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Published online 11 May 2005
Published in J Environ Qual 34:1131-1137 (2005)
DOI: 10.2134/jeq2004.0378
© 2005 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
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TECHNICAL REPORTS

Waste Management

Proton Binding by Humic and Fulvic Acids from Pig Slurry and Amended Soils

César Plazaa,*, Juan C. García-Gila, Alfredo Poloa, Nicola Senesib and Gennaro Brunettib

a Centro de Ciencias Medioambientales, Consejo Superior de Investigaciones Científicas, Serrano 115 dpdo., 28006 Madrid, Spain
b Dipartimento di Biologia e Chimica Agroforestale ed Ambientale, University of Bari, Via Amendola 165/A, 70126 Bari, Italy

* Corresponding author (c.plaza{at}ccma.csic.es)

Received for publication October 8, 2004.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The knowledge of acid–base characteristics of humic acid (HA) and fulvic acid (FA) fractions of organic amendments and amended soils is of considerable importance for assessing their agronomic efficacy and environmental impact. In this work, the acid–base properties of HAs and FAs isolated from pig slurry, soils amended with either 90 or 150 m3 ha–1 yr–1 of pig slurry for 3 yr, and the corresponding nonamended control soil were investigated by using a current potentiometric titration method. The nonideal competitive adsorption (NICA) model that describes proton binding by two classes of binding sites (carboxylic- and phenolic-type groups) was successfully fit to titration data. With respect to the control soil HA and FA, pig-slurry HA and FA were generally characterized by smaller carboxylic-type group contents, slightly smaller phenolic-type group contents, larger affinities for proton binding by the carboxylic-type groups, and much smaller, in the case of the HA fraction, or similar, in the case of the FA fraction, affinities for proton binding by the phenolic-type groups. Amendment with pig slurry determined a number of modifications in soil HAs and FAs, including decrease of acidic functional group contents, and slight increase of the proton affinity of the carboxylic-type groups. Further, a slight decrease of the affinities for proton binding by the phenolic-type groups of HAs was observed. These effects can have a large impact on the biological availability, mobilization, and transport of macro- and micronutrients, toxic metal ions, and xenobiotic organic cations in pig slurry–amended soils.

Abbreviations: FA, fulvic acid • HA, humic acid • NICA, nonideal competitive adsorption • PS, pig slurry • PS0, control soil • PS90 and PS150, soils amended with either 90 or 150 m3 ha–1 yr–1 of pig slurry for 3 yr


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
HUMIC SUBSTANCES are quantitatively and qualitatively the major components of the mixture of materials that comprise soil organic matter responsible of soil fertility and soil protection from degradation. The principal fractions of humic substances are humic and fulvic acids, which feature a colloidal, polydispersed, and polyelectrolytic character, a mixed aliphatic and aromatic nature, and the presence of various chemically reactive functional groups, including carboxyls and phenolic and alcoholic hydroxyls, which confer a prevalent acidic character to these substances. Despite the general structural similarity, the soil HA fraction is characterized by greater molecular weight, carbon content, and aromaticity, and smaller content of oxygenated functional groups and hydrophilic character than the FA fraction (Stevenson, 1994; Senesi and Loffredo, 1999).

Recycling in agriculture of organic wastes produced by various animal breeding, such as pig slurry, is a common practice throughout the world, which can significantly affect, not only the total content, but also the chemical and biochemical nature and functions of native soil organic matter, including its HA and FA fractions (Plaza et al., 2002, 2003). The presence of acidic functional groups in the molecular structure of HAs and FAs contributes largely to the acid–base buffering capacity of soils and confers to HAs and FAs the capacity to bind and exchange cations, thus playing an important role in the retention and release, biological availability, and mobility of metal ions in soils (Senesi, 1992; Stevenson, 1994). Consequently, an in-depth acid–base characterization of the HA and FA fractions of pig slurry and pig slurry–amended soils in terms of acidic functional group contents and proton-binding affinities is important for achieving a better understanding of the agronomic efficacy and environmental impact of pig-slurry amendment.

The complex mixture of acidic functional groups involved in proton binding by HAs and FAs makes their acid–base characterization quite difficult (Perdue and Lytle, 1983; Perdue, 2001). The typical polyelectrolytic nature of HAs and FAs further complicates the description of proton binding because of electrostatic interactions that give rise to ionic strength effects (Perdue and Lytle, 1983; Milne et al., 1995; Koopal et al., 2001). These problems have prompted the use of models in which various assumptions are included.

