Evaluation of the possibilities of using humic acids obtained from lignite in the production of commercial fertilizers

Purpose

Technological progress and high market demand contributed to a significant interest in the production of fertilizers based on humic acids. The aim of this study was to evaluate the possibilities of using humic acids obtained from lignite in the production of new commercial products. For this purpose, it is necessary to determine the quality standard requirements for such material. Properties of humic acids depend on source of origin as well as method of its extraction.

Materials and methods

The humic acids were extracted from polish deposit of lignite–Sieniawa Lubuska by alkaline extraction using for this purpose six kinds of extractants: 0.1 M NaOH and 0.25 M NaOH, 0.1 M KOH and 0.25 M KOH, and 0.1 M Na₄P₂O₇ and 0.25 M Na₄P₂O₇. The humic samples were used in solid powder form and characterized by UV-Vis spectroscopy, ¹³C NMR spectroscopy, fluorescence spectroscopy, and thermal analysis.

Results and discussion

The determining factor influencing a degree of humic acids extraction from lignite and their structure is type of extractant. The largest efficiency of extraction (about 50%) was obtained with the use of NaOH solutions. All examined humic acids were generally characterized by simple and heterogeneous molecularly structure with low molecular weight and low aromatic polycondensation. Therefore, it can be concluded that humic acids extracted with NaOH and KOH solutions are less condensed than those extracted with Na₄P₂O₇ solutions. It can suggest that humic acids obtained from lignite using solutions of Na₄P₂O₇ are characterized by a low transformation degree and greater amount of carboxyl groups.

Conclusions

Low rank coal can be successfully used in agriculture as a rich source of humic acids. Reagent used in their extraction, apart from high efficiency should have a neutral impact on their structure. Studies on the physicochemical properties of humic acids can be helpful in predicting behaviors of such fertilizer components in the environment and in inventing new products taking the principles of sustainable development into consideration.

     

Cyanidadsorption an Sesquioxiden,Tonmineralen und Huminstoffen

Cyanide adsorption on sesquioxides, clay-minerals and humic substances The adsorption of cyanide (KCN) on sesquioxides, clay minerals, and humic substances at different pH-values was studied. Moreover we looked for the CN-adsorption on L-layers of the humus forms mull, moder and mor. Cyanide was only adsorbed by humic acid. The amount of CN adsorbed increased with increasing pH of the reaction solution. IR-spectroscopic investigations of CN treated humic acids revealed that the cyanide was adsorbed at low pH (<7) as HCN-molecules by formation of hydrogen bonds with COOH-, COH-, OH- and NH₂-groups of the humic acid. At pH > 7 the cyanide was mainly adsorbed as CN^? by charge transfer with acceptor-molecules such as chinones. The cyanide adsorption of L-layers of humus forms decreased in the order mor > mull > moder. It is surmised that the HCN-molecules were not only adsorbed by humic acids in these layers but also by oxidation products of lignin, pectin, protein, cellulose, and carbon-hydrates of fulvic acids. Solutions of K₂HPO₄ did not desorb cyanides from humic acids to any great extent.     

Characterization of lake sediment humic acids from the Ćelije Lake system near Kruševac (Central Serbia) by13C and 1H solution NMR and 13C cpmas NMR

Humic acids were extracted from the bottom sediment of the three lakes (Zlatari, Vasici and Vodozahvat) that together comprise Lake ?elije a regional drinking water reservoir. The humic acids were examined by high resolution ¹³C and ¹H solution NMR and ¹³C CPMAS. The aromaticity expressed as the number of benzene rings per 1000 g of humic acid is 1.8 for Zlatari, 2.3 for Vasici and 1.9 for Vodozahvat. Oxygen-substituted aromatic carbon structures in humic acids from the first two lakes (Zlatari and Vasici) are similar and resemble p-coumaryl units in lignin while those in Lake Vodozahvat more closely resemble coniferyl or sinapyl units in lignin. The aromatic rings of all three humic acids are highly substituted. Humic acids from the first two lakes have very similar carbohydrate carbon contents (19.9 and 19.5% respectively) compared to 23.3% for Vodozahvat humic acid. Zlatari and Vodozahvat humic acids have a higher alkyl chain content which indicates a greater potential for forming hydrophobic cavities. The carboxyl and aromatic OH group content of each humic acid influences their capacity for forming complexes with metal ions.     

