Aromaticity and humification of dissolved organic matter (lysimetric experiment)

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1 Aromaticity and humification of dissolved organic matter (lysimetric experiment) Elizaveta Karavanova 1, Evgeny Milanovskiy 2 1 LomonosovMoscowStateUniversity, Faculty of Soil Science,Department of soil Chemistry 1 2 Department of Soil Physics and Reclamation 2, Moscow, Russia Key word: dissolved organic matter, aromaticity, HIX, lysimetric waters Introduction The optical properties of dissolved organic matter (DOM) are widely used to characterize its chemical nature. Ultraviolet (UV) and fluorescent spectrometry are swift and noninvasive methods which require little sample preparation and a small sample volume. Absorption in UV range is governed by presence of C double-bond of benzene-type structures in the DOM. Many authors proposed the UV absorbance to assess a variety of DOM properties, such as aromaticity, hydrophobic content, humification [2]. Specific ultraviolet absorbance (SUVA) is defined as the UV absorbance of a solution sample at a given wavelength normalized for dissolved organic carbon (DOC) concentration. Numerous data sources confirm that SUVA, determined at 254 nm (SUVA 254 ), is strongly correlated with percent aromaticity of DOM as determined by standard direct methods (such as 13 C NMR), which proves that SUVA is a useful parameter for estimating the dissolved aromatic carbon content in aquatic systems: rivers, soil solutions, lysimetric waters etc. Fluorescent organic components of DOM include humic substances, derived from the break-down of plant material and aromatic amino acids (in free form or in proteins and peptides). Fluorescence spectra can be used to assess the origin and transformation degree of DOM through calculation of several fluorescence indices. The humification index (HIX) is a sensitive and simple parameter which is often used to characterize DOM; it is calculated from the fluorescence emission spectra obtained at excitation wavelength of 254 nm. HIX index was introduced by Zsolnay et al. [8] in order to estimate the degree of maturation of soil DOM. It was based on the fact that humification is associated with an increase inthe C/H ratio [7] and with a resulting shift of fluorescence maximum to longer emission wavelength [6]. HIX is defined as the area of the emission peak in nm divided by the area of the peak in nm. When the degree of DOM aromaticity increases, the emission spectrum (measured at excitation wavelength of 254 nm) is shifted to the red zone and thus the HIX index increases. In order to avoid the dependence of HIX values on DOM concentration Ohno [5] proposed to define the humification index as the fluorescence intensity in the nm region divided by the sum of intensity in the nm and nm regions. In this case HIX values characterize the degree of humification in such a way that avoids the sensitivity to the magnitude of the denominator (so named inner filter effect correction). The aim of our work was to investigate the aromaticity and humification of DOM in the lysimetric experiments by use of UV and fluorescence indexes. 178