The recently developed NICA (nonideal competitive adsorption) family of models have shown considerable success for describing proton and metal binding to humic substances over a wide range of conditions (Koopal et al., 1994; Benedetti et al., 1995, 1996; Kinniburgh et al., 1996, 1999; Milne et al., 2001). These models assume a continuous distribution of ion-binding sites and recognize the experimentally observed fact that the affinity distributions are ion-specific. In the basic NICA model for proton binding, the intrinsic chemical heterogeneity of humic materials and proton-specific nonideality are, therefore, considered explicitly. Further, the polyeletrolytic nature of ion adsorption to humic substances is accounted for in a generic way by the nonideality parameters (Koopal et al., 1994; Benedetti et al., 1995).

The objectives of this work were to (i) determine and discuss comparatively the acid–base properties of PS-HA and PS-FA with respect to unamended soil HA and FA and (ii) investigate the effect of PS applications on proton binding by soil HAs and FAs. To reach these objectives, potentiometric titration and the NICA model were used.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Humic and Fulvic Acid Samples
The HA and FA samples used in this work were isolated according to conventional procedures from a pig slurry (PS-HA and PS-FA), two soils amended with pig slurry at a rate of either 90 or 150 m3 ha–1 yr–1 for 3 yr (PS90-HA and PS90-FA, and PS150-HA and PS150-FA, respectively), and the corresponding nonamended control soil (PS0-HA and PS0-FA), which were collected from a pig-breeding farm and an experimental field located in the Toledo province (Spain). The sampling site, the experimental design, the main chemical properties of pig slurry, pig slurry–amended soils and unamended control soil, and the isolation procedures and major compositional, structural, and functional properties of HAs and FAs are described in detail elsewhere (Plaza et al., 2002, 2003).

Potentiometric Analyses of Humic and Fulvic Acids
Potentiometric titrations were performed using a Model DL77 titrator equipped with a Model DG-111-SC pH electrode (Mettler Toledo, Columbus, OH), which was previously calibrated with standard buffers at pH 4.00, 7.00, and 10.00. Aliquots of 30 to 40 mg of freeze-dried HAs and FAs were dissolved in 17.5 mL of 0.05 M NaOH in 50-mL thermostatic vessels under N2 gas. A volume of 0.025 M H2SO4 was added to each solution, which was sufficient to neutralize the excess NaOH and to lower the pH from approximately 11 to approximately 3. Standardized 0.05 M NaOH was then dispensed to each solution in aliquots of 0.05 mL by using an automatic syringe. The volume of titrant added and pH (when stable for 10 s with a drift of no more than ±0.002 pH units) were recorded after each addition of titrant. Samples were maintained at a constant stirring speed, at a temperature of 298 K, and under N2 atmosphere throughout the titrations.

Theory
The concentration of organic charges in solution at any point during titration can be calculated by the charge-balance equation:

[1]
where Orgi– represents the ith organic charge and brackets denote molar concentrations. Assuming the HA or FA samples as mixtures of monoprotic acids, the ionic strength (I) of solution may be estimated by:

[2]
Combination of Eq. [1] and [2] yields the ionic strength:

[3]

The concentrations of H+ and OH– in Eq. [1] were calculated from the pH and activity coefficients of both ions, which were, in turn, calculated by the iterative use of Eq. [3] and the Davies Eq. [4] (Davies, 1962):

[4]
where {gamma}i and zi are, respectively, the activity coefficient and charge of i.

As expected, changes in ionic strength during titration were less than 5% of the initial value of 0.04 M. The value of in Eq. [1] was normalized to the weights of HA and FA on a moisture- and ash-free basis to obtain the charge (Q) of each HA and FA sample expressed as mol kg–1. Titration data at a pH of <3.5 or >10.5 were considered unreliable, and were not used for subsequent calculations.

Once the charge was calculated as a function of pH, the carboxyl group content was estimated as the value of Q at pH 8, and the phenolic OH group content was estimated as two times the change in Q between pH 8 and 10 (Ritchie and Perdue, 2003).

The affinity distribution for each HA and FA, that is, the probability of finding sites with an affinity in the range log KH + d log KH, can be estimated by using methods based on the local isotherm approximation. Among various available methods, the condensation approximation method (Nederlof et al., 1992) was used to calculate the distribution function, FCA (log KH), from the first derivative of the curve Q vs. proton concentration:

[5]

[6]
where log KH is the affinity constant for the protonation reaction.

