CC BY
47
УДК 631.41
Z. Sokoiowska, L. Szajdak, P. Warchulska EFFECT OF PH ON THE RELEASE OF ORGANIC MATTER FROM MUCKS
Изучена роль pH в высвобождении органического вещества (ОВ) из торфяников. Исследования проводились для H - моноионных форм семи торфяников (Terric Histosols). Концентрацию растворенного органического вещества (РОВ) определяли колориметрически (465 нм). На основании полученных спектров поглощения были рассчитаны параметры Q2/6 и Q. Растворенность ОВ существенно зависит от величины рН. Зависимость может быть описана следующим уравнением: РОВ = 0.01 exp(fy*pH). Полученные результаты подтверждают зависимость между параметрами Wp Q и b1.
Ключевые слова: растворенное органическое вещество (РОВ), pH, степень вторичной трансформации.
Introduction
Peatlands are large sources of dissolved organic matter. Peat represents a highly organic material, which in nature contains typically 80-90 %. The importance of humic substances in water management of peat is suggested by the fact that peat contains about 25-30 % humic acids (HA) on a dry weight basic [1]. The melioration of peatlands led to the biotic and abiotic changes, which implicated the degradation of organic matter and organic compounds. Kalbitz et al. [2] showed that the land use of peatlands effects on fulvic acids (FA) properties, which account for the major fraction of dissolved organic matter. The above mentioned authors suggested that long-term intensive land use (from 50 to above 200 years) resulted in larger proportion of the aromatic structures and a larger degree of polycondensation of FA [2]. However it is unknown what changes in the units of the structure of FA they cause. Leinweber et al. [3] reported that in water-soluble FA, which are the main component (about 60 %) of dissolved organic matter, the proportion of carbohydrates and phenols together with lignin monomers increased with increasing intensity of soil tillage, aeration and peat degradation.
Most of the peatlands in Poland have been drained and subjected to agricultural use. Draining melioration changes natural peat soil evolution characterized by processes among which mineralization and secondary humification are of the greatest importance. The above processes lead to the changes in the morphological, chemical, biological and physical properties of peat soils. Taking into account the degree of the peat transformation, the muck mass can be divided into three categories: Z1 - peaty muck, Z2 - humic muck and Z3 - proper muck [4]. In the peat-muck soil, the muck constitutes the upper part and primary peat the lower part of the profile. The thickness of the muck layer depends on the intensity of drainage. Based on the thickness of this layer, peat-muck soils are next divided into three groups, weakly transformed (MtI), medium transformed (MtII) and strongly transformed (Mtlll) [4].
While water conditions change, soil mass loses its sorption abilities and gains more hydrophobic character. The mechanism of the above changes is called secondary
transformation. It causes a decrease in the humus content and changes its quality and release of ash elements. As the consequence of the secondary transformation, soil degradation and lost of fertility occurs. One of important factor of these phenomena can be release of organic matter which can affect not only the soil but the quality of groundwater and surface water. To improve agricultural properties of muck soils and protect of groundwater and surface water from pollution and rapid degradation, knowledge of the influence of pH, temperature, ionic content of soil solution on mobility of organic matter is necessary. In particular, pH of organic soil is very important for coagulation and peptization processes of organic matter. Changes in pH affect the electrostatic charge that induces attraction - repulsion of negatively charged surfaces of humic acids to other soil components.
Important characteristics of organic matter is its ability to form water-soluble and water insoluble complexes with metal ions and hydrous oxides and to interact with organic compounds such as alkanes, fatty acids, dialkyl phthalates, pesticides etc. Of special concern is the formation of water-soluble complexes of FA with toxic metals and organics.
The effect of pH on organic matter release from mineral and organic-mineral soils has been the subject of many investigations [5]. However, there is still to little of studies connecting results of optical measurements (concentrations, Q-factors) with kinetic investigations of alkalization processes.
The purpose of studies was to carry out the model investigations to look for effect of the degree of the secondary transformations on the DOM release at various pH in peat-muck soils. In that studies, spectrophotometric measurements of the concentration of released total dissolved organic carbon and indexes calculated on its basis (Q2/6, Q4/6) were supported by mathematical formula of organic carbon dynamics, which was established in order to improve of method sensitivity.