2 International Congress on Soil Science in International Year of Soils Materials and methods Field experiment was carried out at the Soil Science Department of the Lomonosov Moscow State University. Zero-tension lysimeters filled by soil and/or plants residues were used for the study. Soil samples were taken from the arable horizons of chernozem (lysimeter III), plant residues were presented by oak leaves (lysimeter I) and perennial grasses (lysimeter II). Lysimeters IV and V had two components: arable horizons of chernozem (the same as in the lysimeter III) and oak leave or grasses, respectively (the same as in the lysimeters I and II) placed on the soil surface. All samples were taken at Voronezh region, Russia (51 36'21.8" N 38 58'11.1" E). Samples of the lysimetric waters were collected 6 times in the spring 2015: March samples N4-6, April samples N10, 11; May sample N12. After collection water samples were filtered through the 0.45 m membrane filters and stored at 4 C in the dark before analysis. Carbon and nitrogen contents in lysimetric waters were determined by dry combustion in Vario EL element analyzer (Hanau, Germany). Inorganic C was not present in any sample, thus all of discovered carbon corresponds to organic carbon. UV absorption spectra were measured in a quartz cell with a 1-cm optical path length between 200 and 500 nm with a uniform data point interval of 1 nm at a constant temperature (25 C) with UV-visible spectrophotometer (SF-2000). Index SUVA (specific UV absorbance at 254 nm) was calculated as a value of DOM solution absorption at 254 nm divided by dissolved organic carbon (DOC) concentration. Fluorescence analysis was performed on a spectrometer Perkin Elmer LS 51. Emission spectra (EM) were obtained from 280 to 500 nm with the excitation wavelength of 254 nm; scan speed was 500 nm/min. In order to avoid an innerfiltering effect (the fluorescence intensity is proportional to the concentration of fluorescent compounds only for low absorbance: D< 0.5 [1]) the UV absorbance at 254 nm of the measured samples was systematically recorded. When the maximum absorbance was higher than 0.5, samples were diluted. Humification index HIX was calculated according to Ohno [5] as sum of fluorescence intensivities from 435 to 480 nm divided by the sum of fluorescence intensivities from 435 to 480 nm and from 300 to 345 nm. HIX values in this formula ranges from 0 to 1 with increasing degree of humification. All data was statistically analysed (Statistica 6). Results and discussion The time dynamics of specific ultraviolet absorbance (SUVA 254 ) is given at the fig. 1; variation parameters (mean, median and limits) were calculated taking to the account all of the measured values (Table 1). Table 1 Mean, limits and median values of SUVA 254 of DOM Lysimeter Sourse of SUVA254 statistics DOC mean minimum maximum Median I leaves 0,035 0,023 0,041 0,037 II grass 0,029 0,020 0,039 0,030 III soil 0,031 0,012 0,062 0,022 IV soil+leaves 0,031 0,008 0,044 0,031 V soil +grass 0,033 0,012 0,067 0,

3 Fig. 1. Dynamics of SUVA254 in the lysimeters leachates. DOMs of different samples do not differ much in the specific adsorption SUVA 254 through springtime. Mean values vary from 0,029 to 0,035. This is partly due to the significant differences in DOM optical properties in the early and late spring. In March, during snow melting (sampling time 4-6) specific adsorption of the soil DOM is significantly lower as compared with DOM from the other sources (fig. 1). In the April and May (samples 10 and 12) SUVA values from soil DOM are increased significantly in accordance with the temperature growth and intensification of microbial activity, that lead to the partial decomposition of soil humus. At the same time the dissolution and transfer of these substances into the lysimetric waters are limited by the precipitation rate, volume and type of water movement (matrix or base flow) through the soil. According to Kaiser [3] the composition of DOM in streams and ground waters varies with different hydrological conditions. At the end of the April (sample 11) solutions had organic components with very low SUVA. Probably they are represented by the hydrophilic DOM fraction, that dominate in solutions at the low flow conditions [3], because sorptive interactions of DOM with mineral surfaces result in preferential removal of aromatic compounds (with greater SUVAvalues). Such compounds were sorbed by soil as determined from the drastic drop of SUVA 254 of the DOM from leaves and grasses as compared with the same components from soil (fig 1). Sample 11 (end of April) revealed the substantial increase of the :N ratio of DOM in comparison with the other late spring samples (12 and 10) that confirms the change of the DOM nature. Values of C:N of DOM from plant residues increased 2-4 times or evern 10 times for DOM from plant residues with soil. The observed change is likely due to the changes in organic residues transformation or to the income of some allochthonic organics (including substanses of the anthropogenic origin). Median and mean values of SUVA (Table 1) show that plants decomposition leads to the appearance of the water soluble aromatic compounds. Such substances dominated in the DOM from the oak leaves, likely due to the presence of tannines. On the contrary, solutions from the soil arable horizon contained a minimum amount of the aromatic compounds. SUVA of plants DOM is 1,4-1,7 greater than soil DOM (lysimeters I, II, III). After the interaction with soil, SUVA of plants DOM 180