Titration data were also examined by the NICA model using the basic equation for proton binding by i-type sites (Koopal et al., 1994):

[7]
where {theta}H,i denotes the degree of protonation of the i-type sites, {joenq1131-eqn9} H,i represents the median value of the affinity distribution for proton binding by the i-type sites, and the value of mi determines the width of the affinity distribution. Equation [7] can be easily extended to include two or more distributions of proton binding sites by weighted summation of charge contributions of different site types:

[8]
where Qmax,i is the total amount of i-type sites.

The optimum set of NICA parameters (i.e., Qmax,i, {joenq1131-eqn9} H,i, and mi) for each HA or FA sample was obtained by nonlinear fit of the NICA model to titration data in the form of Q as a function of proton concentration. The initial charge was estimated by including it as an additional fitting parameter (Milne et al., 1995). Full, unconstrained optimization was possible when using a hybrid algorithm based on the Marquardt–Levenberg approach to minimize the weighted residual sum of squares (Kinniburgh et al., 1999). The FIT 2.5 program was used for calculations (Kinniburgh, 1993).

Although it is possible to extend the NICA model to account specifically for ionic strength effects (e.g., by combination with a Donnan description of the electrostatic behavior of the HA or FA particle [NICA–Donnan model] [Kinniburgh et al., 1996]), in this work only a single ionic strength was considered; thus, further calculation of the salt-dependence of proton binding was not performed.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Chemical Characteristics of Humic and Fulvic Acids
The compositional, structural, and functional chemical properties of HAs and FAs examined in this paper have been previously described (Plaza et al., 2002, 2003). Briefly, PS-HA had larger S and N contents, aliphatic character, and molecular heterogeneity, and smaller acidic functional group and organic free radical contents and smaller degrees of aromatic polycondensation and humification than PS0-HA. The PS amendment increased C, S, and COOH contents, C to N ratios, and aliphaticity, and decreased extraction yields and contents of N, O, phenolic OH, and organic free radicals in soil HAs.

Compared with the corresponding HAs, the FAs featured a different elemental and acidic functional group composition, much larger E4 to E6 ratios, and relative fluorescence intensity values, much smaller organic free radical concentrations, and markedly different Fourier transform infrared and fluorescence spectra. Compared with PS0-FA, PS-FA was characterized by a prevalent aliphatic character, large contents of S- and N-containing and polysaccharide components, very small organic free radical concentration, great molecular heterogeneity, and small degrees of aromatic ring polycondensation, polymerization, and humification. Further, PS application to soil increased the contents of C, N, S, total acidity and phenolic OH, and aliphaticity, and decreased extraction yields, O and COOH contents, O to C ratio, E4 to E6 ratio, and organic free radical concentration of soil FAs.

Charge versus Proton Concentration Curves
The experimental data for proton binding by HAs and FAs isolated from pig slurry and control and amended soils expressed as negative charge on the HA and FA as a function of proton concentration are shown in Fig. 1 and 2 , respectively. In agreement with previous findings (Milne et al., 2001; Ritchie and Perdue, 2003), the HAs have smaller charge densities across the entire range of proton concentration than the corresponding FAs. Furthermore, the charge densities of PS-HA across the range of proton concentration are much smaller than the values of any soil HA, whereas the charges of PS-FA are only slightly smaller than the values of soil HAs. The charges of amended soil HAs and FAs are smaller than those of the corresponding PS0-HA and PS0-FA, and tend to diminish with increasing the PS-amendment rate, especially in the case of the soil HAs.



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  		Fig. 1. Negative charge of humic acids (HAs) isolated from pig slurry (PS), control soil (PS0), and soils amended with pig slurry at a rate of either 90 or 150 m3 ha–1 yr–1 for 3 yr (PS90 and PS150, respectively) versus proton concentration in solution.

 


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  		Fig. 2. Negative charge of fulvic acids (FAs) isolated from pig slurry (PS), control soil (PS0), and soils amended with pig slurry at a rate of either 90 or 150 m3 ha–1 yr–1 for 3 yr (PS90 and PS150, respectively) versus proton concentration in solution.

