Low molecular weight organic acid adsorption in forest soils effects on soil solution concentrations

Low molecular weight organic acid adsorption in forest soils:effectson soil solution concentrations and biodegradation ratesP.A.W.van Hees a ,S.I.Vinogradoff b ,A.C.Edwards c ,D.L.Godbold b ,D.L.Jones b,*aDepartment of Chemistry and Process Technology,Mid Sweden University,85170Sundsvall,SwedenbSchool of Agricultural and Forest Sciences,University of Wales,Deiniol Road,Bangor,Gwynedd LL572UW,UKcMacaulay Institute,Craigiebuckler,Aberdeen AB158QH,UKReceived 7August 2002;received in revised form 17March 2003;accepted 19March 2003AbstractLow molecular weight (LMW)organic acids are believed to play a key role in many rhizosphere and pedogenic processes;However,their efficiency is likely to depend on their susceptibility to sorption and biodegradation.The sorption characteristics of three organic acids (citrate,oxalate and acetate)and phosphate were examined over the concentration range 0–1000m M in three coniferous forest soil profiles.Sorption to the soil’s solid phase could be adequately described by the Langmuir equation with sorption capacity following the horizon series:B .C .E .O.The strength of anion sorption followed the series:phosphate .oxalate $citrate q acetate.Calculations indicated that between 50and 95%(O and E horizons)and .93%(B horizons)of these LMW organic acids entering the soil will become sorbed to the solid phase.The amount of organic acids predicted to be present on the solid phase at typical soil solution concentrations ranged from ,1to 1100nmol g 21yielding adsorbed-to-solution ratios (adsorption coefficients)of between ,0.1and 3100.In the case of citrate,sorption to the solid phase significantly reduced its biodegradation potential by 35–99%depending upon the degree and type of sorption surface.The findings of this work are discussed in the context of the quantitative effects of adsorption on organic acids,their ecological functions and role in soil forming processes.q 2003Elsevier Science Ltd.All rights reserved.Keywords:Adsorption;Biodegradation;Citrate;Forest soil;Organic acids;Oxalate;Phosphate;Podzols1.IntroductionBiogeochemical processes that involve low molecular weight (LMW)organic acids (,500MW)can be of considerable ecological importance (Ryan et al.,2001).These simple carboxylic acids (e.g.acetate,citrate and oxalate)are present in most soil systems with concentrations normally in the low micromolar range (1–20m M;Strobel,2001).Some of the key roles of organic acids in the soil environment are thought to include the mobilisation of nutrients by plants and microorganisms (e.g.Fe,Zn,P)and the detoxification of metals by plant roots (e.g.Al,Cd,Zn;Jones,1998).Organic acids can also act as efficient weathering agents and promote the dissolution of primary minerals in soils (Drever and Stillings,1997).Conse-quently,they have been implicated in pedogenic processessuch as podzolisation,where LWM organic acids complex up to a third of the Al in soil solution from coniferous forestsoils (Lundstro¨m et al.,2000;van Hees et al.,2000;Farmer and Lumsdon,2001).During leaching through the soil profile the organic components of the organo-metallic complexes are susceptible to biodegradation,which can result in the subsequent precipitation of secondary minerals which may be of great importance for Bs horizon formation(Lundstro¨m et al.,2000;Farmer and Lumsdon,2001).Since many LMW organic acids are very reactive,their concentration in soil solution is influenced by a number of factors relating to the properties of the soil as well as the vegetation and seasonality (Jones,1998;Strobel,2001).Moreover,these factors may directly determine the efficiency with which they participate in various soil processes.Certain organic acids show a strong affinity for certain mineral surfaces,and these have been used extensively as model anions for sorption studies (Parfitt et al.,1977a,b;Jones et al.,1996a,b;Karltun,1998;Jones0038-0717/03/$-see front matter q 2003Elsevier Science Ltd.All rights reserved.doi:10.1016/S0038-0717(03)00144-5Soil Biology &Biochemistry 35(2003)1015–1026/locate/soilbio*Corresponding author.Tel.:þ44-1248-382579;fax:þ44-1248-354997.E-mail address:d.jones@ (D.L.Jones).and Brassington,1998;Geelhoed et al.,1998;Christofaro et al.,2000).Furthermore,adsorption has been shown to significantly influence the concentrations of organic acids in soil solution(Jones et al.,1996b;Jones