Properties of humic acids developed during humification process of post-harvest plant residues

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Environment International, Vol. 24, No. 5/6, pp. 603-608, 1998 Copyright 01998 Elsevier Science Ltd Printed in the USA. All rights reserved 0160-4120/98 $19.00+.00

Pergamon

PII S0160-4120(98)00034-8

PROPERTIES OF HUMIC ACIDS DEVELOPED DURING HUMIFICATION PROCESS OF POST-HARVEST PLANT RESIDUES Slawomir S. Gonet and Bozena Debska University of Technology and Agriculture, Department of Soil Chemistry, 6 Bernardynska st., PL-85-029 Bydgoszcz, Poland

E19706-153 M (Received27 June 1997; accepted 28 February 1998)

The objective of the study was to determine elemental composition and spectrometric properties of humic acids (HAs) developed in the process of decompositionof post-harvestresidues of wheat, lucerne, and maize. The selected plant materials differed in their chemical composition. HAs were extracted before and after 30 and 360 d of incubation. Properties of the HAs studied depended on chemicalcomposition of post-harvestresidues used. The correlationcoefficientsbetween elemental composition and spectrometric properties of HAs from post-harvest residues indicated that correlation dependencies commonly accepted for humic soil substances are not relevant in case of plant residues. In the case of the plant material derivated HAs, no significant correlation between the absorbance values in the visible light spectrum and the carbon content was noted. Additionally, no significant correlation between the absorbancecoefficientsand the H:C ratio was f o u n d . 01998 Elsevier Science Ltd

INTRODUCTION The humus content in soils results from the processes of humus substances decomposition and the influx of fresh organic matter. The post-harvest residues, left in arable soil after harvesting the crops, are the basic source of organic matter (Jurcova and Bielek 1996; Kolbe and Stumpe 1975). Mazur (1995) stated that the influences of specific post-harvest residues on the balance of organic matter are not the same. This poses a problem in the amount and, moreover, the quality (of chemical composition) of the biomass introduced to the soil. Chemical composition of plant material is one of the basic factors determining the process of humic substances development and their physico-chemical properties (Debska and Gonet 1995; Zaujec 1980). The basic parameter characterizing humic substances is their elemental composition. Many studies on postharvest residues decomposition have shown that

"young" humic acids (HAs) are characterized by higher content of hydrogen and lower content of carbon and oxygen. The plant material humification process is connected with a decrease of hydrogen content and an increase of oxygen and carbon content (Aleksandrova 1980; Gomah et al. 1978; Zigunov and Simakov 1977). One of the main criteria defining humic substances of different types of soil is the absorbance value at 465 nm and the values of the E4/6 parameter. These parameters are also used to characterize newly-formed HAs (Aleksandrova 1980; Gonet et al. 1992) and interpreted according to the dependencies proved in reference to soil humic substances. Together with an increase of carbon content in soil HA molecules, the absorbance value of humates solution increases in the UV-VIS range (Kumada 1975; 1985; 1987). The value o f E4/6 is Dositively correlated with the value of H:C 603

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calculated on the basis of elemental composition (Gonet 1989; Kumada 1987) and negatively correlated with the value of the oxygen-hydrogen ratio O:H (Kumada 1987). Kumada (1987) also obtained negative correlation between the value of AlogK (difference between logarithm of absorbance measured by 400 nm and 600 nm) and the carbon content and the value of the O:H ratio, as well as a positive correlation between AlogK and the H:C ratio. The aim of the present study was to investigate how the quality of plant materials affects the properties of HAs formed during the humification processes of maize, lucerne, and wheat plant residues. MATERIALS AND METHODS

The studies were carried out as a pot-model incubation experiment. Post-harvest residues of maize, lucerne, and wheat were used as a source of organic matter. Plant materials were collected from the field experiment after the harvest as described by Zaujec (1980). The samples were incubated at 25°C and in constant humidity of 60% for periods of 1-360 d. Incubation of a sample taken from the plough layer of brown soil was used as the control. The incubation experiment was realised in plastic pots according to the following scheme: Variant (symbol) Incubated material Soil (control) S Maize (mixture of straw and roots Ma in 2:1 w/w ratio) Lucerne (mixture of hay and roots Lu in l : 1.5 w/w ratio) Wheat (mixture of straw and roots Wh in 4:1 w/w ratio) HA fractions from dried samples were extracted with 0.1 M NaOH, precipitated at pH 2.0 with 5.0 M HCI, and then purified with an HF-HCI mixture, according to the Schnitzer and Skinner (1968) method. The extracted HAs were analysed for elemental composition (Perkin Elmer - CHN Analyser 2400). On the basis of elemental composition, the values of atomic ratios: H:C, O:C, O:H, and N:C were calculated. The spectra in the UV-VIS region were made for 0.2% of HA solutions in 0.1 M NaOH (Hewlett Packard - UV-VIS spectrometer). Absorbance measured at the wavelengths 280 nm (A280), 400 nm (A40o), 464 nm (A464), 600 nm (A6o0), and 664 nm (A664) were used for calculation of coefficient values:

