Molecular weight and humification index as predictors of adsorption for plant- and manure-derived dissolved organic matter to goethite

Sorptive retention of organic matter is important in maintaining the fertility and quality of soils in agricultural ecosystems. However, few sorption studies have been conducted that use dissolved organic matter (DOM) characteristic of agricultural amendments. We investigated the sorption to goethite (α‐FeOOH) of DOM extracted from: (i) above‐ground biomass of wheat straw (Triticum aestivum L.), maize residue (Zea mays L.), soybean residue (Glycine max (L.) Merr.), and hairy vetch residue (Vivia billosa L.); (ii) below‐ground biomass from maize, soybean, canola (Brassica napus L.), and green bean (Phaseolus vulgaris L.); and (iii) beef, dairy, poultry, and pig animal manures. The apparent molecular weight (MW_AP) of the DOM was measured by high performance‐size exclusion chromatography and ranged from 312 to 1074 g mol⁻¹. The carboxyl‐group content of the DOM measured by potentiometric titration ranged from 4.84 to 21.38 mmol₍₋₎ g⁻¹ carbon. The humification index (HIX) determined by fluorescence spectrometry varied from 1.15 to 4.33. Sorption was directly related to both MW_AP and HIX values of the DOM. Molecular weight analysis of the solution prior to and after sorption indicated that the DOM molecules > 1800 g mol⁻¹ were preferentially sorbed, resulting in fractionation of the DOM upon reaction with goethite. The multiple regression equation, based only on MW_AP and HIX parameters, explained 76% of the variance in amount of DOM sorbed. The results indicate that MW_AP and HIX are important factors in controlling the sorption of DOM to mineral surfaces. Amendment with materials that release DOM of higher molecular weight and greater humification will result in enhanced initial sorption of DOM to soil solids, thereby contributing to accumulation of a larger soil organic C pool.     

SORPTION OF INORGANIC PHOSPHATE BY IRON- AND ALUMINIUM- CONTAINING COMPONENTS

The amounts of inorganic P sorbed by a range of Fe- and Al- containing components varied appreciably and decreased in the order allophane > fresh Al gel > Fe gel pseudoboehmite > aged Al gel > dried Fe gel > Fe-coated kaolinite > haematite > goethite > akaganeite > gibbsite = ground kaolinite > dispersed kaolinite. Al gel sorbed 30 to 70 times more P than gibbsite, and Fe gel sorbed approximately 10 times more P than its crystalline analogues (haematite, goethite. and akaganeite). Despite large differences in the extent of P sorption, the form of the isotherm was essentially the same for each sorbent. The ability of freshly-prepared Al gel suspensions to sorb P decreased with ageing, a property not shown by Fe gel. Drying of Fe gel at 80°C, however, caused an approximately 4-fold decrease in P sorption. Precipitation of Fe gel (2% Fe) on the surface of kaolinite increased P sorption by a factor of 10. The occurrence of Fe gel as a coating apparently presents more sorption sites to solution per unit weight of Fe gel than Fe gel alone. A linear relationship (r= 0.98) was obtained between the amount OH^? sorbed per unit increase in pH value (‘hydroxyl buffering’) and the overall P sorption maximum for each sorbent. Hydroxyl buffering provided a better index of P sorption potential than specific surface area. Except for the crystalline Fe sorbents, isotherms obtained by plotting fractional sorption saturation against final solution P concentration for the sorbents were essentially coincident with those for several contrasting soils. For crystalline Fe components a lower relative amount of weaker sorption, as opposed to chemisorption, of the overall sorption maximum was obtained. Differences in the extent of P sorption. however, appear to be primarily related to the number of functional M-OH groups presented at the solid-solution interface.     

