Humic Matter in Soil and the Environment: Principles and Controversies - Chapter 5 pot

CHAPTER 5 CHEMICAL COMPOSITION OF HUMIC MATTER 5.1 ELEMENTAL COMPOSITION 5.1.1 Weight Percentage As indicated in Chapter 1, the elemental composition of humic matter is a very big issue among scientists, with one group being very critical about the presence of an elemental composition, and the other group proclaiming the existence of an elemental composition charac- terizing humic matter. A number of scientists in the first group above fail to see the significance of an elemental composition, indicating that extraction procedures may have effected changes in the elemental composition of humic substances. However, Steelink (1985) declares that this issue is not clear yet and needs to be resolved by more detailed research. Except for a generous number of criticisms, no further information of interest is available from this group, and as stated by Ziechmann (1994), elemental composition is a dead issue among these scientists. In contrast, a lot of information has been supplied by the second group. Many of them have analyzed the C, H, 0, N, and S contents of humic and fulvic acids extracted from a variety MARCEL DEKKER, INC. 270 Madison Avenue, New York, New York 1001 6 TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. 128 Chapter 5 Table 5.1 Summary of Elemental Composition of Humic Acids (HA) and Fulvic Acids (FA) Extracted from Tropical and Temperate Region Soils and Miscellaneous Environments. (The figures are averages of weight percentages from multiple analyses of various sources.)" CHONS CAY Tropical Region Soils HA-Alfisols 52.3 5.2 37.2 3.6 n.d 14.5 HA-Andosols 58.5 3.9 34.8 3.8 0.3 15.4 HA-Oxisols 54.5 4.4 38.0 3.1 n.d 17.5 FA-Andosols 48.9 4.3 44.5 2.3 n.d 21.3 Temperate Region Soils HA-Alfisols 56.8 5.0 33.6 4.6 n.d 12.3 HA-Aridisols (Solonetz) 54.5 4.1 36.4 5.0 n.d 10.9 HA-Histosols (Peat-bogs) 58.7 5.0 32.9 3.4 n.d 17.3 HA-Inceptisols 51.4 5.4 39.1 4.1 n.d 12.5 HA-Mollisols 53.7 4.3 36.3 3.7 n.d 14.5 HA-Spodosols 53.1 4.9 39.2 3.5 0.4 15.2 HA-Ultisols 50.5 5.2 40.0 3.9 0.5 12.9 FA-Inceptisols 47.9 5.2 44.3 2.6 n.d 18.4 FA-Mollisols 41.6 4.0 51.9 1.1 n.d 37.8 FA-Spodosols 50.6 4.0 44.1 1.8 0.3 28.1 FA-Ultisols 45.1 4.5 48.0 2.2 0.3 20.5 Geologic Deposits Lignite 52.6 2.8 31.8 2.0 0.7 26.3 HA-lignit e 57.4 3.9 36.8 1.2 0.8 47.8 FA-lignite 46.4 4.4 45.8 1.5 0.6 30.9 Rivers and Swam~s HA-aquatic 49.6 4.8 43.5 2.1 n.d 23.6 FA-aquatic 50.8 4.3 43.1 1.6 n.d 37.8 Merck Chemical. Co. HA-artificial 59.8 4.8 33.6 1.8 n.d. 33.2 MARCEL DEKKER, INC. 270 Madison Avenue, New York, New York 1001 6 TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. Chemical Composition of Humic Matter 129 Table 5.1 Continued CHONS C/N q, - - - - - - - - - - - - - - - - 'Reference' humic acid 56.2 4.7 35.5 3.2 0.8 17.6 Peat 55.5 5.4 33.1 3.0 n.d 18.5 Plant Residue 49.6 6.3 41.6 2.5 n.d 19.8 "Sources: Lobartini et al. (1997; 1992; 1991); Tan et al. (1991); Kumada (1987); Thurman and Malcolm (1981); Orlov (1985); Steelink (1985); Schnitzer et al. (1991); Schnitzer and Mathur (1978); Schnitzer and Khan (1972); Cranwell and Haworth (1975); Tan and Van Schuylenborgh (1959). n.d = not determined. of soils in tropical and temperate regions. These are considered the major elements in humic matter, and a summary of the data is given as examples in Table 5.1. The lignite samples listed in the table above were from the deposits in North Dakota, and the data for the lignite- humic and fulvic acids are the average figures of data reported by Mathur and Parnham (19851, Steelink (1985), Tan et al. (1991), Lobartini et al. (1992), and from unpublished data of the present author. The elemental composition of aquatic humic and fulvic acids are also the average figures of data supplied by Lobartini et al. (1991), Thurman and Malcolm (1981), and from unpublished data of the present author. The black water samples for extraction of these humic substances are from the Okefenokee swamps, Satilla, Ohopee, and Suwannee rivers in the southeastern United States. Historical data reported as early as 150 years ago by Mulder and other pioneers in humic acid science, for %C (56-62%), %H (2.0- 5.5%), and %N (2-8%), are remarkably within range of those listed in Table 5.1. However, Orlov (1985) cautions their use for comparison, because they are in the higher ranges, due perhaps to the habit in the old days MARCEL DEKKER, INC. 270 Madison Avenue, New York, New York 1001 6 TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. 