and Brassington, 1998;Ponizovsky et al.,1999;Hu et al.,2001).Dissolved LMW organic acids also form an important source of labile C for soil microorganisms(Boudot,1992; Jones and Darrah,1994;van Hees et al.,2002).Typically, LMW organic acids are rapidly degraded in soil with half-lives ranging from0.5to12h(Jones and Darrah,1994; Jones et al.,2001).The rate of mineralization,however,is likely to be dependent upon factors such as microbial biomass and community structure as well as microbial transporter expression(Jones,1998).Adsorption has been found to significantly reduce the decomposition rates of organic acids in model systems(Boudot,1992;Jones and Edwards,1998).Few detailed studies have been undertaken where the sorption characteristics of a soil are measured in the context of realistic soil solution organic acid concentrations. Furthermore,our understanding of the interaction between adsorption and organic acid biodegradation is limited and controversial(Jones et al.,2002).Again,this interaction must be assessed using environmentally realistic concen-trations.Our aim was to examine the quantitative effects of adsorption in three coniferous forest soil profiles.The effect of adsorption on the biodegradation of citrate was evaluated in a model system with mineral phases known to be present in these soil profiles.2.Materials and methods2.1.SoilsSoil was collected from three sites as described in Van Hees et al.(2002).Thefirst two,Nya¨nget(Ny)and Heden (He),were located in the Svartberget Research Park,70km NW of Umea˚,Sweden(648140N,108460E).Both sites are predominantly forested with Scots pine(Pinus sylvestris) and Norway spruce(Picea abies).The understory layer consisted of dwarf Bilberry(Vaccinium myrtillus),Cow-berry(Vaccinium vitis-idaea)and mosses.Both soils are classified as Haplic Arenosols(FAO-Unesco,1988)and are developed on either a glacial till(Ny)or sediment(He) parent material.The third site was located at Delamere(De), Cheshire,UK(538140N,28400W).The site was predomi-nantly forested with Scots pine(P.sylvestris)with a small amount of oak(Quercus robur)also present.The soil was classified as an Orthic Podzol(FAO-Unesco,1988) developed on triassic sandstone parent material.The understory layer consisted of bracken(Pteridium aquili-num)and a sparse mixture of grasses.The upper organic soil layer(O horizon;0–8cm)was collected and living matter such as pieces of moss,grass and herbs removed.The organic soil layer was then divided into two layers,O1(upper,0–4cm)and O2(lower,5–8cm)for Ny and He,and Ol(0–4cm)and Of(5–8cm)for De.At the Ny and Hefield sites,samples were taken in the middle of the E horizon(8–15cm)and below the E horizon at15–20cm(Bs1),20–35cm(Bs2),40–45cm(C1)and80–85cm(C2).At the Defield site soil samples below the O horizon were taken at depths of0–5cm(Ah),10–15cm (E),40–45cm(Bh),60–65cm(Bs)and90–95cm(B/C).Soil for the sorption studies was sampled from the mineral horizons in three vertical transects located50cm apart.Soil for centrifugal removal of soil solution was taken by driving5cm dia cylindrical,plastic sampling devices 7cm horizontally into the side of the profile with one sample removed from each vertical transect(i.e.three independent replicate samples per depth per site).All soil samples were kept at48C until soil solution extraction, which was performed within24h of sample collection.The samples for the microbial decomposition studies were stored in plastic bags at either88C(Ny and He)or128C (De).2.2.Soil solution extraction and chemical analysisThe centrifugation drainage technique described by Giesler and Lundstro¨m(1993)and van Hees et al.(2002) was used to obtain soil anic acids and inorganic anions in the soil solution were analysed by capillary electrophoresis(Go¨ttlein,1998;Dahle´n et al., 2000).2.3.Sorption isotherms of soilsSorption isotherms were measured for citrate,oxalate, acetate and phosphate by the method of Jones and Brassington(1998).A14C-radiolabeled organic acid (specific activity0.07kBq ml21)or KH2PO4solution (2.5ml)was added to0.50g of chloroform-fumigated (48h)field-moist soil contained in6ml plastic vials(soil-to-solution ratio1:5(w/v)).Three organic acids,14C-citric acid(1,5-14C;ICN Pharmaceuticals Inc.,Irvine,CA;3.7GBq mmol21),14C-acetic acid(1-14C;ICN Pharmaceu-ticals Inc.,Irvine,CA;2.2GBq mmol21)and14C-oxalic acid(1,2-14C;Sigma Chemical Co.,St Louis,MO;0.2GBq mmol21)were used in the sorption assays.The initial solution anion