)was the least adsorbed and the most bioavailable since it was mostly adsorbed via the electrostatic mechanism.
The coexistence of low molecular weight(LMW)organic acids and EPS in soils can either be beneficial or not with regards to heavy metals bioavailability.The single or combined effects of citrate,oxalate,and EPS(117mg L-1)on Pb sorption by an acidic Ultisol(from Guangzhou)was studied.Zeta potential investigation revealed that both EPS and the organic anions had a positive effect on the negative zeta potential of the soil.The increment in the negative zeta potential caused by the ligands was oxalate＞citrate＞EPS.By extrapolation fiom zeta potential-pH curve,it was observed that the isoelectric point of the Ultisol shifted from4.8to3.2when it interacted with EPS and below3.0with citrate and oxalate.The adsorption of Pb was highly promoted in the presence ofoxalate compared to citrate and EPS.Of all the ligands,EPS exhibited a more positive effect on Pb adsorption at higher concentrations(800mg L-1)whereas citrate inhibited adsorption at this concentration(and pH＞5.5).Pb adsorption was governed by both electrostatic attraction and complexation.In combination,EPS-citrate-oxalate in the same system was more effective in enhancing Pb adsorption compared to EPS-oxalate and EPS-citrate.
Acidification in variable charge soils is the leading cause of soil infertility and decreased agricultural productivity.In a laboratory setting,we investigated the role played by P.fluorescens(93.95g kg-1)in the acidification of two Ultisols(from Yingtan and Liuzhou)using HNO3as model acid rain.It was observed that the pH of the bacteria-treated soil was higher than that of control,implying that P.fluorescens inhibited acidification.The mechanism for inhibiting acidification was the interaction of organic anions(-COO-or-O-)on the bacteria surface with protons to form neutral molecules(-COOH or-OH).Also,the quantities of soil soluble and exchangeable Al3+were markedly reduced during acidification in the bacteria-treated soil.This was attributed to the high pH in the bacteriatreated soil coupled with the ability of bacteria to promote complexation of free Al3+.Generally,bacterial adhesion increased soil effective cation exchange capacity(ECEC)and exchangcable base cations at each pH.The result of this study demonstrates the important role rhizobacteria play during soil acidification in enhancing the pH buffering capacity(pHBC)of the soil.
The indiscriminate use of nitrogenous fertilizers in agriculture is one of the major reasons for accelerated soil acidification.The functions of ammonia-oxidizing bacteria(AOB)and ammonia-oxidizing archaea(AOA)are central in the nitrogen cycle as they oxidize NH3to NO3-.Besides,the efficiency and abundance of AOA and AOB depend on soil pH and NH3concentration.Importantly,very little research has been done regarding how other non-nitrogen fixers such as P.fluorescens and B.subtilis can affect the nitrification process either directly or indirectly.In this study,the effect of P.fluorescens(27.4g kg-1)on the abundance of AOA/AOB and nitrification of urea(202mg N kg-1soil)was investigated within a70d incubation period using an acidic Ultisol(from Langxi).It was observed that P.fluorescens inhibited soil acidification,increased soil pH,increased the content of NH4+-N,and favored the growth of AOB by32.2％while decreasing the abundance of AOA by36.5％.The bacteria inhibited soil acidification by(i)consuming protons released during nitrification,(ii)enhancing soil pH through the release of OH-from the soil by ligand exchange reactions and(iii)prolonging the hydrolysis of urea and delaying nitrification.Also,P.fluorescens inhibited nitrification within the first36days of incubation and promoted it thereafter.The net nitrification rate was negative for bacteriatreated soil(-9.94mmol N kg-1d-1)compared to a positive value(1.77mmol N kg-1d-1)for the control.This suggested that the proton consuming ability of the bacteria was superior to the proton producing ability of nitrification.
The role bacterial EPS play in bacterial adhesion in variable charge soils was studied using four bulk variable charge soils;an Oxisol from Haikou,three Ultisols(Guangzhou,Liuzhou,and Jinxian)and one constant cbarge Alfisol from Nanjing.The bacteria were cultured in beef extract-peptone and had their EPS removed following a24h magnetic stirring with cation exchange resin(CER).The effect of EPS removal on the zeta potential and adhesion behavior o f Pseudomonas fluorescens,Bacillus subtilis,and Escherichia coli were studied at varied and constant pH,with varying amounts of bacteria(0-68mg mL-1for100mg soil).It was observed that the adhesion ability of the bacteria was reduced after removal of EPS with CER at pH5.6.This suggested increased bacterial mobility in soils under EPS-deficient conditions,with E.coli demonstrating the highest mobility.Although EPS removal reduced the bacterial adhesion,their electrokinetic properties were only slightly affected.Nevertheless,bacterial adhesion increased the negative zeta potential of the soils and also enhanced Mg2+adsorption.The spectroscopic analysis demonstrated that the reduced adhesion observed for EPS-deficient cells was due to a reduction in the concentration of important functional groups needed for bonding with soils.The adsorption capacities for the untreated bacteria were significantly(p＜0.05)different,and corresponded to609.3,1113.9,2169.2mg g-1(Ultisol-Guangzhou);434.7,837.5,2067.9mg g-1(Oxisol-Haikou);and134,632.3,408.5mg g-1(Alfisol-Nanjing)for E.coli,P.fluorescens,and B.subtilis,respectively.For these soils,the content of EPS,soil pH,CEC,and Fe2O3content greatly influenced the adhesion ability of the bacteria.