130 Chapter 5 of drying humic acids at 140°C or higher, and the use of different values in atomic weights. Both the old and especially the modern data show the elemental composition to be within relatively fixed limits, meaning that it does not exhibit an erratic or a very wide range of variation, as would have been expected with fake materials or artifacts. The differences noticed may be due to differences in origin or to the types of humic substances. Orlov (1985) believes that variation in the elemental composition is affected by (1) variability in soils, (2) variability of humic substances in time and space, (3) different isolation techniques, and (4) errors in sampling and analyses. Nevertheless, the general composition of humic matter, as listed in Table 5.1, is still displaying a close relation with that of the plant material from which it has been derived. The observed divergence from the plant residue is apparently the result of the humification process and other soil factors. The composition is also in the range of that of humic acid, listed as a 'reference' in the table. This particular sample is considered the ideal example of humic acid in soils by Steelink (1985) and Schnitzer and Mathur (1978). With few exceptions, the carbon content of humic acids is similar to slightly higher than that in plant residue. On the other hand, fulvic acids exhibit carbon contents almost similar to slightly lower than that in plant residue. The differences in carbon contents between the two types of humic substances are in agreement with data supplied by Steelink (1985), showing carbon contents of 53.8 to 58.7% in humic acids and that of 40.7 50.65 % for fulvic acids. This may indicate that fixation of carbon or carbon sequestration takes place during the synthesis of humic matter and goes up slightly with an increased rate of humification. The decomposition of organic residue is characterized generally by a loss of C in the form of CO,, but when humification steps in, some of the C will be incorporated into humus and humic matter. It is estimated that approximately one-third of the C from plant residues is retained in the soil and stabilization of nonaromatic C is expected to take place through microbial transformation into aromatic substances used in the synthesis of humic matter. By comparison with the elemental composition of the plant residue listed at the bottom of the table, the carbon contents of humic and fulvic acids indicate that losses of C from decomposition of plant residue should have been MARCEL DEKKER, INC. 270 Madison Avenue, New York, New York 1001 6 TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. Chemical Composition of Humic Matter 131 minimal. Most of the liberated carbon is apparently retained in the soil in the form of humic matter, since both humic and fulvic acids exhibit C contents in the range of that in plant residues. The hydrogen content of humic and fulvic acid does not differ much from each other. The data for both of the humic substances are also in the range of the average hydrogen values of 3.2 - 6.2% and 3.8 - 7.0% in humic and fulvic acids, respectively, as reported by Steelink (1985). The oxygen contents listed in the Table 5.1 above for klvic acids tend to be slightly higher than the average values of 39.7-49.8% as reported by Steelink, but the big difference is in the nitrogen contents. The data in Table 5.1 display nitrogen contents of 2.6 to 5.05 % for soil humic acids, which are considerably higher than Steelink's average values of 0.8 -4.3% N, whereas those of fulvic acids, ranging from 1.1 -2.6%, tend to be lower than the average values of 0.9 -3.3 % as reported by Steelink. It is conspicuous that soil humic acid is substantially higher in N than soil fulvic acid, which is not apparent from Steelink's average ranges. The exceptions to the above are the geologic and aquatic humic substances, which are characterized by low N contents. The differences in N content are also not too obvious between aquatic humic and fulvic acid. 5.1.2 The C/N Ratio The carbon to nitrogen ratio is often considered as an index of a decomposition process of crop residue. Its value varies from 13 to 20 in legume crops, to 40 in cornstalks, or 80 in straw of cereal crops, and has been reported as high as 500 to 800 in sawdust (Brady, 1990; Miller and Gardiner, 1998). The decomposition ofplant residues results in some of the carbon and nitrogen being lost, processes considered part of the carbon and nitrogen cycles. However, some of the C and a considerable amount of the N liberated are incorporated into microbial cells or fixed in substances used for formation of humic matter. These processes lead to decreasing the values of the C/N ratios, eventually reaching relatively constant values in soils. In most soils, the C/N ratio falls within narrow limits to about 10 to 15, when decomposition is virtually 'completed', meaning organic matter decomposition is in MARCEL DEKKER, INC. 270 Madison Avenue, New York, New York 1001 6 TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. 