concentration was50,250,500or 1000m M and the pH adjusted to4.5with1.0mM KOH prior to addition to the soil.The isotherms were measured in a background of1mM KCl.Following addition,the sample was shaken for10min on a reciprocating shaker operating at a speed of320rpm.The samples were subsequently centrifuged(16000g;5min)and the supernatant solution recovered.The equilibrium solution organic acid concen-tration was determined by liquid scintillation counting (Wallac1409scintillation counter,Wallac EG&G Ltd, Milton Keynes,UK)using Wallac Optiphase3scintillation fluid(Wallac EG&G Ltd,Milton Keynes,UK).P.A.W.van Hees et al./Soil Biology&Biochemistry35(2003)1015–1026 1016The equilibrium solution phosphate concentration was determined by capillary electrophoresis as described above.The Langmuir equation was thenfitted to the experimental results whereA¼ðK s£S max£CÞ=ð1þK s£CÞin which A is the amount of anion adsorbed(m mol g21), S max is the maximum anion sorption capacity(m mol g21),C is the equilibrium solution concentration(m mol cm23),and K s is the affinity constant.The soil’s adsorption coefficient (b)was calculated using the following equation(Barber, 1995):b¼C tot=C where C tot¼ðC£QÞþðA£gÞwhere C tot is the total amount of anion in the soil (m mol cm23),C is the soil solution concentration (m mol cm23),Q is the volumetric water content (cm3cm23)and g is the soil bulk density(g cm23).2.4.Biodegradation of citrate in soil in the presenceof added ferrihydriteThe effect of sorption on organic acid biodegradation was assessed by altering the sorption capacity of the Nya¨nget E horizon soil(intrinsically low sorption capacity) through the addition of different amounts of ferrihydrite (high anion adsorption capacity).Different amounts of(1) a synthetic ferrihydrite(Fe(OH)3content in suspension¼42g l21;Jones et al.,1996a)and(2)a natural siliceous ferrihydrite(T.Fraser m.) collected from a groundwater well in Aberdeenshire,UK, were mixed with1g offield moist Nya¨nget E horizon soil contained in50ml polypropylene tubes.A14C-radio-labeled citrate solution(100m M;specific activity 0.5kBq ml21)was added to the soil according to two procedures.In Procedure1,the ferrihydrite was incorpor-ated with the soil by shaking,left to stand for1h and then the14C-citrate solution added to the soil.In Procedure2,the14C-citrate solution wasfirst added to the ferrihydrite,the mixture allowed to equilibrate for 10min,and the suspension added to the soil.Following14C-citrate addition,the soil was gently shaken to ensure mixing and incubated at128C in gas-tight tubes.Placing a vial containing 1.0ml of1M NaOH inside the tube collected the14CO2produced during the biodegradation of the14C-citrate.The14CO2 trapped as NaH14CO3in the NaOH was determined by liquid scintillation as described above.NaOH traps were sampled at1,3,6,24,48and72h after14C-citrate addition.The two ferrihydrites were added to the Nya¨nget E horizon soil at different rates to achieve sorption capacities of25,50,75and100%of that present in a strongly adsorbing Bs horizon from the same soil profile.In a separate experiment,KH2PO4was added to the synthetic ferrihydrite tofill up the sorption sites with the strongly bound PO432anion.From the sorption isotherm (Section2.6below),KH2PO4was added to saturate25, 50or100%of the measured sorption capacity of the synthetic ferrihydrite.Citrate biodegradation in the presence of the differentially P-loaded ferrihydrite was then measured as described above using both Procedure1 and2.The ferrihydrite was added at a level equivalent to 25%of the Bs horizon sorption capacity.2.5.Sorption isotherms of ferrihydriteFor both the synthetic and natural ferrihydrite,sorption isotherms were determined for citrate and phosphate in the concentration range50–5000m M(pH 4.5)and the Langmuir equationfitted to the results.Briefly,50m l of artificial ferrihydrite suspension(Fe(OH)3content of suspension¼42g l21)or100mg natural ferrihydrite were shaken for10min(320rpm)with1ml of KH2PO4 or14C-citrate solution as described above.The samples were then centrifuged(16000g;5min)and the supernatant removed for equilibrium solution concentration analysis as outlined above.2.6.Calculations and statisticsLangmuir adsorption parameters were determined using a BASIC computerized least squares optimization pro-cedure(N.J.Barrow,m.).Testing for statistical significanceðP,0:05Þwas made using F and t-tests ANOVA(MINITAB1.3,Minitab Inc.).Most experiments