 
Proton-Affinity Distributions
In agreement with previous studies on humic substances of various origin and nature (Milne et al., 1995, 2001; Fiol et al., 1999), the proton-affinity distribution derived from the charge vs. proton concentration curves of the HAs and FAs examined (Fig. 3 and 4 , respectively) clearly feature two broad peaks. The peak at small log KH reflects sites with small proton affinity (log KH = approximately 4.5), and can be assigned to carboxylic moieties, whereas the peak at large log KH reflects sites with great proton affinity (log KH = approximately 9.8), and is assigned preferentially to phenolic OH and possibly to other very weakly acidic groups such as alcohols and enols (Perdue, 1985; Benedetti et al., 1995). However, PS-HA and PS-FA contain large amounts of S and N groups (Plaza et al., 2002, 2003) possibly in the form of proteinaceous amino sugar and bacterial cell-wall peptidoglycan components. These peptidoglycans contain protonated basic amino acids such as lysine in which the basic group deprotonates in the same log KH range of phenols. Thus, the latter peak may also be ascribed, at least in part, to protonated amino groups of basic amino acids such as lysine of which these substances are particularly rich (Plaza et al., 2002, 2003). For sake of simplicity, in the following text we will refer to "phenolic-type" groups that include all other kinds of very weakly acidic groups in PS-HA and PS-FA.



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  		Fig. 3. Affinity distributions for proton binding by humic acids (HAs) isolated from pig slurry (PS), control soil (PS0), and soils amended with pig slurry at a rate of either 90 or 150 m3 ha–1 yr–1 for 3 yr (PS90 and PS150, respectively).

 


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  		Fig. 4. Affinity distributions for proton binding by fulvic acids (FAs) isolated from pig slurry (PS), control soil (PS0), and soils amended with pig slurry at a rate of either 90 or 150 m3 ha–1 yr–1 for 3 yr (PS90 and PS150, respectively).

 
In agreement with Milne et al. (2001), the distributions of proton-binding sites are broader and flatter for the HAs than for the FAs, which indicates a larger degree of heterogeneity in the former. Furthermore, the first maximum is centered for the HAs, especially for PS-HA, at slightly larger log KH values than for the FAs, whereas the opposite occurs for the second peak. Results provided by the condensation approximation method do not permit further distinction and comparative discussion between the proton-affinity distributions of the soil HAs and those of the PS-FA and soil FAs, since there is considerable overlap between the curves.

Nonideal Competitive Adsorption Model Parameters
Because the proton-affinity distributions clearly indicate the presence of two different types of binding sites, the NICA equation for a bimodal distribution was fitted to the experimental curves of charge vs. proton concentration. Table 1 shows the best-fit parameters obtained for each HA and FA sample, that is, Qmax,1, Qmax,2, log {joenq1131-eqn9} H,1, log {joenq1131-eqn9} H,2, m1, and m2, where the suffixes 1 and 2 denote the two classes of acidic functional groups that are assumed to be, respectively, the carboxylic-type groups and the phenolic-type groups, as stated previously, and consistently with the conventional interpretation of acid–base properties of humic substances.


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  		Table 1. Fitting parameters of the nonideal competitive adsorption (NICA) model for humic acids (HAs) and fulvic acids (FAs) isolated from the pig slurry (PS), control soil (PS0), and soils amended with pig slurry at a rate of either 90 or 150 m3 ha–1 yr–1 for 3 yr (PS90 and PS150, respectively).{dagger}

 
The large values found for the coefficient of determination (r2) and the small residual mean square errors (RMSE) indicate that the NICA model fits very well to the experimental datasets for the HAs and FAs examined, as it was previously demonstrated in other works on similar systems (Benedetti et al., 1995; Milne et al., 2001). The total acidity (Qmax,1 + Qmax,2), carboxyl and phenolic OH group contents (Qmax,1 and Qmax,2, respectively), the median values of the affinity distributions for proton binding by carboxyl groups and phenolic OH groups (log {joenq1131-eqn9} H,1 and log {joenq1131-eqn9} H,2, respectively) , and the width of proton-affinity distributions of carboxyl and phenolic OH groups (m1 and m2, respectively) of HAs and FAs estimated by the NICA model are described and discussed comparatively in the following text.