132 Chapter 5 equilibrium with the synthesis and accumulation of new organic materials. This decrease in C/N ratio, reaching a constant value, is now extended to also indicate degree or rate of a humification process. As indicated above, part of the liberated carbon and a substantial amount of the organic nitrogen are 'sequestered' by the humic molecule. Hence, it is generally believed that the C/N ratio will also decrease with increased rate or degree of humification, and C/N ratios between 10 to 15 are often considered to be characteristic for well-developed humic acids. The data listed in Table 5.1 show some support for the opinion above by displaying C/N ratios of soil humic acids between 12.3 and 17.3. Higher C/N ratios are exhibited by aquatic and geologic humic acids, which is perhaps caused by disturbance in the humification process due to the different environments in aquatic or geologic systems from that in the soil ecosystem, e.g., lower N contents and reduced condition in aquatic systems. Of interest to note is the high C/N ratios of soil fulvic acids, ranging from 18.4 to 37.8, in comparison to those of humic acids. As discussed above, the differences in carbon contents between fulvic and humic acids were rather small and would not have caused the values of C/N to differ that much. In contrast, the nitrogen content is approximately 2 to 3 times higher in humic acids than in fulvic acids, which may indicate that fixation of N increases with increased humification from fulvic to humic acid. Increasing amounts of nitrogenous compounds are apparently being sequestered in the process of the synthesis of a humic acid molecule. This raises the possibility that the polymerization or condensation theory of formation of humic acids is not quite correct. Polymerization only of fulvic acids is not adequate, since it would not be able to increase the N content so that it causes the C/N ratio of the polymerized product (humic acids) to decrease that much. The polymerization of two moles, 10 moles, 100 moles (or even higher) of fulvic acids will change neither the composition nor the C/N ratio. Therefore, in addition to polymerization, other reactions, such as interaction, adsorption, and chelation of nitrogenous substances are apparently also involved. For raising the nitrogen content in humic acid, it is necessary to also invoke these reactions for the inclusion of the needed nitrogen sources in the humic MARCEL DEKKER, INC. 270 Madison Avenue, New York, New York 1001 6 TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. Chemical Composition of Humic Matter 133 acid molecule. The differences in N content between fulvic acids and humic acids can perhaps be explained partly by invoking the biopolymer degradation theory. Since in this theory humic acid is formed first, it is more likely that the degradation process yields humic acids with less N than the plant residue, or maximal with N contents similar to that of the parent material. This will not explain the N contents of humic acids, which are higher than that in the plant residues. However, by further degradation of humic acids into fulvic acids, the possibility arises that in this process large amounts of the nitrogenous constituents have been broken down and removed from the humic acid molecule. The losses of N by comparison to C must be substantial, causing the N contents to be very low and the C/N ratio to become high in fulvic acids. 5.1.3 Atomic Percentage A number of scientists believe that elemental composition based on weight percentages cannot be used to explain the molecular structure of humic substances. For the purpose of studying and devising structural formulas for humic substances, they suggest the use of atomic percentages. This method of expressing the elemental composition of humic matter is especially popular in eastern European countries (Cieslewicz et al., 1997; Debska, 1997). Orlov (1985) is of the opinion that atomic percentages give a better picture than weight percentages on the composition of these substances and on the role they play in molecular structure. He has used them for the determination of molecular weights and formulas, and in distin- guishing two groups of humic acids, one with an atomic percentage of 40-42% C, and the other with an atomic percentage of 37-38% C. Steelink (1985) also believes that atomic percentages and atomic ratios are useful as guides in the identification of different types of humic acids, and for drafting structural formulas of the humic substances. MARCEL DEKKER, INC. 270 Madison Avenue, New York, New York 1001 6 TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. 