An Experimental and Modelling Study of Cu2+ Binding on Humic Acids at Various Solution Conditions. Application of the NICA-Donnan Model

Humic substances are characterized by a strong binding capacity for both metals and organic pollutants, affecting their mobility and bioavailability. The understanding of the mechanisms of proton and metal binding to humic substances is of fundamental importance in geochemical modelling and prediction of cation speciation in the environment. This work reports results on copper binding on humic acids obtained through a thorough experimental and modelling approach. Two humic acids, a reference purified peat humic acid isolated by the International Humic Substances Society (IHSS) and a humic acid from a Greek soil, were experimentally studied at various pH values (4, 6 and 8), humic acid concentrations (ranging from 20 to 200 mg?L^?1) and ionic strength (0.1 and 0.01 M NaNO₃). The binding of copper to humic acids was determined over wide ranges of copper ion concentrations using a copper ion selective electrode. The copper binding isotherms obtained at different conditions have shown that copper binding is dependent on the pH and ionic strength of the solution and on the concentration of both humic acids. Copper binding experimental data were fitted to non-ideal competitive adsorption NICA-Donnan model and the model parameter values were calculated. Both Cu²⁺ and CuOH⁺ species binding to humic acid with different binding affinities were considered. Two sets of the NICA-Donnan parameters have been calculated: one for humic acid concentrations of ??100 mg?L^?1and one for humic acid concentration of 20 mg?L^?1. The meaning of the parameters values for each concentration level is also discussed.     

Determination of DTNB-reactive sulphhydryl groups in soil humic acids: their enrichment in humic acids from volcanic acid soils

Thirteen acid soils were collected from typical volcanic regions in Japan (S content: 0.9–4.1, mean = 2.2g kg^?1; pH (H₂O): 2.81–3.93, mean = 3.33), as well as nine reference soils (S: 0.6–1.7, mean= 1.1 gkg^?1; pH(H₂O): 4.10–4.74, mean = 4.47). Humic acids were extracted from the soils separately with 0.1 M NaOH and precipitated by acidification (pH = 2, HCl). After purification, the humic acids were subjected to colorimetric analysis using a DTNB reagent [5,5′-dithiobis(2–nitrobenzoic acid] for the active -SH group. Since humic acids have significant absorption at 4I2 nm, the coloured compound (5–mercapto-2–nitrobenzoic acid) was separated from the humic acids by ultrafiltration or solvent extraction prior to the colorimetric measurement. Humic acids also caused discoloration of the coloured compound when they coexisted in the reaction solutions. Thus, the reproducible determination of -SH was accomplished by employing a standard addition technique (-SH standard: cysteine). Although -SH contents obtained by the ultrafiltration method were considerably higher than those by the solvent-extraction method, probably due to the denaturation of humic acids by the higher buffer concentration used in the ultrafiltration method, they correlated well with each other. The humic acids from acid soils contained apparently higher concentrations of -SH (120–510, mean = 270mg S kg^?1 by the ultrafiltration method; 8–110, mean = 38mg S kg^?1 by the solvent-extraction method) compared to those from reference soils [20–260, mean = 90mg S kg^?1 by the former; not detectable (<5)-34, mean = 11 mg S kg^?1 by the latter]. This -SH enrichment in the humic acids from acid soils may result from the degradation and subsequent humification of S-rich debris of plants and micro-organisms and/or direct incorporation of volcanic acid gas (H₂S) into the humic acids.     

New methodology to assess the quantity and quality of humic substances in organic materials and commercial products for agriculture

Purpose

The traditional method to determine humic content (humic and fulvic acids) in commercial fertilizers, biostimulants, and organic materials is based on the oxidation of the organic carbon contained in the basic-soluble but acid-insoluble fraction (humic acids) and the basic-acid soluble fraction (fulvic acids) of their alkaline water extracts. This methodology, merely operational, makes it impossible to distinguish if the quantified carbon corresponds to substances with “humic” chemical nature or to non-humic organic matter but with similar solubility properties to those of humic matter. The aim of this work is to develop a new methodology that not only quantifies the humic content in commercial products (and raw materials) but also assesses the humic quality of the quantified organic matter.

Materials and methods

To this end, humic and fulvic (-like) fractions have been isolated/purified from several humic and non-humic materials and characterized by means of elemental analysis and UV-visible, fluorescence, and infrared spectroscopies, and these data have been used to perform a discriminant analysis (DA).

Results and discussion

The model obtained from the DA is able to discriminate humic and fulvic fractions from apparently humic or fulvic ones and provides discriminant classification functions that have proven to successfully predict the “humic quality” of the fractions isolated from commercial products, after their elemental and spectroscopic characterization.