were performed in duplicate or triplicate unless otherwise stated.3.Results3.1.Adsorption isotherms of organic acids and phosphateSorption isotherms for the three soil profiles are shown in Fig.1.Generally,the data conformed well to a single Langmuir equation with r2values exceeding0.99for all soil horizons with the exception of phosphate in the O horizons. The degree of sorption to the soil’s solid phase was dependent on both anion type,soil horizon and site.Across almost all the soil samples tested,the degree of sorption followed the seriesphosphate.oxalate$citrate q acetate:Among the soil horizons sampled,the sorption capacity was generally in the orderB.C.E.O:Generally,B horizons containing high sesquioxide contents possessed a high affinity for citrate and phosphate sorption.P.A.W.van Hees et al./Soil Biology&Biochemistry35(2003)1015–10261017This was most pronounced for the Nya¨nget and Heden Bs1and Bs2horizons.Oxalate sorption was substantial in all the organic soil horizons (Fig.1),particularly in comparison to citrate.Across the three sites examined here,the adsorptioncapacity followed the order Nya¨nget .Heden .Delamere.The amount of anion sorption ðA Þpredicted from the isotherm and the ratio of anion held on the solid phase to that in soil solution (adsorption coefficient)at actual soil solution concentrations were calculated and the results are presented in Table 1.For the three organic acids examined here and across all sites,the mean (^SEM)organic acid ‘solid-to-solution’ratio (adsorption coefficient)was 3.5^0.4for the O horizon, 3.8^0.4for the A/E horizons,349^221for the B horizons and 88^32for the C horizons.The adsorption coefficientsfollowedFig.1.Concentration-dependent sorption of citrate,oxalate,acetate and phosphate in different horizons from three coniferous forest soil profiles (Nya¨nget,Heden and Delamere).Symbols denote experimental points while the curves represent Langmuir isotherms fitted to the experimental data.Only mean data points are shown,(SEM ,5%of the mean value for the organic acids;SEM ,10%of the mean value for phosphate).P.A.W.van Hees et al./Soil Biology &Biochemistry 35(2003)1015–10261018the trends of the sorption isotherms with the highest values observed for oxalate and phosphate.The results also showthat the O horizons of the Nya¨nget and Heden profiles in particular constitute a substantial sink for oxalate.The soil solution composition is discussed in greater detail in van Hees et al.(2002).Adsorption coefficients can be used to provide a simple comparable estimate of the degree to which the solid phase buffers soil solution anion concentrations and are routinely used in mathematical models describing solute flow in the soil (Barber,1995).We have calculated the solid-to-solution adsorption coefficients at two concentrations which reflect those in the bulk soil (‘low’,10m M)and in the rhizosphere (‘high’;500m M;Jones,1998).The results are shown in Table 2and confirm that solid phase sorption has a significant effect in lowering soil solutionconcentrations.Fig.1(continued )P.A.W.van Hees et al./Soil Biology &Biochemistry 35(2003)1015–10261019We predict that65–95%of oxalate and50–65%of citrate released into the soil will become adsorbed in the O and E horizons while in the B horizons.99%will become adsorbed on the solid phase.3.2.Effects of sorption on organic acid biodegradationTwo different ferrihydrites were added to Nya¨nget E horizon soil together with14C-labelled citrate.The initial concentration of citrate corresponded to100m M which is within the range reported forfield soils(Table1;van Hees et al.,2000).The amounts of synthetic or natural ferrihydrite added to the E horizon soil were calculated to achieve different degrees of citrate sorption potential.The actual amounts of material added were related(in percent)to the maximum sorption capacityðS maxÞvalue for phosphate in the Nya¨nget Bs1horizon.The Nya¨nget Bs1horizon was chosen as a reference since this soil showed the highest adsorption of all soils investigated.Of the two adsorbents, synthetic ferrihydrite showed the highest anion sorption capacity(Table3).In the control,low citrate sorbing soil(Nya¨nget E horizon),without any ferrihydrite amendment,citrate was rapidly biodegraded and.60%was recovered as14CO2within72h(Fig.2).This is in close agreement with investigations of this and similar forest soils by Jones et al. (2001)and Van Hees et al.