EPS is ubiquitous in soils and has oxygen-containing functional groups capable of deprotonating or protonating as soil pH changes.This study investigated the role EPS from Escherichia coli plays on the adsorption behaviors of Cu(Ⅱ)and Cd(Ⅱ)on variable charge soils collected from different regions in China.The soils included Oxisol-Haikou,UltisolGuangzhou,and Alfisol-Nanjing,which were interacted with EPS suspension(40mg L-1for500mg sofl)and varying amounts of heavy metal cations(0.5-3.0mM).It was observed that EPS enhanced the negative surface charge of the soils and shifted the zeta potentials in a negative direction:from-18.6to-26.4mV for Alfisol,+5.1to-22.2mV for Oxisol,and+0.3to-28.0mV for Ultisol at pH5.0.By adsorbing EPS,the fraction of heavy metals adsorbed through the specific mechanism(formation of complexes)was increased and their bioavailability in the soils decreased.For both metals,Cd(Ⅱ)was the least adsorbed and the most bioavailable since it was mostly adsorbed via the electrostatic mechanism.
The coexistence of low molecular weight(LMW)organic acids and EPS in soils can either be beneficial or not with regards to heavy metals bioavailability.The single or combined effects of citrate,oxalate,and EPS(117mg L-1)on Pb sorption by an acidic Ultisol(from Guangzhou)was studied.Zeta potential investigation revealed that both EPS and the organic anions had a positive effect on the negative zeta potential of the soil.The increment in the negative zeta potential caused by the ligands was oxalate＞citrate＞EPS.By extrapolation fiom zeta potential-pH curve,it was observed that the isoelectric point of the Ultisol shifted from4.8to3.2when it interacted with EPS and below3.0with citrate and oxalate.The adsorption of Pb was highly promoted in the presence ofoxalate compared to citrate and EPS.Of all the ligands,EPS exhibited a more positive effect on Pb adsorption at higher concentrations(800mg L-1)whereas citrate inhibited adsorption at this concentration(and pH＞5.5).Pb adsorption was governed by both electrostatic attraction and complexation.In combination,EPS-citrate-oxalate in the same system was more effective in enhancing Pb adsorption compared to EPS-oxalate and EPS-citrate.
Acidification in variable charge soils is the leading cause of soil infertility and decreased agricultural productivity.In a laboratory setting,we investigated the role played by P.fluorescens(93.95g kg-1)in the acidification of two Ultisols(from Yingtan and Liuzhou)using HNO3as model acid rain.It was observed that the pH of the bacteria-treated soil was higher than that of control,implying that P.fluorescens inhibited acidification.The mechanism for inhibiting acidification was the interaction of organic anions(-COO-or-O-)on the bacteria surface with protons to form neutral molecules(-COOH or-OH).Also,the quantities of soil soluble and exchangeable Al3+were markedly reduced during acidification in the bacteria-treated soil.This was attributed to the high pH in the bacteriatreated soil coupled with the ability of bacteria to promote complexation of free Al3+.Generally,bacterial adhesion increased soil effective cation exchange capacity(ECEC)and exchangcable base cations at each pH.The result of this study demonstrates the important role rhizobacteria play during soil acidification in enhancing the pH buffering capacity(pHBC)of the soil.
The indiscriminate use of nitrogenous fertilizers in agriculture is one of the major reasons for accelerated soil acidification.The functions of ammonia-oxidizing bacteria(AOB)and ammonia-oxidizing archaea(AOA)are central in the nitrogen cycle as they oxidize NH3to NO3-.Besides,the efficiency and abundance of AOA and AOB depend on soil pH and NH3concentration.Importantly,very little research has been done regarding how other non-nitrogen fixers such as P.fluorescens and B.subtilis can affect the nitrification process either directly or indirectly.In this study,the effect of P.fluorescens(27.4g kg-1)on the abundance of AOA/AOB and nitrification of urea(202mg N kg-1soil)was investigated within a70d incubation period using an acidic Ultisol(from Langxi).It was observed that P.fluorescens inhibited soil acidification,increased soil pH,increased the content of NH4+-N,and favored the growth of AOB by32.2％while decreasing the abundance of AOA by36.5％.The bacteria inhibited soil acidification by(i)consuming protons released during nitrification,(ii)enhancing soil pH through the release of OH-from the soil by ligand exchange reactions and(iii)prolonging the hydrolysis of urea and delaying nitrification.Also,P.fluorescens inhibited nitrification within the first36days of incubation and promoted it thereafter.The net nitrification rate was negative for bacteriatreated soil(-9.94mmol N kg-1d-1)compared to a positive value(1.77mmol N kg-1d-1)for the control.This suggested that the proton consuming ability of the bacteria was superior to the proton producing ability of nitrification.