Conclusions

Therefore, the combination of the fractionation, characterization, and evaluation by the DA is proposed as an effective methodology for quantifying and assessing the quality of the humic content claimed in the labels of commercial products.

     

Reduction of Cr(VI) by soil humic acids

The rate of hexavalent chromium reduction by a soil humic acid (SHA) was investigated in aqueous solutions where concentrations of Cr(VI), H⁺, and SHA were independently varied. Rate experiments were done with a large excess of SHA over Cr(VI). Rates of reduction depend strongly on [H⁺], increasing with decreasing pH. Typical Cr(VI)-SHA reactions display a nonlinear reduction of Cr(VI) with time that cannot be modelled with simple first- or second-order rate equations. An empirical rate equation is developed for Cr(VI)-soil humic acid reactions over a range of experimental conditions. The model is in part based on a reactive continuum concept developed for soil fulvic acids. The rate equation describing Cr(VI) reduction by SHA is: R= -(k₀+k[H⁺]^1/2)[HCrO₄^?]^1/2X_e^?1, where k₀ is (8·3 ± 1·2) × 10^?12, s^?1k is (2·04 ± 0·05) × 10^?9 l^1/2 mol^?1/2 s^?1, and X_e is the equivalent fraction of SHA oxidized. The rate equation adequately models Cr(VI) reduction in an experiment with [Cr(VI)]₀ four times greater than the maximum concentration used in its derivation. Cr(VI) reduction at pH 3 by two other SHAs can also be modelled using the rate equation. The difference between the rate coefficients for the humic acid and the fulvic acid from the same soil was greater than the difference in the rate coefficients for humic acids from different soils.     

Effect of soil invertebrates on the formation of humic substances under laboratory conditions

The complete polymerization of phenols and proteins (one of the processes involved in the formation of humic substances) was explained. It was shown that fly (Bibio marci) larvae and earthworms (Aporrectodea caliginosa) participate in the complete polymerization of phenols and proteins. In a laboratory experiment, invertebrates participated in the degradation of organic matter and the synthesis of humic substances, which was proved in experiments with ¹⁴C-labeled phenols and proteins. The same organic substances (phenols and proteins) without the impact of invertebrates were used as the control substances. The distributions of the ¹⁴C isotope in alkaline extracts separated by solubility in acids (humic and fulvic acids) was compared to those of the control substances. The portion of the ¹⁴C isotope in the humic acids in the excrements of Bibio marci was higher than that in the control substances. The content of ¹⁴C-labeled humic substances in the excrements of the earthworm Aporrectodea caliginosa exceeded the control values only in the experiment with proteins. When clay material was added to the organic substances, the portion of the ¹⁴C isotope in the humic acids increased in both experiments with phenols and proteins. When these substrates passed through the digestive tracts of the invertebrates, the polymerization of organic substances and the inclusion of proteins and phenols into humic acids occurred.     

EXPERIMENTAL CONDITIONS CONCERNING POTENTIOMETRIC TITRATION OF HUMIC ACID

The humic acids of four soils from Denmark have been extracted by EDTA. The humic acids were separated from EDTA by precipitation with Ba²⁺. The ash content was lowered by treatment with HF and HC1. The ash content in the four humic acids was less than 0.6 per cent. Chemical analysis of the humic acids supply some evidence for the suggestion that EDTA extracted organic matter is a more definite fraction than humic acid extracted by NaOH. It was shown that an alkaline humate solution stored in sterile, sealed ampoules under N₂ does not change its initial pH, indicating that the protolytic equilibrium is established instantly. It is therefore proposed that the titration of humic acid should be carried out as quickly as a normal acid-base titration.     

Analysis of the variable charge of two organic soils by means of the NICA-Donnan model

We have tested to see if the generic set of NICA‐Donnan model parameters, used to describe isolated humic substances, can also describe soil humic substances in situ. A potentiometric back‐titration technique was used to determine the variable surface charge of two organic peat soils at three different ionic strengths. The non‐ideal, competitive‐adsorption NICA‐Donnan model was used to simulate the surface charge, by assuming a bimodal distribution of H⁺ affinity on the soil solid phase. The model provided an excellent fit to the experimental data. The Donnan volume, V_D, varied slightly with ionic strength, although the variation was less than for humic substances in solution. The values obtained for the parameters that define the affinity distributions, the intrinsic proton binding constant (log K_i^int) and the heterogeneity of the site (m_i), were similar to those observed for isolated soil humic acids. The abundance of carboxylic groups in the whole soil represented 30% of the typical value for isolated soil humic acids. The composition of the organic matter of the whole soils, obtained by ¹³C CPMAS NMR, was comparable to the characteristic composition of soil humic acids.     