Influence of foliar phosphorus and nitrogen contents on chemical properties of water extractable organic matter derived from fresh and decomposed sugar maple leaves

The effects of watershed-scale experimental acidification on the macronutrient content and decomposition of sugar maple (Acer saccharum Marsh) leaves were investigated. Bear Brook Watershed in Maine (BBWM) is a paired forest watershed study where the West Bear (WB) watershed has been treated bi-monthly with 1800 eq ha^?1 yr^?1 of (NH₄)₂SO₄ since 1989, and the adjacent East Bear (EB) watershed has acted as a reference. Leaf samples collected from the treated WB watershed had significantly higher concentrations of N and P than leaves from the reference EB watershed. Leaves from both watersheds were decomposed for a 10-day laboratory incubation. Extractable total soluble carbon (C_TS) content of the leaves decreased following decomposition to a greater extent in WB leaves than in EB leaves. Spectroscopic and chromatographic chemical analyses indicated similar chemical properties for the fresh WB and EB WEOM. However, after decomposition, the WB WEOM was more humified as compared to EB WEOM indicating that the watershed treatment resulted in leaves which were more biodegradable than those in the reference watershed. Multi-dimensional fluorescence spectroscopy with parallel factor analysis (PARAFAC) modeled five components: tyrosine-like, three humic substance-like, and terrestrial/anthropogenic associated-like fluorophores. Following decomposition, the relative concentrations of two of the humic-associated components increased to a significantly greater extent for WB than for EB WEOM. These observations were consistent with greater decomposition-related changes to the WEOM from WB samples relative to EB samples. Pearson correlation analysis showed that foliar N and P concentrations were positively correlated with indices of humification. Adsorption of WEOM to goethite and gibbsite was significantly greater for decomposed WB WEOM than EB WEOM. These results demonstrate that greater leaf N and P contents can increase short-term decomposition, accelerate production of more humic-like WEOM, and thereby potentially influence the distribution of organic matter within the soil carbon pool.     

Dissolved organic matter sorption on sub soils and minerals studied by 13C-NMR and DRIFT spectroscopy

Sorption on the mineral matrix is an important process restricting the movement of dissolved organic matter (DOM) in soils. In this study, we aimed to identify the chemical structures responsible for the retention of DOM by sorption experiments with total DOM and acidic humic substances (AHS), containing humic and fulvic acids, on soil samples and minerals (goethite, ferrihydrite, and amorphous Al(OH)₃). The AHS remaining in solution after sorption were studied by ¹³C nuclear magnetic resonance (NMR) analysis, and total DOM and AHS for bed on the surfaces of minerals by diffuse reflectance Fourier-transform infrared (DRIFT) spectroscopy. The soil samples were taken from strongly sorbing Bw horizons of two Inceptisols rich in pedogenetic Fe (29 and 35 g kg ^?1) and containing little C (7 and 22 g kg^?1). The ¹³C-NMR spectra showed that sorption causes a preferential removal of aromatic and carboxyl C from the solution, whereas alkyl-C accumulates in the solution. No change was observed for O-alkyl C. The DRIFT spectra of sorbed total DOM and AHS showed a relative increase of the band intensity of carboxyl groups compared to DOM in the initial solution, confirming the importance of those groups for the sorption to mineral surfaces. The spectra also indicated reactions of carboxyl groups with metals at the mineral surfaces. The extent to which the carboxyl groups are bound depended on the surface coverage with DOM and the type of mineral.     

Retention of Ni,Zn and Cd by Si-associated goethite

Synthetic goethite used to study the effects of reaction time and temperature on the pH-dependent sorption of Ni, Zn and Cd was associated with amorphous silica. Ni interacted with dissolved Si and formed a Ni/Si precipitate on the goethite surface. Individual metals added at a concentration of 0.5 μmol g^?1 and sorbed during a reaction period of 504 hours (21 days) at 35°C were extracted by 0.7 M HNO₃ for 14 days. At the end of this period 11,28 and 40 percent of Ni, Cd and Zn, respectively, were not extracted whereas 20 percent of the total Fe content of the goethite and 39 percent of the associated Si were dissolved. During the sorption process metals became immobilized in the goethite particles. This effect can be related to a diffusion of metal ions into micropores. A total mobilization of sorbed metals can only be achieved by a complete dissolution of the goethite. The strong fixation of Ni, Zn and Cd by goethite suggests that additions of this Fe oxide could be used to ameliorate highly contaminated sludges or soils.     