134 Chapter 5 Table 5.2 Summary of Elemental Composition of Humic Acids (HA) and Fulvic Acids (FA) Extracted from Tropical and Temperate Region Soils and Miscellaneous Environments in Atomic Percentages Tropical Region Soils HA-Alfisols HA-Andosols HA-Oxisols FA-Andosols Temperate Region Soils HA-Alfisols HA-Aridisols (Solonetz) HA-Histosols (Peat-bogs) HA-Inceptisols HA-Mollisols HA-Spodosols HA-Ultisols FA-Inceptisols FA-Mollisols FA-Spodosols FA-Ultisols Geologic Deposits Lignite A-lignite FA-lignite Rivers and Swamps HA-aquatic FA-aquatic Merck Chemical Co. HA-artificial 'Reference' humic acid MARCEL DEKKER, INC. 270 Madison Avenue, New York, New York 1001 6 TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. Chemical Composition of Humic Matter 135 Table 5.2 Continued Peat 37.6 43.9 16.8 1.7 -0.138 Plant residue 29.4 44.7 24.6 1.3 0.286 Hence, the data in Table 5.1 are converted by the present author into atomic percentages and the results are listed in Table 5.2. By comparison with weight percentages, the variation in elemental composition expressed in terms of atomic percentages is also relatively small. The humic and fulvic acids from soils and aquatic environments exhibit C, H, 0, and N atomic percentages within relatively narrow fixed limits, also providing strong credentials for the presence of real natural compounds, instead of fake or operational substances. The data suggest that for every C atom there is at least one H atom. It is also more evident now that the composition of humic substances contains approximately one atom of 0 to two atoms of C. Again, one can notice that in general the N content is larger for humic acid than for fulvic acid. The nitrogen atomic percentages are approximately twice that large in hurnic acids than in fulvic acids of terrestrial soils. 6.1.4 Internal Oxidation of Humic Substances, o The parameter in Table 5.2, designated by the symbol a, refers to the internal oxidation value of humic substances. This is considered a very important value, especially in Europe, for studying diagenetic changes of humic substances. A number of scientists believe that MARCEL DEKKER, INC. 270 Madison Avenue, New York, New York 1001 6 TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. 136 Chapter 5 diagenetic changes of humic acids are closely related to degradation and oxidation reactions. This hypothesis, as discussed by Orlov (1985), starts with the degradation of plant residue to form a humic acid-like substance. The degradation process involves the loss of CH, groups. The transformation of the humic substance above to a humic acid characteristic -for example - in mollisols, is postulated to take place by a continuation of the degradation process, involving now also partial oxidation and further losses of CH, groups. An increase or decrease in the number and length of aliphatic chains in the humic molecule is considered to be inherent to this basic process of addition or loss of especially terminal CH,- or CH2-groups. The resulting difference between the number of oxygen and hydrogen atoms is used as a measure of the degree of the oxidation processes. This difference can be calculated by several different, though related, methods. The simplest is the method using a revised formula below, adapted from Orlov (1985): in which A = difference between numbers of oxygen and hydrogen atoms, 0 = number of oxygen atoms, and H = number of hydrogen atoms. For a water molecule (H20), a fundamental basis of this hypothesis, the difference equals zero: To compare the degree of oxidation in different organic substances, Orlov (1985) proposes to calculate the difference on the basis of 1 gram atom of carbon or per unit amount of carbon atoms: MARCEL DEKKER, INC. 270 Madison Avenue, New York, New York 1001 6 TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. [...]