S.S. Gonet and B. Debska

A2/4 - the absorbance values ratio at 280 nm and 464 nm; A2/6 - t h e absorbance values ratio at 280 nm and 664 nm:

and A4/6 - the absorbance values ratio at 464 nm and 664 nm. AlogK = log A4oo - log A600. RESULTS

Chemical composition of the examined material residues is given in Table 1. The results of the analyses of HAs isolated from the examined plant materials before incubation (in tables indicated 0), after 30 d of incubation (indicated 30), and after 360 d of incubation (indicated 360) are presented in Tables 2, 3, and 4 and on Fig. 1. The calculated correlation coefficients between the elemental composition and spectrometric properties of the examined HAs are presented in Table 5. In comparison with HAs of the brown soil, HAs extracted from the incubated post-harvest residues were characterized by lower carbon and oxygen contents, but a higher content of hydrogen (Table 2). During the incubation, a decrease of hydrogen content and an increase of oxygen content were noticed. The content of carbon before and after the incubation period did not change significantly, which indicated that the humification process of plant materials cannot be univocally connected with the increase of carbon content. The nitrogen content in HA molecules depended on the amount of nitrogen in post-harvest residues. Therefore, the highest content of this element was found in HAs separated from the lucerne postharvest residues, while was found in HAs from wheat residues. On the basis of the value of the hydrogen-carbon ratio (H:C - aromaticity index), it was shown that HAs from lucerne and maize post-harvest residues, separated after 360 d of incubation, were characterized by a higher content of aromatic structures in comparison to HAs of residues separated before incubation (Table 3). Such a dependence was not found for the Wh variant, so one may conclude that the humification process of wheat post-harvest residues was not connected with the increase of the aromatic degree of HA molecules. The increase of oxygen content, as well as the values of the O:C ratio, indicated that the process of plant material decomposition is connected with the increase of HA carboxylation degree. However, the diagram of H:C vs. O:C atomic ratios (Fig. 1) shows that the carboxylation process in HA molecules or lucerne and maize post-harvest residues was more intensive than that of wheat residues.

HAs in post-harvest plant residues

605

Table 1. Chemical composition of post-harvest residues (g/kg). Post-harvest residues