(2002).For reasons of comparison,citrate decomposition was also determined in the Nya¨nget and Heden Bs1horizons that are known to possess a high citrate sorption capacity(Table1).In comparison to the Nya¨nget E horizon,the Bs1horizonTable2Calculated soil adsorption coefficients(total anion in soil/total anion in soil solution)for organic acids and phosphate at two equilibrium solution concentrations relecting typical concentrations found in the bulk soil solution(10m M)and those predicted to occur in the rhizosphere(500m M)in three coniferous forest soil profiles(Ny,He and De)Citrate Oxalate Acetate PhosphateBulk soil Rhizosphere Bulk soil Rhizosphere Bulk soil Rhizosphere Bulk soil Rhizosphere Nya¨nget(Ny)O11112611ND NDO2219611ND NDE32653110838Bs19987216526853191945129Bs275270902473313188291C14663732632158144271C2401762154326437Heden(He)O11113621ND NDO22114522ND NDE431064249747Bs1281815322851132121Bs23416781883505101C111029109245470657C2153201391912535747 Delamere(De)Ol119621ND NDOf215331ND NDAh42643211E63544132Bh1261364214441Bs3820412375344119B/C1063411230167573112 Values were calculated using the Langmuir isotherms(Fig.1).ND indicates not determined.Table3Optimised values for the Langmuir isotherm parameters,maximumsorption capacityðS maxÞand the sorption affinity constantðK sÞ;for citrateand phosphate for mineral phases and soils employed in the biodegradationexperimentsCitrate PhosphateS max(m mol g21)K s S max(m mol g21)K sMineral additivesSynthetic ferrihydrite12077.0£10231496 3.0£1022Natural ferrihydrite12 1.4£102391 2.5£1023SoilsNya¨nget E(control)0.37.4£1024 3.7 5.0£1023Nya¨nget Bs17.2 3.6£1022268.3£1023Heden Bs1 2.7 1.3£102312 1.7£1022 P.A.W.van Hees et al./Soil Biology&Biochemistry35(2003)1015–10261021demonstrated a 70-fold slower rate of citrate mineralization (,5%recovered as 14CO 2over 72h).In contrast,the rate of citrate mineralization in the Heden Bs1soil was intermedi-ate with approximately 40%recovered as 14CO 2.The addition of even small amounts of synthetic ferrihydrite to the soil almost completely inhibited citrate decomposition (Fig.2a )in agreement with the results of Jones and Edwards (1998).The natural ferrihydrite also caused a significant decline in citrate mineralization with increasing additions to the soil,but was much less than that observed with synthetic ferrihydrite (Fig.2b ).The two methods of citrate addition to the soil (citrate added to the soil after the ferrihydrite had been added (Procedure 1),or citrate pre-mixed with the ferrihydrite phase and then the ‘mix’added to the soil (Procedure 2))did not modify subsequent mineralization rates significantly except for the natural ferrihydrite added at the 50%level (Fig.2;P ,0:05).The effects of filling the sorption sites of syntheticferrihydrite with a strongly sorbing anion (PO 432)prior to performing the citrate decomposition experiment was also evaluated.Synthetic ferrihydrite was loaded with sufficient phosphate to satisfy 25,50and 100%of its measured adsorption capacity prior to its addition to soil.Only in thecase where the ferrihydrite should have been fully PO 432saturated was a considerable increase in citrate degradationapparent when compared to the non-PO 432saturated ferrihydrite (Figs.2a and 3).However,the total percentage of citrate mineralized was still only one-third of that of the non-ferrihydrite amended (control)soil.Small but significant differences between the two procedures of citrate addition (Procedures 1and 2)were observed for this treatment.Attempts were made to predict the citrate mineralization curves shown in Fig.2by mathematically combining the Langmuir adsorption isotherm (Table 3)with apreviouslyFig.2.Cumulative soil 14CO 2production after the addition of 14C-labelled citrate (50m mol kg 21)to a low sorption capacity E horizon soil either before (Control)or after the addition of strongly sorbing mineral phases to the soil.The mineral phases employed (Panel A,synthetic ferrihydrite;Panel B,natural ferrihydrite)were either added to soil before the addition of 14C-citrate (closed symbols)or together with the 14C-citrate (open symbols).In both panels A and B,the minerals were added at different rates to achieve sorption capacities of 25,50,75and 100%of that present in a strongly sorbing Bs horizon of the samesoil.14C-citrate mineralization data for the strongly citrate sorbing Nya¨nget Bs1and Heden Bs1soil horizons are also shown for comparison.Data points represent means ^SEM ðn ¼2Þ:For further details see Section 2.P.A.W.van Hees et al./Soil Biology &Biochemistry 35(2003)1015–10261022parameterised Michaelis–Menten kinetic equation that describes concentration-dependent citrate decomposition (van Hees et al.,2002).Values for the maximum microbial citrate uptake rate (V max ;17nmol g 21h 21)and the affinity constant (K M ;329m M)were obtained from van Hees et al.