磷和柠檬酸共存对高岭石和针铁矿吸附铅的影响

通过平衡吸附试验及矿物电动电位(Zeta电位)的变化分析,研究了磷(P)和柠檬酸(CA)共存对针铁矿和高岭石吸附铅的影响。结果表明:(1)针铁矿和高岭石对铅的吸附量随柠檬酸浓度的升高呈现"峰形"曲线变化,铅吸附量达到峰值的柠檬酸浓度均为0.5 mmol L~(-1),不同浓度磷存在下柠檬酸对矿物吸附Pb2+量有不同程度增加。(2)随着磷添加浓度的增加,两种矿物对铅吸附量均呈增加趋势,磷添加浓度分别为1 mmol L~(-1)和0.6 mmol L~(-1)时,针铁矿和高岭石吸附铅量达到平衡;当处理中添加不同浓度柠檬酸,两种矿物均表现为对铅的吸附量增加,且随着柠檬酸浓度增加促进铅吸附的作用增强,说明在磷及试验浓度柠檬酸存在下促进了矿物对铅的吸附。(3)高岭石体系中,加入磷或(和)柠檬酸后,Zeta电位-pH曲线向负值方向位移,降低程度大小顺序为1.0 mmol L~(-1) P+0.5mmol L~(-1) CA0.5 mmol L~(-1) CA1.0 mmol L~(-1) P,说明高岭石表面增加的负电荷也部分增加了其对铅的电性吸附;添加磷和柠檬酸处理针铁矿的Zeta电位显著降低,且随着体系pH的升高其Zeta电位没有明显变化,表明磷和柠檬酸均主要是通过吸附到针铁矿表面而增加对铅的专性吸附。     

Changes in P fractions and sorption in an Alfisol following crop residues application

With the emphasis on sustainable agriculture, attention has been increasingly turning to recycling of crop residues as a component of fertility management strategies for tropical soils. We assessed the effects of soybean residue (SR) and wheat residue (WR) applied either alone or in combination with fertilizer P (FP) on dynamics of labile P, distribution of P fractions, and P sorption in a semiarid tropical Alfisol by conducting a 16 w long incubation experiment. The amount of P added through crop residues, FP or their combinations was kept constant at 10 mg P (kg soil)^–1. Addition of SR or WR resulted in net increase of labile inorganic (P_i) and organic P (P_o) and microbial P throughout the incubation period, except that the WR decreased labile P_i during first 2 w due to P_i immobilization. The P immobilization associated with WR addition was, however, offset when fertilizer P was combined with WR. Generally, the increases in labile‐P fractions were larger with the SR and SR+FP than with the WR and WR+FP. The sequential fractionation of soil P at the end of 16 w indicated that a major part of added fertilizer P transformed into moderately labile and stable P fractions as evident from the increased NaOH‐P_i and HCl‐P in the FP treatment. In contrast, the addition of SR and WR alone or in combination with FP favored a build‐up in NaHCO₃‐P_i and ‐P_o and NaOH‐P_o fractions while causing a decrease in NaOH‐P_i and HCl‐P fractions. The addition of these crop residues also effectively decreased the P‐sorption capacity and hence reduced the standard P requirement of the soil (i.e., the amount of P required to maintain optimum solution P concentration of 0.2 mg P l^–1) by 24%–43%. Results of the study, thus, imply that soybean and wheat crop residues have the potential to improve P fertility of Alfisols by decreasing P‐sorption capacity and by redistributing soil P in favor of labile‐P fractions and promoting accretion of organic P.     

Factors affecting phosphate sorption along a Mediterranean, dolomitic soil and vegetation chronosequence