Acidic Functional Group Contents
The total acidity and carboxyl group contents (Qmax,1 + Qmax,2 and Qmax,1, respectively, in Table 1) of HAs are markedly smaller than those of FAs, whereas the phenolic OH group contents (Qmax,2 in Table 1) are similar. In agreement with recently published data (Milne et al., 2001), carboxyl groups constitute from 64 to 66% of the total acidity for soil HAs, and from 77 to 78% for soil FAs, which indicates that most of the acidity of the HAs and especially of the FAs is attributable to carboxyl groups. In contrast, PS-HA and PS-FA exhibit relatively larger phenolic OH group contents (66 and 24%, respectively) than soil HAs and FAs. According to Ritchie and Perdue (2003), HAs and FAs formed in systems rich of water or poorly aerated have significantly larger phenolic contents than their terrestrial counterparts.

As a consequence of the smaller total acidity and carboxyl and phenolic OH group contents of PS-HA and PS-FA, with respect to those of nonamended soil HA and FA, the acidic functional group contents of amended soil HAs and FAs decrease with increasing PS-amendment rate but remain larger than those of PS-HA and PS-FA, respectively.

The acidic functional group contents of HAs and FAs isolated from PS and nonamended and amended soils measured either in this work by direct titrations or previously obtained by indirect titrations using the Ba(OH)2 and Ca(CH3COO)2 methods (Plaza et al., 2002, 2003) are listed in Table 2. Although the values of carboxyl and phenolic OH group contents obtained by the direct titration method are, respectively, slightly larger and slightly smaller than the corresponding values obtained by using the NICA model (Table 1), both sets of values lead to the same general conclusion. However, in agreement with results of previous studies (Perdue, 1985; Stevenson, 1994; Ritchie and Perdue, 2003), the total acidity and carboxyl and phenolic OH group contents obtained by indirect titration are much larger than those obtained by direct titration. In consequence, the comparison of the data obtained by indirect titration with those estimated by the NICA model reveals substantial inconsistent trends, both in the total acidity and in the carboxyl and phenolic OH group contents.


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  		Table 2. Acidic functional group contents of humic acids (HAs) and fulvic acids (FAs) isolated from the pig slurry (PS), control soil (PS0), and soils amended with pig slurry at a rate of either 90 or 150 m3 ha–1 yr–1 for 3 yr (PS90 and PS150, respectively) as measured by direct titrations using the pH-based method and indirect titrations using the Ba(OH)2 and Ca(CH3COO)2 methods.

 
According to Ritchie and Perdue (2003), it is likely that acid-generating side reactions may occur when total acidity is measured after a 24-h exposure of a humic substance to extremely alkaline conditions, and that part of the humic material may not be removed as insoluble Ba-salt during filtration. Furthermore, the carboxyl group contents obtained by Plaza et al. (2002)(2003) might be affected by ion exchange processes possibly occurring during the Ca(CH3COO)2 procedure, which implies that acidic OH groups of HAs and FAs can ionize below the pH of the acetate reaction mixture (Perdue, 1985; Stevenson, 1994). Furthermore, as in the case of total acidity measurements by the Ba(OH)2 method, incomplete removal of humic matter during filtration might have occurred.

Proton-Binding Affinities
In all cases, the median values of affinity distributions for proton binding by carboxylic-type groups (log {joenq1131-eqn9} H,1) are larger for the HAs than for the corresponding FAs (Table 1), which confirms that carboxyl groups of FAs are generally more acidic than those of HAs (Ritchie and Perdue, 2003). The log {joenq1131-eqn9} H,1 values of PS-HA and PS-FA are larger than the corresponding values of PS0-HA and PS0-FA, which are, in turn, slightly smaller than those of the corresponding amended soil HAs and FAs.

In contrast, the median value of affinity distributions for proton binding by phenolic-type groups (log {joenq1131-eqn9} H,2) is much smaller for PS-HA than for PS-FA, and slightly smaller for soil HAs than for soil FAs. Furthermore, the log {joenq1131-eqn9} H,2 values of PS-HA are much smaller than the corresponding values of PS0-HA, whereas the log {joenq1131-eqn9} H,2 values of PS-FA are similar to the value of PS0-FA. Pig-slurry amendment appears to cause a slight increase of acidity of phenolic-type groups of soil HAs, and not to affect substantially the acidity of these groups in soil FAs.

Width of the Affinity Distributions
The widths of distributions (i.e., the ranges of log {joenq1131-eqn9} H values) of carboxylic-type groups (m1) of HAs are quite similar to, and those of phenolic-type groups (m2) are smaller than the corresponding values of FAs. Further, the m1 values of PS-HA and PS-FA are larger and the m2 values are smaller than those of any soil HA and FA.