Temporal changes in humic acids in cultivated soils with continuous manure application

Abstract

Although the application of manure to upland fields is believed to induce changes in the quality of humic substances in soil as well as the quantity, the direction and extent of these changes have not been elucidated. To understand temporal variations in humic acids, periodically collected soil samples from two fields, a Typic Hapludult (Togo) and a Pachic Melanudand (Kuriyagawa), with cattle manure and chemical fertilizer (CF) were examined. The content and degree of humification (darkening) of the humic acids were distinctly greater in Kuriyagawa than in Togo soil. Corresponding to the difference in the degree of humification, molecular size distribution, elemental composition, infrared (IR) spectra, and ¹³C cross polarization/magic angle spinning nuclear magnetic resonance (CPMAS NMR) spectra of humic acids differed between the two soils. Manure application at 40 Mg ha^?1 year^?1 for 16 years (Togo) and at 80 or 160 Mg ha^?1 year^?1 for 19 years (Kuriyagawa) resulted in greater humic acid content compared with plots with CF only because of its increase in the manured plots and/or decrease in the CF plots. Manure application at an extremely high rate (160 Mg ha^?1 year^?1) resulted in higher H content and greater signal intensities of alkyl C, O-alkyl C and amide C=O in the ¹³C CPMAS NMR and/or IR spectra. Although humic acids with larger molecule sizes increased in all the manured plots, differences between the humic acids from the plots with and without manure applied at practical levels in the elemental and spectroscopic analyses were small or scarce. These results were considered to be because of the similarity between the indigenous soil humic acids and the manure-derived ones in Togo soil (a low degree of humification) and because of the abundance of highly-humified humic acids in Kuriyagawa soil.     

Physiological action of humic substances on microbial cells

By incorporating molecular weight fractions of humic acids of various origins into selective substrates designed for the enumeration of physiological groups of microorganisms, it was found that the presence of humic acids at concentrations of up to 30mgl^?1 normally resulted in increased numbers of soil microbes active within a particular physiological group. Observed increases could be as much as 2000-fold. Microbes in an organic humus-rich soil were more stimulated by humic substances than organisms from a sandy soil.In certain microbes humic substances appeared to induce a change in metabolism, allowing the organisms to proliferate on substrates which previously they could not utilize. Indications were obtained that within the 10–30 mgl^?1 concentration range lower molecular weight humic fractions (approx. 5500 dallons) were more effective than higher molecular weight material. At higher concentrations the reverse was sometimes noticed. Similarly, fulvic acids at concentrations of up to approximately 50mgl^?1 would appear to have a more pronounced physiological effect than humic acids, whereas the latter might be more effective at higher concentrations.The response of certain physiological groups to humic products of natural origin appeared to be comparable to that of surfactants such as Tween and Brij. This would suggest that the physiological action of humic substances is, at least partly, the result of their surface activity, making the membrane one of the prime targets of the physiological action of humics on living cells.     

Selective digestion of the proteinaceous component of humic substances by the geophagous earthworms Metaphire guillelmi and Amynthas corrugatus

Humic substances play a key role in the global carbon cycling and the sequestration of micropollutants in soil. The transformation of these substances by earthworms, the dominant soil macroinvertebrates of many terrestrial ecosystems, and the mechanisms involved are still obscure. We prepared two chemically identical humic model compounds that were specifically ¹⁴C-labeled either in the aromatic or the proteinaceous component, and added them to soil incubated with the geophagous earthworm species Metaphire guillelmi (anecic) and Amynthas corrugatus (endogeic). In the absence of the earthworms, both the aromatic and the proteinaceous components were mineralized at similarly low rates (5−8% after 9 days of incubation). In the presence of the earthworms, mineralization rate of the proteinaceous component was strongly stimulated (2-fold by M. guillelmi and 1.4-fold by A. corrugatus). The mineralization rate of the aromatic component was (slightly) stimulated (1.2-fold; P < 0.05) only by A. corrugatus. In all cases, the stimulated mineralization was accompanied by a transformation of radiolabeled humic acids to fulvic acids within the earthworm guts and by an incorporation of radiolabel into the earthworm tissues. Digestion of the proteinaceous component of humic acids by the earthworms was corroborated also by a decrease of extractable humic acids in fresh cast and a stimulated mineralization of soil nitrogen; in the case of M. guillelmi, the fresh cast contained sixfold more NH₄⁺ than the non-ingested soil. Our study provides direct evidence for the selective digestion of humic components by earthworms. Considering the ubiquity of geophagous earthworms and their large biomass, the alteration of the chemical structure of humic substances by the earthworms through their selective digestion of peptidic components may have significant impacts on the stability of humic substances and the bioavailability of micropollutants in soil.     