Saccharides

Proteins

Hemicellulose and starch

Cellulose

Lignin

C:N 14.5 52.9

Lucerne

43.6

164.4

148.5

322.5

296.0

Maize Wheat

67.2 46.8

47.5 43.1

426. I 233.0

232.9 421.2

182.7 230.0

Table 2. Elemental composition of HAs. Weight percentage Sample

Atomic percentage

C

H

N

O

C

H

N

O

S-0

55.9

4.7

4.0

35.4

39.2

39.8

2.4

18.6

S-360

54.7

4.8

4.1

36.4

40.4

38.8

2.6

18.2

Ma-0

60.0

7.4

5.2

27.4

34.6

50.9

2.6

11.9

Ma-30

,56.9

6.1

3.2

33.8

36.0

46.2

1.8

16.0

Ma-360

56.8

6.4

3.5

33.3

35.2

47.4

1.9

15.5

Lu-0 Lu-30

56.5 59.1

7.6 6.6

7.3 4.3

28.6 30.0

32.3 36.0

51.9 48.0

3.6 2.3

12.2 13.7

Lu-360

53.7

6.6

4.7

35.0

32.9

48.5

2.5

16.1

Wh-0

59.9

6.7

2.1

31.3

36.3

48.4

1.1

14.2

Wh-30

60.7

6.6

1.6

31.1

36.9

48.0

0.9

14.2

Wh-360

56.1

6.1

2.0

35.8

35.5

46.5

1.1

17.0

Table 3. Atomic ratios of HAs. Sample

H/C

O/H

O/C

N/C

S-0

1.01

0.47

0.47

0.061

S-360

0.96

0.47

0.45

0.064

Ma-0 Ma-30

1.47 1.28

0.23 0.35

0.34 0.44

0.075 0.050

Ma-360

1.35

0.34

0.44

0.054

Lu-0

1.61

0.24

0.38

0.111

Lu-30

1.33

0.28

0.38

0.064

Lu-360

1.47

0.33

0.49

0.076

Wh-O

1.33

0.29

0.39

0.030

Wh-30 Wh-360

1.30 1.31

0.30 0.37

0.38 0.48

0.024 0.031

57.0

606

S.S. Gonet and B. Debska

Table 4, Optical properties of HAs. Sample

A280

A400

A464

A2..+

S-0 S-360

5.75 5.56

2.16 2.05

1.38 1.30

4.17 4.28

Ma-0 Ma-30 Ma-360

3.04 3.84 4.01

0.876 1.19 1.15

0.275 0.351 0.393

Lu-0 Lu-30 Lu-360

2.12 3.19 2.94

0.830 0.985 0.820

Wh-0 Wh-30 Wh-360

3.44 3.55 4+17

0.892 0.839 1.12

A2/6

A4/6

AlogK

15.9 16.3

3.81 3.80

0.577 (;,577

1 I, I 109 1(!.2

116.9 101.0 95.5

10.6 9.24 9.36

1.28 1,24 122

0.214 0.305 0.312

9.~1 1().5 9 42

75.7 88.6 79.5

7.64 8.47 8.43

129 1.24 1.(/7

0.234 0.217 0.400

14.7 16.4 10.4

229.3 295.8 85.1

15.6 18.1 8.16

1.44 1.48 1.04

1,65 H/C

1,55

1,45

1,35

O/C

1,25 0,30

i

0,35

0,40

•

-

Maize

0,45 • - Lucerne

0,50

0,55

0,60

• - Wheat

Fig. 1, Atomic H/C vs. O/C diagram for HAs. (The arrows mark direction of ttA transformation in the incubation perind of 0-360 d.

H o w m u c h H A s separated from post-harvest residues differ f r o m typical H A s is p r o v e n by the results o f the s p e c t r o m e t r i c a n a l y s e s (Table 4). The obtained absorbance values in the U V - V I S range were m u c h lower than those for the H A s o f the b r o w n soil. Also, the calculated values o f absorbance coefficients (A2/4, A2/6, A4/6, and A l o g K ) o f h u m a t e s f o r m e d f r o m plant m a -

terial were m u c h higher than those o f soil genesis, and similar to values o f soil fulvic acids. The obtained values o f the A4,6 coefficient suggested that H A s o f low m o l e c u l a r w e i g h t were f o r m e d in the p r o c e s s o f plant material decomposition. The high values o f the A2/+ and A2~.6 coefficients indicated that the H A s originated from plant material freshly d e c o m p o s e d to a small degree,

HAs in post-harvest plant residues

607

Table 5. Correlation between elemental composition and spectrometric data. C A28o A40o A464 A664

0.719"* 0.390 0.110 -0.239 m2/4 0.671" A2/6 0.620 A4/6 0.621 AIo~:K 0.546

H -0.887** -0.729* -0.677* -0.420 -0.102 -0.079 -0.073 0.256

O 0.724* 0.599 0.737* 0.591 -0.155 -0.145 -0.154 -0.560

H:C

N:C

-0.829** -0.596 -0.418 -0.077 -0.439 -0.398 -0.399 -0.120

-0.829'* -0.347 -0.222 0.148 -0.661 -0.637 -0.640 -0.244

O:C 0.441 0.432 0.693 0.685* -0.416 -0.335 -0.394 -0.742*

O:H 0.789** 0.687* 0.763* 0.590 -0.119 -0.116 -0.126 -0.500

* Significance level at tz = 0.05. ** Significance level at cc = 0.01.

Moreover, the spectrometric parameters values depended on the type of the incubated plant residues, so they were determined by their chemical composition. This can be clearly seen in the first stage of the decomposition process of the examined plant residues. The humification process is connected with an increase in the absorbance value in the UV-VIS range and a decrease in the value of the absorbance coefficients (A2/4, A2/6, A4/6, and AlogK). Interesting results were obtained from the correlation between the spectrometric parameters and the elemental composition of the investigated HAs (Table 5). In the case of the examined HAs, no important correlation between the absorbance values in the visible spectrum and the carbon content was noted, except for the absorbance value at 280 nm (A2s0), which correlated positively with the carbon content. A negative correlation was also obtained for the A280 value with the hydrogen content and the H:C ratio value. However, no significant correlation between the absorbance coefficients (A2/4, A2/6, A4/6, and AlogK) and the H:C ratio was found. The correlation coefficients obtained between O:H ratio values and A4/6and AlogK did have negative values but statistically were not significant. DISCUSSION