Phosphate sorption by calcareous soils has been studied mainly on heavily fertilized agricultural soils and soils with calcite as the main carbonate mineral. We examined factors affecting phosphate adsorption in the soils of a semi-arid, mediterranean, dolomitic, soil and vegetation chrono-sequence in southeastern Spain. The youngest soils are highly eroded, Sandy Regosols (Typic Xerorthents) under gorse-scrubland vegetation. These have small P sorption capacities, large Mg-Ca carbonate contents but small amounts of Fe and Al oxides. Small total P (HNO₃/HClO₄ digestion) concentrations (30–130 μg P g^?1), of which up to 90% is Ca-bound (HCl-extractable), are typical of these young soils. P sorption markedly increased when Ca²⁺ was added to the solution. The fractionation of previously sorbed P indicates that the fate of most of this extra-sorbed P is the labile-P fraction sorbed on to (carbonate) surfaces and the apatite-like fraction (NaHCO₃-extractable and HCl-extractable fractions). At the other extreme, older more-intensively weathered, sandy-clay-loam rendzinas (Entic Haploxerolls), supporting dense mature garrigue, have a much greater P adsorption capacity and larger clay and Fe and Al oxide concentrations. They have more total P (ca 400 μg P g^?1), much of it in occluded form (residual fraction). These soils show no significant differences in P sorption whether or not CaCl₂ was used as a background electrolyte. Considering the overall variations within the chronosequence, dithionite extractable Fe and Al are the properties best correlated with P sorption. This support the general finding that crystalline Fe-oxides (e.g. goethite and haematite) appear to be the most important P-sorbing component for soils in the Mediterranean region, rather than amorphous Fe-oxides (e.g. ferrihydrite) as is reported for more mesic areas. Stepwise multiple regression and fractionation data, however, suggest that, provided the soil solution is rich in Ca²⁺, carbonate may also be a significant contributing factor to P sorption, especially in the youngest of these dolomitic soils.     

Sorption of iron‐cyanide complexes on goethite investigated in long‐term experiments

The iron‐cyanide complexes ferrocyanide, [Fe^II(CN)₆]^4–, and ferricyanide, [Fe^III(CN)₆]^3–, are anthropogenic contaminants in soil. We investigated their sorption on goethite, α‐FeOOH, in batch experiments in a time range from 1 d to 1 yr, their desorption by phosphate and chloride as well as their surface complexes on goethite by Fourier‐transform infrared spectroscopy (FTIR). The sorption of both complexes continued over the whole time range. Percent desorption of ferricyanide by phosphate decreased, whereas that of ferrocyanide increased until it amounted to approximately 87% for both complexes. By FTIR spectroscopy inner‐sphere complexation of both complexes on the goethite surface was indicated. With both complexes, a Berlin‐Blue‐like layer (Fe₄[Fe(CN)₆]₃) was formed initially on the goethite surface which disappeared with increasing reaction time. After at least 30 d reaction time, ferricyanide was the only sorbed iron‐cyanide complex detected even when ferrocyanide was initially added. This resulted from slow oxidation of ferrocyanide, most probably by dissolved oxygen. Based on all results, we propose that ferricyanide forms monodentate inner‐sphere complexes on the goethite surface.     

不同pH、电解质浓度条件下草酸对针铁矿吸附Cd(II)的影响及机制

用吸附平衡法研究了不同草酸浓度、体系pH对针铁矿 (G)吸附Cd2+的影响与机制以及电解质 (KNO3)浓度对针铁矿、草酸化针铁矿 (G+40 )吸附Cd2+的影响差别及原因。结果表明 ,低浓度草酸 (1mmolL-1 )促进Cd2+的吸附 ;高浓度草酸 (1mmolL-1/sup )抑制Cd2+的吸附。已吸附在针铁矿表面的草酸对Cd2+ 吸附的影响与液相中草酸的影响不同 ,这主要与草酸引起的针铁矿表面电荷性质的变化、草酸在固液两相间的分配、草酸与Cd2+的配合作用和竞争作用有关。电解质 (KNO3)浓度对针铁矿和草酸化针铁矿吸附Cd2+的影响明显不同 ,随KNO3 浓度的提高 ,针铁矿的Cd吸附率由 44.5%增至 95%以上 ,而草酸化针铁矿吸附率由 29%降至6.2% ,这主要决定于二者的电荷零点 (PZC)和体系pH变化的不同。     

Interaction of phytases with minerals and availability of substrate affect the hydrolysis of inositol phosphates