In-source pyrolysis-field ionization mass spectrometry and Curie-point pyrolysis-gas chromatography/mass spectrometry of amino acids in humic substances and soils

With the aid of in-source pyrolysis-field ionization mass spectrometry (Py-FIMS) and Curie-point pyrolysis-gas chromatography/mass spectrometry (cPy-GC/MS) in the conventional electron impact mode, characteristic signals of 23 amino acid standards were described. Thermal and mass spectrometric fragmentation pathways of these amino acids differed with each method and complemented each other. Pyrolysis products assigned by Py-FIMS extended the range of signals for N-containing compounds in humic substances and soil organic matter and gave marker signals for free amino acids and their subunits in proteinaceous materials. These characteristic signals were correlated with the amino acid content in N-rich humic fractions consisting of seven fulvic acids and eight humic acids. The selected marker signals reflected 25–84% of the variances of the molar distribution of acidic, neutral, neutral aromatic, and basic amino acids in the humic fractions. In addition, a well described agricultural soil (0.08% amino acid N) was spiked with a standard amino acid mixture (0.08 mg amino acid N 100 mg^-1 dry soil) and produced enhancements of the relative abundances of the corresponding amino acid signals. Moreover, for 27 samples of whole agricultural soils of widely different origins, soil types, and vegetations, 15 selected amino acid indicators were correlated significantly with -amino N (r=0.76^***) and total N (r=0.65^***).     

Decomposition and distribution of residual activity of some 13C-microbial polysaccharides and cells,glucose, cellulose and wheat straw in soil

Studies were made to determine the rate of decomposition of some ¹⁴C-labeled microbial polysaccharides, microbial cells, glucose, cellulose and wheat straw in soil, the distribution of the residual ¹⁴C in various humic fractions and the influence of the microbial products on the decomposition of plant residues in soil. During 16 weeks from 32 to 86 per cent of the C of added bacterial polysaccharides had evolved as ¹⁴CO₂. Chromobacterium violaceum polysaccharide was most resistant and Leuconostoc dextranicus polysaccharide least resistant. In general the polysaccharides, microbial cells, and glucose exerted little effect on the decomposition of the plant products. Upon incubation the ¹⁴C-activity was quickly distributed in the humic. fulvic and extracted soil fractions. The pattern of distribution depended upon the amendment and the degree of decomposition. The distribution was most uniform in the highly decomposed amendments. After 16 weeks the bulk of the residual activity from Azotobacter indicus polysaccharide remained in the NaOH extracted soil. From C. violaceum polysaccharide both the extracted soil and the humic acid fraction contained high activity. About 50–80 per cent of the residual activity from the ¹⁴C-glucose, cellulose and wheat straw amended soils could be removed by hydrolysis with 6 n HCl. The greater part of this activity in the humic acid fraction was associated with the amino acids and that from the fulvic acids and residual soils after NaOH extraction with the carbohydrates. About 8 16 per cent of the activity of the humic acid fraction was present in substances (probably aromatic) extracted by ether after reductive or oxidative degradation.     

THE HUMIC ACIDS EXTRACTED BY VARIOUS REAGENTS FROM A SOIL

Solutions of o.5N NaOH, o.1M pyrophosphate (pH 7), and o.5N Na(CO^2?₃/HCO^?₃) [2:1] extract humic acid and organic matter from a soil with decreasing effectiveness. Pre-treating the soil with o.1N HC1 increased the yield of humic acid obtained with the alkaline extractants. An additional pre-treatment with a mixture, which was normal with respect to HC1 and HF, gave a slight reduction in yield. Increasing the temperature of extraction increased the yield of humic acid. The total C extracted was usually in excess of the humic acid recovered. The difference was obtained as ‘humins’. The sum of the Fe₂O₃, SiO₂, and A1₂O₃ contents of the humic acids was always less than 2 per cent. Where the extraction was carried out at room temperatures the SiO₂/Al₂O₃ ratio suggested that they might be present as clay mineral. When extraction was carried out at an elevated temperature this ratio was altered. Humic acids of low ash content (0.1–0.5 per cent) could be obtained by the use of hot reagents. Of all the extractants used at room temperature, pyrophosphate produced the humic acid of lowest ash content (～ 0.2 per cent). The Fe₂O₃ content of the humic acids was not correlated with their SiO₂ or A1₂O₃ content. The N-content of the humic acids was substantially independent of the method of extraction. The cation-exchange capacities (C.E.C.), average pK values and range of pK values, have been determined from the titration curves of the humic acids. These quantities vary with the method of extraction. There are good correlations between cation exchange capacity and both average pK values and the range of pK values. The within-molecule variation of pK values appears to be greater than the between-molecule variation. No correlation exists between C.E.C. and Fe, Al, Si, and N content of the humic acids.     