Matherials and methods
The study was conducted on 7 meadow muck samples (Terric Histosols) at different states of secondary
transformation. The samples were collected at the depth of 5-20 cm from sites located in a low moor area of the Polesie Lubelskie. The soils investigated represented three stages of moorshing process, i. e. MtI - weakly moorsed, MtII - medium moorshed and MtIII - strongly moorshed [4]. The samples were classified as peaty-mucks - Z1 and proper (granulated) mucks - Z3. The degree of transformation of the peat was characterized by the value of water holding capacity (W1) determined according to Gawlik method [6] (Table 1).
W1 index segregates all the samples into four classes of secondary transformation state. Values of W1 less than 0.36 characterize non-transformed peat fraction. Values of W1 ranging from 0.36 to 0.61 are characteristic for weakly and medium transformed peats. W1 between 0.61 and 0.82 are found in strongly transformed peats and W1 larger than 0.82 are typical for totally degraded hydrogenic soils. W1 values of mucks in that study ranges between 0.44-0.74. Table 1 shows W1, Z, Mt values for the studied materials and also selected physical properties.
Homoionic forms of studied soils were obtained as follows: the samples of each soil were placed in glass columns with a permeable bottom and were contacted three times with 0.1 M HCl. The ratio of solid to liquid phase was equal 1:50 w/w. The suspensions were centrifuged after each equilibration and washed four times with distilled water. Then, hydrogen forms of fresh materials containing exactly 0.4 g of dry organic matter
Firstly, UV-VIS absorption of alkalic extracts for a few muck samples, on the basis of the 4-th derivatives analysis, was investigated. Our data show, in agreement with literature, that the maxima wavelengths were at about ^=470 nm. So, it was stated that the measurement of the concentrations of DOM can be done at ^=470 nm. Moreover, absorption spectra obtained for a few studied mucks were almost the very same to that for sodium humate solutions (Aldrich H1, 675-2). A typical absorption spectrum of the organic matter extracted from the studied soils is presented in Fig. 1.
Measurements of A470 were made with Jasco V-500 UV/VIS spectrophotometr after extracts were collected to dark vessels, by using 0.45 ^m filtrates. pH measurements were done with Radiometer Copenhagen model pH 240 pH/Ion-meter equipped with a combined electrode. A calibration curve was based on a series of 10 sodium humate standard solutions (Aldrich H1, 6752). Concentrations of C in standard solutions from 0.005 to 0.15 mg/ml ranged of absorbance from 0.4 to 0.03.
From the obtained absorption spectra, absorption values at the following wavelengths ^=280, 465 and 665 nm were read. Q2/6 and Q4/6 indexes were calculated on the basis of measured absorbances.
Results and discussion
All of soil samples are characterized by low (< 25 %) ash content (Table 1) [7]. Soils examined greatly differ for volumetric densities, which are undoubtedly related
Table 1
Selected physical properties of investigated samples
No sample W1 Kind of muck formation Mt pH in H2O pH in 1 N KCl Ash content. (%) d.m Bulk density (g cm-3) Total porosity (% vol.)
1 0.44 Z1 I 5.1 4.5 22.7 0.21 88.5
2 0.48 Z1 II 4.7 4.2 20.5 0.28 84.7
3 0.60 Z3 II 5.4 5 21.2 0.34 81.4
4 0.61 Z1 II 5.8 5.3 15.1 0.24 85.2
5 0.65 Z3 II 5.5 5 18.9 0.31 80.9
6 0.71 Z3 III 6.2 5.8 22.8 0.3 83.6
7 0.74 Z3 II 5.8 5.3 21.5 0.29 84.1
Where: d.m - dry mass.
were treated by NaOH solutions at pH = 5,6,7,8, respectively. The ratio of solid to liquid was 1:100 w/w.
Concentrations of DOM in the extracts were determined from absorbances of alkalic extracts. Colorimetric studies of alkalic extracts obtained by using NaOH solution (for example 0.5 M NaOH) is widely used by soil scientists for the characterization of these materials. The ratios of absorbances of dilute aqueous HA and FA solutions at wavelengths ^=280 and 665 nm, and ^=465 and 665 nm are the most often used. These ratios are independent of concentrations of humic materials, but vary for humic materials extracted from different soil types.
wavelength [nm]
Fig. 1. Typical absorption spectrum of humic substances
to the degree of mucking and to the secondary transformation of the soil mass. The lowest density (0.21-0.28 g-cm-3) is observed in mucks, which posses the lowest soil water adsorptivity index (0.41<W1 <0.48). In the strongly transformed mucks, the values of W1 range from 0.60 to 0.74, the bulk density is higher (from
0.24 to 0.34 g-cm-3). Most of the soils are acidic (samples 1, 3, 4, 5), one is slightly acidic (sample 6) and one - very acidic (samples 2).