The stability and activity of phytases in the soil environment may be affected by their sorption on soil particle surfaces and by substrate availability with important consequences for P cycling and nutrient bioavailability. This work evaluated the interaction of phytases with goethite, haematite, kaolinite, montmorillonite and two oxisol clays and investigated how this interaction is affected when myo-inositol hexakisphosphate (InsP₆) was sorbed on the mineral surfaces. phyA histidine acid phosphatases of fungal origin were used and their ability to release orthophosphate from the InsP₆-saturated minerals was evaluated.The phytases showed a high affinity for the mineral surfaces, with a loss of enzyme activity generally being observed over 24 h (up to 95% of the initially added activity). The loss of phytase activity was dependent on the type of mineral, with kaolinite and montmorillonite showing the greatest effect. Retention of enzyme activity was higher with the two oxisol clays, suggesting that the heterogeneous nature of clay surfaces and the presence of endogenous organic matter may limit the inhibition caused by interaction with minerals.In the presence of mineral surfaces saturated with InsP₆, the partitioning of enzyme activity between the solution and the solid phase was shifted more towards the solution phase, presumably due to the mineral surfaces being occupied by the substrate. However, phytases were not able to release any orthophosphate directly from InsP₆-saturated goethite and haematite, and hydrolysed InsP₆ that was desorbed from haematite. Conversely, in the case of kaolinite and of the oxisol clays, where desorption was limited, phytases appeared to be able to hydrolyse a small fraction of the InsP₆ adsorbed on the surfaces. These findings suggest that the bioavailability of P from inositol phosphates is governed to a large extent by the mineral composition of soil and by competitive effects for sorption on reactive surfaces among inositol phosphates and phytases.     

Biochemical properties and biodegradation of dissolved organic matter from soils

Various biologically mediated processes are involved in the turnover of dissolved organic matter (DOM) in soil; however, relatively little is known about the dynamics of either the microbial community or the individual classes of organic molecules during the decomposition of DOM. We examined the net loss of DOC, the mineralisation of C to CO₂ and the degradation of DOC from six different soils by soil microorganisms. We also quantified the changes in the concentrations of protein, carbohydrate and amino acid C during microbial biodegradation. Over a 70-day incubation period at 20°C, the mineralisation of DOC to CO₂ was described by a double exponential model with a labile pool (half-life, 3–8 days) and a stable pool (half-life, 0.4–6 years). However, in nearly all cases, the mass loss of DOC exceeded the C released as CO₂ with significant deviations from the double exponential model. Comparison of mass DOC loss, CO₂ production and microbial cell counts, determined by epifluorescence microscopy, showed that a proportion of the lost DOC mass could be accounted for by microbial assimilation. Carbohydrate and protein C concentrations fluctuated throughout the incubation with a net change of between 3 to 13 and −30 to 22.4% initial DOC, respectively. No amino acid C was detected during the incubation period (level of detection, 0.01 mg C l⁻¹).     

The Contrasting Effect of Goethite and Hematite on Phosphate Sorption and Desorption by Terre Rosse

Phosphate sorption and desorption in soils are markedly influenced by iron oxides, although little is known on how the common iron oxides differ in their behaviour towards added phosphate. In this study, we investigated phosphate sorption and desorption in the clay fractions of 12 Terre Rosse that ranged widely in Fe oxide content, had very low contents of oxalate-extractable Fe oxides and different hematite/goethite ratios. Phosphate sorption at an equilibrium concentration of 1 mg P 1^?1 was correlated with the goethite but not with the hematite content of the clay fractions. When phosphate was desorbed by electro-ultrafiltration, the difference in desorption half-time between untreated and deferrified clays was positively correlated with the goethite but not with the hematite content. These results suggest that goethite is more active than hematite in phosphate sorption and retention by soils.     