(2002)for the Nya¨nget E horizon.For this test,the unamended E horizon soil (control),25%synthetic ferrihydrite amended soil (1)and 50%natural ferrihydrite amended soil (2)were chosen as examples.Calculated from the maximum sorption capacity ðS max Þvalues for citrate (Table 3),the additions of the two ferrihydrites should yield a similar (1.5–1.8mmol kg 21)theoretical S max for citrate in the samples.The mineralization curves predicted by the model (dotted/dashed lines in Fig.4)could reproduce the general trend observed in the experiments that citrate decomposition was significantly inhibited by the presence of synthetic ferrihydrite and natural ferrihydrite (Fig.4).However,the absolute fit with the experimental values was generally poor with the exception of the initial 6h for the control sample which showed very close agreement.Generally,the equations overestimated the amount of citrate mineralization.The calculated total amount of citrate mineralized to CO 2over 72h was higher than that observed in the experiments by 20%for the ‘25%synthetic ferrihydrite’treatment,75%for the ‘50%natural ferrihy-drite’treatment and 27%for the control unamendedsoil.Fig.3.Cumulative soil 14CO 2production after the addition of 14C-labelled citrate (50m mol kg 21)to a low sorption capacity E horizon soil either before (control)or after the addition of P-loaded synthetic ferrihydrite to the soil.The synthetic ferrihydrite was treated with KH 2PO 4to fill 0(control),25,50or 100%of its sorption capacity with PO 432and either added to soil before the addition of 14C-citrate (closed symbols)or together with the 14C-citrate (open symbols).14C-citrate mineralization data for the strongly citrate sorbing Nya¨nget Bs1and Heden Bs1soil horizons are also shown for comparison.Data points represent means ^SEM ðn ¼2Þ:For further details see Section2.Fig.4.Predicted and measured cumulative amounts of 14CO 2production following the addition of 14C-labelled citrate (50m mol kg 21)to a low sorption capacity E horizon soil which has either been untreated (control)or amended with a strongly sorbing mineral phase (ferrihydrite or natural ferrihydrite).The minerals were added at rates to achieve sorption capacities of 25%(ferrihydrite)or 50%(natural ferrihydrite)of that present in a strongly sorbing Bs horizon of the same soil.Symbols represent experimental data points (means ^SEM,n ¼2)while the dashed lines represent the modelled cumulative decomposition,calculated by combining the adsorption isotherm and decomposition kinetics using the parameters in Table 3.P.A.W.van Hees et al./Soil Biology &Biochemistry 35(2003)1015–102610234.Discussion4.1.Effects of sorption on organic acid behaviourOur study confirmed the important role that sorption processes play in affecting mobility and bioavailability and therefore longevity of certain LMW organic acids and reactive inorganic anions such as phosphate in forest soils. Both the maximum sorption capacity and the shape of the isotherm are of importance for predicting the quantitative effects of anion behaviour in these soils.The results also demonstrate that these key sorption properties varied widely not only between experimental sites but also for horizons within individual soil profiles.The highest sorption potential was observed for the subsoil Bs horizons,which is in accordance with the studies of Jones et al.(1996b)and Jones and Brassington(1998)on acidic grassland soils.For citrate and acetate sorption at the Nya¨nget and Heden sites there was a significant relationship between the amounts of oxalate-extractable Al plus Fe(Fe ox)(from Karltun et al. (2000))and the maximum sorption capacityðS maxÞvalues determined here(r2¼0:74to0.91).The study also indicated that the natural Fe-(hydr)oxides in these(Nya¨nget and Heden)profiles,which are dominated by ferrihydrite (Karltun et al.,2000),are significantly less reactive than the synthetic source of ferrihydrite employed here.Assuming that all the Fe ox in the Nya¨nget Bs1soil(252mmol kg21) behaves as the synthetic ferrihydrite,this would theoreti-cally provide a maximum P sorption capacity