Very similar spectra were observed for the DOM extracted at various pH values. These absorption spectra had neither maximum nor minima and the optical density usually decrease as the wavelength increased. The composition of organic matter is frequently characterized by the ratio of absorbances at ^=465nm (Q4) and at ^=665 nm (Q6) [8] and also by 280/665nm ratio. As the absorbance at ^=465 nm is due to smaller molecules, and at ^=665 nm to larger molecules, the Q4/6 ratio is expected to be larger for FA of low molecular weight and smaller for humic acids of greater molecular weight studies. The Q4/6 ratio is <6 for HA and 6-18.5 for FA [9]. Kononova [9] believes that the magnitude of the Q4/6 is related to the degree of condensation of the aromatic C network, with a low ratio indicative of a relatively high degree of condensation of aromatic condensation and infers the presence of relatively large proportions of aliphatic structures. Conversely, a high Q4/6 ratio reflects a low degree of aromatic condensation and infers the presence of relatively large proportions of aliphatic structures. Q2/6 index characterizes content of compounds of lignin types. Those ratios have been reported to vary for humic materials extracted from different soil types and to be independent on concentrations of humic materials [9-10]. The Q4/6 and Q2/6 ratios for the organic matter released from the studied soils at pH=8 and in relation to secondary transformation index are shown in Fig. 2.
The Q4/6 values ranging between 5.3 to 8 show that in the studied mucks the relative content of FA and HA is very similar and that the FA is prevailed.
The amounts of DOM measured in the extracts at different pH are shown in Fig. 3. The dissolution of soil organic matter was significantly affected by pH. The
Q
100 -I 80 -60 -40 -20 -0 -
□ q4/6,pH8
□ q2/6,pH8
0.44 0.48 0.6 0.61 0.65 0.71 0.74
W,
Q
100 1 80 60 -40 -20 -
0
□ q4/6,pH8
□ q2/6,pH8
0.44 0.48
0.6
0.61 0.65
W-,
0.71 0.74
Fig. 2. Relationship between Q2/6, Q4/6 of extracted organic matter and secondary transformation degree
Fig. 3. Concentration of dissolved organic matter released at different pH from extracts vs secondary transformation index of studied mucks
results of[11] suggest that the key factor in humic release is the net electrical charge which is governed principally by pH. Data in Fig. 3 show the dissolved organic matter concentration was small that at low pH values. Generally the dissolution of organic matter increases with an increase in pH. An increase in pH leads to an increase in the negative surface charge of organic substances that causes their repulsion by each other and the other negatively charged soil components. Also the organic-organic and organic-mineral bonds by multivalent cations may be broken by cations neutralization, e. g. aluminum or precipitation, e.g. magnesium. The above processes cause migration of organic particles from the solid to the liquid phase. Low Q4/6 ratio reflects high proportion of strongly colored humus substances. So, it can be stated that muck formation process enriched the investigated soils with humic acids. Low molecular weight organic substances could be leached out from the soils during the transformation process. The contents of fulvic acids were higher in less transformed samples.
Previous researches of the authors showed, that the organic matter dissolution process during alkalization could be satisfactory described with the following equation [12]:
DOM(pH) = 0.01 exp(b1pH),
where b1 was parameter which could be considered as quantitative index of OM release process under the experimental conditions applied.
It was proved, that index of equation - b1 increases with secondary transformation degree. The highest concentration of DOM was obtained for the strongly secondary transformed sample (W1=0.74). The opposite was obtained for the weakly secondary transformed sample (W1=0.48). The tendency of increasing of DOM with the state of secondary transformation could indicate that the partial disruption of humus aggregates took place during the secondary humification. As a consequence of the above different substances of lower molecular weight, e. g. FA acids were formed. Qualitative index - Q are also correlated with b1 and W1 indexes what was presented in previous investigations [12].
Conclusions
The study showd the impact of pH on the release of DOMom mucks. Cncentrations of DOMfeleased to solution increase iwth pH increasing. Q 2/6 and Q4/6 are suitable parameters to describe of organic matter released
from precipitate. Empiric euation may be used to describe the release of DOMrom muck soils at laboratory conditions. b1 indexcould be used to quantify the DOMelease process and it is correlated wh secondary transformation degree W1.
List of references
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