... Tropical Redon Soils HA-Alfisols HA-Andosols HA-Oxisols FA-Andosols Temperate Redons Soils HA-Afisols HA-Aridisols (Solonetz) HA-Histosols (Peat-bogs) HA-Inceptisols HA-Mollisols HA-Spodosols HA-Ultisols FA-Inceptisols FA-Mollisols FA-Spodosols FA-Ultisols Geoloeic Deposits Lignite HA-lignite FA-lignite Rivers and Swamps HA-aquatic FA-aquatic Merck Chemical Co HA-artificial 'Reference' humic acid TM... Steelink (19 85) , who also reports lower OIC atomic ratios in humic acids (averaging 0 .5) than in fulvic acids (averaging 0.7) Steelink is of the opinion that the OIC ratio is the best parameter for differentiating between types of humic compounds The OIC ratios of aquatic and lignite humic matter also fall within the range of those of soil humic matter These values are noted to be lower in the humic. .. from the figures in Table 5. 2, and the results are presented in Table 5. 3 The atomic ratios of WC of the humic substances, ranging from 0.90 to 1. 35, are in agreement with those reported by Orlov (19 85 )and Steelink (19 85) As indicated earlier by Orlov (19 85) , the figures suggest that for each carbon atom, there is indeed one hydrogen atom in the humic molecule The atomic WC ratios of aquatic humic matter. .. DEKKER, INC 270 MadisonAvenue, New York, New York 10016 Chemical Composition of Humic Matter 157 5. 2.1 Structures Based on the Ligno-ProteinConcept Lignin Monomer Concept ofFlaig - Structural designs of humic acid based on lignin monomer units have been proposed by Flaig (1967) and Steelink (19 85) Flaig has studied the disintegration of lignin into the simpler monomers, and their further disintegration into... York 10016 Chapter 5 H H H 0 - C - C = C- H Soil Humic Acid After Orlov (19 85) H R2 H N-C-C-0-CH b H - 0 ti 11 HO-C+C- 0 G-CH~-E-OH I I HCOH I R3 Aquatic Fulvic Acid After US Geol Survey (1989) Figure 5. 2 Structural models according to the phenol-protein theory adapted from Orlov (19 85) and the U S Geological Survey (1989)' respectively The peripheral part, in Orlov's model has been deleted for brevity... are the methods proposed by Steelink (19 85) and Orlov (19 85) Both have attempted using elemental compositions expressed in atomic percentages and atomic ratios for the calculation of formula weights of humic substances However, in contrast to Steelink (19 851 , who considers only C, H, and 0, Orlov (19 85) includes N in devising chemical formulas for the humic substances As indicated in Chapter 1, the. .. well drained soils, underscoring the aerobic environment prevailing in soils affecting the oxidation of humic acids In contrast, the negative values for o are noticed by Ci6slewicz et al (1997) for humic acids extracted from lagoon sediments, indicating anaerobic conditions in the transformation of humic acids 5. 1 .5 Atomic Ratios For the purpose of creating structural formulas or formula weights, the ratios... empirical formula for humic acid from the elemental composition and atomic ratios in Tables 5. 2 and 5. 3, respectively The average values are used instead of rounding-up the figures to one digit: Average WC = 0.8 - 1.2 OIC = 0.4 -0 .6 N/C = 0. 05 -0 .07 + + + 1.0 0 .5 0.06 The average values for WC and OIC are similar to those used by Steelink (19 85) in his calculations It could be coincidental, but such... acids and aldehydes by analysis with the Py-GC/MS method Although this could be true, it is not irrefutably proven yet that these small phenol units originate from lignin, or pyridine units from protein It is a matter of believing and trusting only, since as discussed in Chapter 4, phenol or benzene-like compounds are derived from two sources in soils, from lignin and from synthesis by soil micro-organisms... but these are neither of the oxygen containing nor of the acidic types Carboxyl Groups - The carboxyl or COOH groups give to the humic molecule its acidic characteristic Their presence is the reason why humic substances exhibit charge properties, and have the capacity to adsorb and exchange cations As indicated in Chapter 7 and as can be noticed in Figure 7.1, these carboxyl groups will dissociate their . Dekker, Inc. All Rights Reserved. Chemical Composition of Humic Matter 129 Table 5. 1 Continued CHONS C/N q, - - - - - - - - - - - - - - - - 'Reference' humic. 54 .5 4.1 36.4 5. 0 n.d 10.9 HA-Histosols (Peat-bogs) 58 .7 5. 0 32.9 3.4 n.d 17.3 HA-Inceptisols 51 .4 5. 4 39.1 4.1 n.d 12 .5 HA-Mollisols 53 .7 4.3 36.3 3.7 n.d 14 .5 HA-Spodosols 53 .1. Dekker, Inc. All Rights Reserved. Chemical Composition of Humic Matter 131 minimal. Most of the liberated carbon is apparently retained in the soil in the form of humic matter, since both humic and