of 36mmol PO432kg21.This is significantly higher than the experimental value of26mmol kg21derived for this soil.We conclude that while synthetic ferrihydrite may provide a model system to examine organic acid reactions in vitro,care must be taken when extrapolating these results to the natural situation where the sorption capacities of aged material will have been strongly modified through long exposure to soil solution containing a range of strongly adsorbing inorganic and organic anions(Table3).The range of calculated organic acid solid adsorption coefficients within the soils was large(0.1–3000; Table1).This can be compared to previous adsorption coefficient estimates for organic acids from acid,neutral and alkaline-based soil extraction procedures(0.2–14; Strobel,2001).Most of the O,E and A horizons examined here are in general agreement with the results of Strobel(2001),however,our results for B and C horizons indicate that this adsorption coefficient range can potentially be much wider.Moreover,the approach of estimations based on isotherms and actual soil solution concentrations used in this work offers two advantages over conventional chemical extraction-based procedures. Firstly,the application of water and harsh chemical extractants may cause significant contamination from damaged microbial cells and plant residues(Jones,1998), and secondly the extraction efficiency might be poor due to irreversible sorption,although this is rarely quantified.For example,an earlier study using the same soil samples reported that only7to30%of citrate sorbed to the solid phase in B and C horizon soils could be recovered when using a 50mM phosphate solution(van Hees et al.,2002).In addition,the use of radioactively labelled organic acids offers great selectivity and sensitivity.A potential limitation of the method used,as in any study with natural soil,is that the experiments are performed against a background of organic acids that are already sorbed.However,calculations of the amount of organic acid initially present on the sorption phase indicate that these are normally very low in comparison to the total adsorption capacity of the soil(Tables1–3;Fig.1).The results indicate that,for Bs horizons,large changes in the amount of oxalate and citrate adsorbed to the solid phase are likely to bring about small changes in soil solution concentrations.The dominance of the sorption process as a controlling mechanism for soil solution concentration in Bs horizons is also supported by observations that LMW organic acid concentrations were very consistent throughout the growing season at the Nya¨nget site(van Hees et al.,2000).Considerably large adsorption coefficient values were obtained for subsoil horizons(Table2).These high values imply that the major part(.50%)of the organic acids,in particular oxalate and citrate,believed to take part in rhizosphere processes such as Al detoxification and nutrient mobilisation(e.g.P and Fe),are adsorbed.Sorption to the solid phase is also known to be a quick process with more than70%of the sorption occurring within1min(Jones et al., 1996a,b;Karltun,1997).Adsorption and the fast biode-gradation of organic acids(van Hees et al.,2002)may therefore strongly reduce the efficiency of organic acids in these rhizosphere processes.Particularly for oxalate,significant amounts of sorption were observed in the surface horizons.We speculate that since the amounts of clay minerals and Al/Fe hydroxides are normally small in these surface horizons,a major part of the observed oxalate removal from solution may be caused by co-precipitation of oxalate with soil solution Ca and exchangeable Ca.The concentration of non-Ca complexed oxalate in equilibrium with Ca-oxalate at mean soil solution concentrations of Ca and pH measured in these three soils(280m M;pH3.7;van Hees et al., 2002)would be27m M.However,since the Ca in soil solution is diluted about8times in a1:5(v/w)soil:water extract it suggests that other Ca pools such as exchange-able Ca may also be involved.This is supported by studies showing that higher adsorption/precipitation occurs in limed soils with larger amounts of exchangeable Ca(van Hees et al.,2003).4.2.Effects of sorption on citrate biodegradationThe protective effect of sorption on biodegradation of organic acids has been shown by Boudot et al.(1989), Boudot(1992)and Jones and Edwards(1998).Additions ofP.A.W.van Hees et al./Soil Biology&Biochemistry35(2003)1015–1026 1024。