Abiotic solubilization of soil organic matter,a less-seen aspect of dissolved organic matter production

Dissolved organic matter (DOM) is a small but reactive pool of the soil organic matter (SOM) that contributes to soil dynamics including the intermediary pool spanning labile to resistant SOM fractions. The solubilization of SOM (DOM production) is commonly attributed to both microbially driven and physico-chemically mediated processes, yet the extent to which these processes control DOM production is highly debated. We conducted a series of experiments using ¹³C-ryegrass residue or its extract (¹³C-ryegrass-DOM) separately under sterile and non-sterile conditions to demonstrate the importance of DOM production from microbial and physico-chemical processes. Soils with similar properties but differing in parent material were used to test the influence of mineralogy on DOM production. To test the role of the source of C for DOM production, one set of soils was leached frequently with ¹³C-ryegrass-DOM and in the other set of soils ¹³C-ryegrass residue was incorporated at the beginning of the experiment into the soil and soils were leached frequently with 0.01 mol L⁻¹ CaCl₂ solution. Leaching events for both treatments occurred at 12-d intervals over a 90-day period. The amount of dissolved organic C and N (DOC and DON) leached from residue-amended soils were consistently more than 3 times higher in sterile than non-sterile soils, decreasing with the time. Despite changes in the concentration of DOC and DON and the production of CO₂, the proportion of DOC derived from the ¹³C-ryegrass residue was largely constant during the experiment (regardless of microbial activity), with the majority (about 70%) of the DOM originating from native SOM. In ¹³C-residue-DOM treatments, after successive leaching events and regardless of the sterility conditions i) the native SOM consistently supplied at least 10% of the total leached DOM, and ii) the contribution of native SOM to DOM was 2–2.9 times greater in ¹³C-residue-DOM amended soils than control soils, suggesting the role of desorption and exchange reactions in DOM production in presence of fresh DOM input. The contribution of the native SOM to DOM resulted in higher aromaticity and humification index. Our results suggest that physico-chemical processes (e.g. exchange or dissolution reactions) can primarily control DOM production. However, microbial activity affects SOM solubilization indirectly through DOM turnover.     

Effect of soil microorganisms on the sorption of zinc and lead compounds by goethite

The objectives of this study were (1) to determine the effect of microorganisms during in‐vitro incubation on the amount of Zn and Pb from solution retained on goethite precipitated as coatings on a sand matrix and (2) to evaluate accumulation of heavy metals in the biomass of soil microorganisms in the fresh soil samples using an extractive approach. A mixture of colonies of cultivated microorganisms extracted from a Haplic Luvisol (Russia) and an Antropi‐urbic Regosol (Germany) were used to prepare the cell and the microbial‐debris suspensions. The concentrations of Zn and Pb in the studied solutions supplied with microbial suspensions and/or goethite coated sand were 0.1 mM (130.8 and 414 mg kg^–1 of sand, respectively). Exchangeable forms of metals were determined by extraction with 10 mL of 1.0 M KNO₃. Nonexchangeable forms of Zn and Pb were recovered using 40 mL of 0.3 M NH₂OH‐HCl in 1 M HNO₃. Concentrations of Pb increased in the solutions and decreased on the surface of the Fe‐mineral due to living microorganisms. In comparison to incubation of heavy‐metal solutions with goethite only, the absolute concentrations of nonexchangeable forms of metal were reduced by microbial suspension to a greater extent than those of the exchangeable forms, whereas the relative content of both fractions decreased by a factor of almost two. Sorption of Pb by goethite was inversely correlated with the concentration of organic C in the solution. Microorganisms clearly influenced the Zn sorption by goethite at concentrations of C_org > 400 mg L^–1. The amount of Zn retained was decreased primarily due to decreasing Zn portions in the exchangeable fraction. Microbial debris prepared by autoclaving reduced the Pb sorption by goethite similar to the results for living cells. Living microorganisms accumulated more Zn than did microbial debris. The data of this paper show that a direct determination of heavy‐metal accumulation in soil microorganisms by extraction with 2.0 M KCl as well as by extraction with 1 M CH₃COONH₄ at the natural pH of the soils after chloroform fumigation of fresh soils samples with different concentrations of organic C was not possible.     