Nitrogen use Rationalization and Boosting Wheat Productivity by Applying Packages ‎of Humic,Amino Acids,and Microorganisms

ABSTRACT

Integrated management of soil organic matter and nutritional status of crop plants is essential to sustain the production of organic farming systems. Thus, a 2–year field experiment was conducted to examine the effects of soil additions (192 kg N ha^–1, humic+192 kg N ha^–1, humic+144 kg N ha^–1 and humic+96 kg N ha^–1) and foliar applications (amino acids, Azotobacter+yeast, and amino acids plus Azotobacter+yeast) as various fertilizer resources on growth and yield of wheat. Results showed that humic+192 kg N ha^–1 × amino acids plus Azotobacter+yeast were the effective combination for producing the highest values of flag leaf area, total dry weight, tiller number m^–2, spike weight m^–2, and grain yield ha^–1. Under foliar application of amino acids plus Azotobacter+yeast, reducing N supply from recommended rate (192 kg N ha^–1) to 144 kg N ha^–1+ humic achieved higher values of all yield traits, with a saving of 25% of applied mineral nitrogen as well as enhancing nitrogen use efficiency.     

Assessment of degradation and generation of humus in a coffee soil affected by weed cover by means of a stable carbon isotopic ratio

We evaluated the effect of soil conservation by weeds on the degradation and generation of humic acids, fulvic acids, and water‐soluble non‐humic substances (WS‐NHS) in a red‐acid soil (Vertic Dystrudept) (Indonesia) from the changes in humus composition and stable carbon isotopic ratio (δ¹³C). Three plots, a weeded plot (T‐1; the common practice), a plot covered with Paspalum conjugatum Berg., a C₄ plant (T‐2), and a plot in which native weeds were allowed to grow (T‐3), were prepared. An incubation experiment determined the δ¹³C values of the humus fractions generated from Paspalum in soil. Based on the increase in δ¹³C value, the proportion of total C that originated from Paspalum C after 4 years under coffee was 16 ± 4% in the T‐2 topsoil (0–10 cm). Humic and fulvic acids in the T‐1 topsoil decreased to 46 and 84%, respectively, whilst both increased or remained constant in the T‐2 and T‐3 soils. The WS‐NHS content varied little and was independent of land management. The preferential loss of the humic acids with a smaller degree of humification as assessed by their darkness in colour was shown in T‐1. The decrease in the degree of humification suggested the accumulation of the weed‐derived humic acids in T‐2 and T‐3. In the T‐2 topsoil, 36 ± 2%, 13 ± 3% and 15 ± 2% of C in the humic acids, fulvic acids and WS‐NHS, respectively, were estimated to be Paspalum‐derived after 4 years. The estimated initial C loss during the same period was 17 ± 3%, 14 ± 2% and 7 ± 2%, respectively, for those fractions, which suggests the fastest turnover rate for the humic acids and significant retardation of their degradation in soil colonized by weeds.     

Acid pyrophosphate extraction of soil fulvic acids

A simple three step method is described for isolation of soil fulvic acids in high yield. The complexing agent H₂P₂O₇^2? (at pH 2) is used to release soil-bound fulvic acids. Extraction of humic acids is minimal. Selective separation of the protonated fulvic acids from the ionic extractant is achieved on a non-ionic polyacrylate resin (Amberlite XAD-7); after washing the resin, fulvic acids were retrieved in >98% yield by adjusting the pH to 6.5. Two problems associated with the classical alkali extraction method are avoided: possible alkaline oxidation of phenolic components, and their oxidation by Fe³⁺ under the acidic conditions employed to precipitate humic acids. The product typically has an ash weight of <0.6% after one XAD treatment. The method has been applied to three soils and one IHSS peat sample.