Elemental composition of HAs suggested that the decomposition process of wheat post-harvest residues was much slower than in the case of lucerne and maize residues. Simultaneously, certain quality parameters (H:C, O:H) showed that, during the decomposition process of wheat post-harvest residues, HAs are most similar to HAs of the brown soil. This seems logical if the chemical composition and the value of the C:N

ratio in the examined post-harvest residues are considered (Table 1). The wheat post-harvest residues had the highest C:N ratio (57.0) and the highest content of lignin and cellulose, compounds which are difficult to decompose, and the elemental composition of which is sometimes similar to the composition of typical HAs. According to Kumada (1975), the advancement of the humification process is connected with the value changes of the AlogK coefficient. On the basis of the HAs spectra in the UV-VIS range, it was possible to divide HAs into three basic types: type A - including HAs of high humification degree, with AlogK achieving the value of 0.6; type B - with AlogK values from 0.6 to 0.8; and type Rp - including HAs with coefficient values from 0.8 to 1.1. The obtained coefficient values indicated that HAs separated before and after 30 d of incubation do not fit into the classification. However, it is the HA of lucerne and wheat residues, separated after 360 incubation d, that can be classified to the Rp type. The given data show that the examined substances, called "humic acids", should be considered as their initial forms (precursors). This is particularly important because the parameter evaluation of newly-formed HAs in literature is generally based on commonly accepted quality parameters applied to soil humic substances. They are also interpreted according to the dependencies demonstrated for this group of compounds. However, these interpretations are not necessarily true for substances extracted from plant materials, indicating the need for careful interpretation. CONCLUSIONS

The properties of HAs isolated from plant material are determined by their chemical composition. There-

608

fore, it can be a s s u m e d that properties o f soil h u m u s can be m o d i f i e d by the origin o f post-harvest residues u n d e r g o i n g mineralization and h u m i f i c a t i o n processes in the soil e n v i r o n m e n t , Firstly, the h u m i f i c a t i o n rate d e p e n d s on the C : N ratio, therefore, the residues rich in nitrogen d e c o m p o s e m o s t rapidly; secondly, the process is related to the content o f c o m p o u n d s p r o v i d i n g carbon easily accessible to m i c r o o r g a n i s m s . Undoubtedly, the h u m i f i c a t i o n process is c o n n e c t e d with the h y d r o g e n c o n t e n t decrease and the o x y g e n content increase in H A molecules. The content o f nitrogen and carbon, as well as the c h a n g e s during h u m i fication, are determined by the origin o f plant material. The results s h o w e d that not all dependencies between quality parameters o f HAs, c o m m o n l y accepted for soil substances, proved true in the case o f organic material o f plant residues.

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s.s. Gonet and B. Debska

Bydgoszcz, Poland: University of Technology and Agriculture: 1989. Gonet. S.S.; Zaujec, A.; Debska, B. Chemiczna charakterystyka produktow rozkladu resztek roslinnych w glebie. (Chemical characteristics of decomposition products of plant residues in soil.) In: Proc. conference on organic fertilizers. Vol. 1. Szczecin. Poland: University of Agriculture; 1992:306-316. Jurcova, O.; Bielek, P. Zabezpiecienie bezdeficitneho hospodarenia s podnou organickou hmotu. (Non-deficit management of soil organic matter.) In: Proc. international conference environmental problems in agriculture. Vol. C. Nitra~ Slovakia: Slovak Agriculture University; 1996: 203-207. Kolbe, G.: Stumpe, H. Nawozenie sloma. (Fertilization with straw.) Warszawa, Poland: Agriculture and Forestry Publ4 1975. Kumada. K. The chemistry of soil organic matter. Technical Bulletin No. 22. Yaipei City, Taiwan: Food and Fertilizer Technology Center; 1975:10-36. Kumada, K. Elementary composition and absorption spectra of humic and fulvic acids. Soil Sci. Plant Nutr. 31(3): 437-448: 1985. Kumada, K. Chemistry, of soil organic matter. Developments in soil science 17. Japan Sc. Soc. Press Tokyo. Amsterdam: Elsevier: 1987. Mazur, I. Stan i perspektywa bilansu substancji organicznej w glebach uprawnych. (The condition and the perspective of the organic matter balance of the cultivated soils.) Zesz. Probl. Post. Nauk Rol. 421a: 267-276; 1995. Schnitzer, M.: Skinner, S.I.M. Alkali versus acid extraction of soil organic matter. Soil Sci. 105(6): 392-396; 1968. Zaujec, A. Chemicka a fizikalno-chemicka charakteristyka produktov rozneho stupna premeny pozborovych zvyskov kukurici, lucerny a psenice. (Chemical and physical characteristics of partially, decomposed plant residues of corn, lucerne and wheat.) PhD Thesis. Nitra, Slovakia: Slovak Agriculture University; 1980. Zigunov. A.W.; Simakov, WH. Sostav i svojstva guminovych kislot, vydzelennych iz razlagajuszczicbsa rastitelnych ostatkov. (The composition and properties of humic acids from decomposing post-harvest residues.) Pochvov. 12: 59-65; 1977.