Phosphate sorption in relation to aluminum and iron oxides of oxisols from Ghana

Abstract

Phosphate (P) sorption characteristics of six natural Ghanaian Oxisols, selected because of their hydrological and topographical suitability for agriculture, were evaluated. Availability of P appears to be adequate for half of the soils as suggested by the Bray P1 test and determination of the standard P requirement (SPR), i.e., the amount of P sorbed at a concentration of 0.2 ppm P (6.46 μM). The SPR was found to be very closely related to P_max (Langmuir P sorption capacity), which in turn, was significantly correlated with oxalate‐extractable aluminum (Al) (Al_o) and iron (Fe) (Fe_o) and related (not significantly) to the difference between dithionite‐citrate‐bicarbonate‐extractable Fe (Fe_d) and oxalate‐extractable Fe. Accordingly, P_max is fairly well predicted by the model of Borggaard: P_ca]e=0.211#lbÀl_o+0.115#lbFe_o+ 0.05#lb(Fe_d‐Fe_o)+0.3, except for one soil strongly enriched in Fe oxides, mainly goethite. This goethite was found by X‐ray diffraction analysis to consist of crystals larger than normally found for pedogenic Fe oxides. The difference between P_max and P_calc for this soil could, therefore, be attributed to the occurrence of these large Fe oxide crystals, because P sorption will decrease with increasing crystal size (decreasing specific surface area).     

Effect of humic amendments on inorganic N, dehydrogenase and alkaline phosphatase activities of a Mediterranean soil

Dehydrogenase activity, alkaline phosphatase activity and NH₄ ⁺, NO₂ ⁻ and NO₃ ⁻ concentrations were monitored in an aridisol treated with three commercially available humic amendments. The materials were of plant residue, lignite and peat origins. The humus plant residues, fulvic acids, with a high content of Kjeldahl-N, sustained high enzyme activities and highest levels of NH₄ ⁺, NO₂ ⁻ and NO₃ ⁻. Humus lignite (mainly humic acids) produced the highest dehydrogenase activity, whereas the alkaline phosphatase activity was not as high as that amendment with humus plant residues. The lower activity of alkaline phosphatase could not be attributed to the higher P content of humus lignite. Nitrification was also low, probably due to the low N content of this fertilizer. The amendment of humus peat origin (only humic acids) did not increase enzyme activity or inorganic N concentrations of soil. Our results show that although these materials are widely utilized and recommended as microbial and plant activators, they all behave very differently, and the effects on soil microbiological activity cannot be predicted solely on the basis of their humic and/or fulvic acid contents.     

Observations and modelling of the reactions of 10 metals with goethite: adsorption and diffusion processes

A sample of goethite was mixed for periods which ranged from 2 hours to 8 weeks with solutions of dilute nitrate salts of Pb, Hg, Cd, Zn, Cu, Ni, Co, Mn, Cr and Al. The amount of sorption after each period was measured for an appropriate pH range for each metal. The sorption behaviour was characterized both by using characteristics of the sorption curves such as the pH at which half of the added metal was sorbed (pH₅₀) and by fitting a model in which sorption was mainly characterized by an affinity constant and by a diffusion constant. Initial sorption, whether characterized by the pH₅₀ or by the affinity constant, was closely correlated with the appropriate dissociation constants of the metals. The greater the affinity of the metals for hydroxide ions, the greater their affinity for the goethite surface. The metals differed in the rate at which they continued to react with the goethite. Lead had the slowest continuing reaction, cobalt the fastest. The continuing reaction was due to diffusion into the particles. It was characterized by the fitted diffusion constant and by the change with time in the pH₅₀. For seven of the eight divalent metals, these were correlated with the ionic radius of the metals: the larger the radius, the slower the reaction. For Al and Cr, rates were slower than would be expected from the ionic radii and we suggest this shows that these ions react as the larger M(OH)²⁺ ions. The behaviour of Ni was consistent with oxidation of the surface species and diffusion of Ni(OH)²⁺ ions. The continuing reaction was similar to that observed when metal ions react with soils and suggests that their reaction with iron oxides is important in soils. The results also show that studies in which sorption is measured at only one period of reaction are incomplete and the application of equilibrium models to such results is misleading.     