The NICA model reveals a substantial heterogeneity of acidic functional groups in the HAs and FAs examined, especially of phenolic OH groups of HAs. These results are in general agreement with previous findings on similar systems (Milne et al., 2001), and are consistent with the general principles of acid–base chemistry of humic substances (Stevenson, 1994). Furthermore, the HA and FA fractions of pig slurry appear to feature a greater homogeneity in carboxyl groups and a greater heterogeneity in phenolic OH groups than soil HAs and FAs.

Environmental Significance
In a previous paper (Plaza et al., 2002), the effect of PS amendment was examined on the main properties of soils of the same field experiment described in this paper. With respect to the control soil, the PS-amended soils have similar or smaller total organic C contents. This "priming effect" can be ascribed to the small amount of easily decomposable organic C and the relatively large N content of PS. After PS application, soil microorganisms have an increased amount of fresh N available for their protein metabolism but not enough fresh C as energy source, thus microbial oxidation of soil native organic C must occur. Previous long-term field studies that have used PS as fertilizer also did not show enhancement of organic C in amended soils (e.g., Rochette et al., 2000).

Results obtained show that the application of PS can determine a number of modifications in the acid–base properties of amended soil HAs and FAs, including a decrease of acidic functional group contents, a slight increase of proton affinity of carboxylic-type groups, and a slight decrease of proton affinity of phenolic-type groups of soil HAs only. These changes together with the effects observed on soil organic matter content can, in turn, influence the acid–base buffering capacity of PS-amended soils and their ability to interact with cations, which can have an important impact on biological availability, mobilization, and transport of macro- and micronutrients and toxic metal ions.

In particular, the capacity of PS-amended soils to buffer acid and base inputs is expected to be affected negatively by the decrease of acidic functional group contents of soil HAs and FAs, the slight decrease of proton affinity of phenolic-type groups of soil HAs, and the similar or smaller content of total organic C in PS-amended soils with respect to the control soil. Furthermore, the decrease of carboxylic- and phenolic-type group contents and the slight increase of proton affinity of carboxylic-type groups would imply a decrease of cation binding capacity of PS-amended soil HAs and FAs.

A major pollution hazard of uncontrolled application of PS may be due to the large toxic trace metal content (Saviozzi et al., 1997; Giusquiani et al., 1998). In particular, copper, which is a common additive in feedlot rations (Nicholson et al., 1999), may accumulate excessively in PS-amended soils. Because of the decreased soil capacity for metal ion and Cu(II) binding, PS application may represent an additional environmental risk for soil and ground water pollution.

However, different effects may be observed on soil organic matter content as a function of PS and soil characteristics and climatic conditions in which experiments are conducted. For example, Hountin et al. (1997) observed an increase of soil total organic C content with increasing rates of amendment with an organic-rich liquid pig manure after 14 yr of application to a poorly-drained, silty loam soil in a rainy and cold area of the province of Quebec (Canada). In such cases, the cation binding capacity on a total soil mass basis could be increased even though the functional-group density of soil HAs and FAs decreases.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The NICA model is shown to describe with a great degree of accuracy the behavior of experimental titration data sets, and highlights important differences in the acid–base properties of HAs and FAs isolated from a pig slurry and nonamended and amended soils. In particular, PS-HA and PS-FA, with respect to soil HAs and FAs, are characterized by smaller carboxylic-type group contents, slightly smaller phenolic-type group contents, larger affinities for proton binding by carboxylic-type groups, and much smaller, in the case of HA, or similar, in the case of FA, affinities for proton binding by phenolic-type groups. Pig slurry application to soil causes a decrease of acidic functional group contents and a slight increase of proton affinity of carboxylic-type groups of soil HAs and FAs, and a slight decrease of proton affinity of phenolic-type groups of soil HAs. These effects, which are more evident for HAs than for FAs and tend to slightly increase with increasing amendment rate, can have a large impact on the biological availability, mobilization, and transport of macroand micronutrients, toxic metal ions, and xenobiotic organic cations in soils.


   ACKNOWLEDGMENTS
 
The authors thank the Consejería de Agricultura y Medio Ambiente de la Junta de Comunidades de Castilla-La Mancha for financial support of this research.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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