Stabilization of dissolved organic matter by sorption to the mineral soil

The main process by which dissolved organic matter (DOM) is retained in forest soils is likely to be sorption in the mineral horizons that adds to stabilized organic matter (OM) pools. The objectives of this study were to determine the extent of degradation of sorbed OM and to investigate changes in its composition during degradation. DOM of different origins was sorbed to a subsoil and incubated for 1 year. We quantified mineralized C by frequent CO₂ measurements in the headspace of the incubation vessels and calculated mean residence times by a double exponential model. Mineralization of C of the corresponding DOM in solution was used as a control to estimate the extent of DOM stabilization by sorption. Changes in the composition of sorbed OM during the incubation were studied by spectroscopic (UV, fluorescence) and isotope (¹³C, ¹⁴C) measurements after hot-water extraction of OM.The fraction of sorbed organic C mineralized during the incubation was only one-third to one-sixth of that mineralized in solution. The mean residence time of the most stable OM sample was estimated to increase from 28 years in solution to 91 years after sorption. For highly degradable DOM samples, the portion of stable C calculated by a double exponential model nearly doubled upon sorption. With less degradable DOM the stability increased by only 20% after sorption. Therefore, the increase in stability due to sorption is large for labile DOM high in carbohydrates and relatively small for stable DOM high in aromatic and complex molecules. Nevertheless, in terms of stability the rank order of OM types after sorption was the same as in solution. Furthermore, the extent of sorption of recalcitrant compounds was much larger than sorption of labile compounds. Thus, sorptive stabilization of this stable DOM sample was four times larger than for the labile ones. We conclude that stabilization of OM by sorption depends on the intrinsic stability of organic compounds sorbed. We propose that the main stabilization processes are selective sorption of intrinsically stable compounds and strong chemical bonds to the mineral soil and/or a physical inaccessibility of OM to microorganisms. The UV, fluorescence and ¹³C measurements indicated that aromatic and complex compounds, probably derived from lignin, were preferentially stabilized by sorption of DOM. The ¹³C and ¹⁴C data showed that degradation of the indigenous OM in the mineral soil decreased after sorption of DOM. We estimated DOM sorption stabilizes about 24 Mg C ha⁻¹ highlighting the importance of sorption for accumulation and preservation of OM in soil.     

Sorption, microbial uptake and decomposition of acetate in soil: Transformations revealed by position-specific C labeling

Many previous studies on transformation of low molecular weight organic substances (LMWOS) in soil were based on applying ¹⁴C and/or ¹³C labeled substances. Nearly all these studies used uniformly labeled substances, i.e. all C atoms in the molecule were labeled. The underlying premise is that LMWOS transformation involves the whole molecule and it is not possible to distinguish between 1) the flux of the molecule as a whole between pools (i.e. microbial biomass, CO₂, DOM, SOM, etc.) and 2) the splitting of the substance into metabolites and tracing those metabolites within the pools.Based on position-specific¹⁴C labeling, we introduce a new approach for investigating LMWOS transformation in soil: using Na-acetate labeled with ¹⁴C either in the 1st position (carboxyl group, -COOH) or in the 2nd position (methyl group, -CH₃), we evaluated sorption by the soil matrix, decomposition to CO₂, and microbial uptake as related to both C atoms in the acetate. We showed that sorption of acetate occurred as a whole molecule. After microbial uptake, however, the acetate is split, and C from the -COOH group is converted to CO₂ more completely and faster than C from the -CH₃ group. Correspondingly, C from the -CH₃ group of acetate is mainly incorporated into microbial cells, compared to C from the -COOH group. Thus, the rates of C utilization by microorganisms of C from both positions in the acetate were independently calculated. At concentrations of 10 μmol l⁻¹, microbial uptake from soil solution was very fast (half-life time about 3 min) for both C atoms. At concentrations <100 μmol l⁻¹ the oxidation to CO₂ was similar for C atoms of both groups (about 55% of added substance). However, at acetate concentrations >100 μmol l⁻¹, the decomposition to CO₂ for C from -CH₃ decreased more strongly than for C from -COOH.We conclude that the application of position-specifically labeled substances opens new ways to investigate not only the general fluxes, but also transformations of individual C atoms from molecules. This, in turn, allows conclusions to be drawn about the steps of individual transformation processes on the submolecular level and the rates of these processes.