4 International Congress on Soil Science in International Year of Soils substantially decrease, due to the absorption of the such substances (they are represented mostly by dissolved lignin-derived compounds), which has been proved before [4]. HIX index (Table 2) demonstrate the extent of the DOM humification: its values correlate with C:H ratio in the molecules of organic matter. Plants derived DOM (lysimeters I, II) are more humificated than soil DOM(lysimeter III) with mean and median values 0,917-0,923 and 0,886-0,866 for plants and soil DO, respectively. Lysimeter Fig. 2. Dynamics of HIX in the lysimeters leachates Table 2 Mean, limits and median values of HIX values Source of HIX statistics DOC mean minimum maximum median I leaves 0,920 0,858 0,978 0,923 II grass 0,923 0,902 0,958 0,917 III soil 0,886 0,834 0,960 0,866 IV soil+leaves 0,919 0,877 0,950 0,923 V soil +grass 0,938 0,919 0,961 0,937 Such a tendency is observed for all of the sampling periods except 10 th, when HIX value for the soil DOM grew up to 0,953, that is typical for the late spring data (fig. 2). In general in the early spring DOM of grasses demonstrate the most extent of humification and in the late spring (May) the same was true for DOM from oak leaves (fig.2). Perhaps more resistant organic substances are subjected to decomposition and further humification as the temperatures increases. HIX values increase from March (mean values 0,856-0,935) to May (0,941-0,978). This trend is particularly evident for soil (lysimeter III): mean HIX of the soil DOM in March is 12% which is lower than in May (0,856 and 0,960). No distinct trends of changes in HIX values after the interaction of the DOM from decomposing plants with soil were discovered. The beginning of March was the only 181

5 exception (sample 4, fig. 2), where the most humificated compounds were sorbed from solutions: HIX of DOM eluted from lysimeters with soil+plants were lower than in lysimeters with plants. By the end of the spring (May) this tendency remain the same only for DOM eluted from oak leaves (sample 12, fig. 2). Conclusions 1. Aromaticity of DOM eluted from plant residues (leaves and grasses) is 1,4-1,7 times greater than DOM from chernozem. DOM from oak leaves show maximum values of UV absorbance, apparently due to the high content of tannins. 2. Mostly aromatic DOM compounds from decomposing leaves and grasses are sorbed by the solid phases of chernozem: SUVA 254 of DOM from the lysimeters IV and V were 20% less comparing with DOM eluted from pure plant residues (lysimeters I and II). 3. Extent of the DOM humification depends on the season and DOM source. In the early spring (March) grasses have a greater value than soil and oak leaves, at the late spring (May) leaves are characterized by the greatest degree of humification. HIX of soil DOM is substantially less than DOM of plant residues. 4. The most humificated compounds of DOM are derived from decomposing plants not preferentially sorbed by soil except for the period of the early spring (beginning of March). Acknowledgements This work was supported by the Russian Foundation for Basic Research, project no References 1. Carstea E. M. Fluorescence spectroscopy as a potential tool for in-situ monitoring of dissolved organic matter in surface water systems//water Pollution Prof. N. Balkis (Ed.), ISBN: , DOI: / Available from: potential-tool-for-in-situ-monitoring-of-dissolved-organic- 2. Jafrain J., Gurard F., Meyer M., Ranger J. Assessing the quality of dissolved organic matter in forest soils using ultraviolet absorption spectrophotometry// SSSAJ V. 71. PP Kaiser K., Guggenberger G. Storm flow flushing in a structured soil changes the composition of dissolved organic matter leached into the subsoil// Geoderma V PP Kaiser K., Guggenberger G., Zech W. Sorption of DOM and DOM fractions to forest soils. Geoderma V.74. PP Ohno T. Fluorescence inner-filtering correction for determining the humification index of dissolved organic matter// Environ. Sci. Technol V. 36. PP Senesi N. Molecular and quantitative aspects of the chemistry of fulvic acid and its interactions with metal ions and organic chemicals. Part II. The fluorescence spectroscopy approach// Analytica Chimica Acta V.232.PP Stevenson F.J. Humus chemistry. genesis, composition, reactions. Wiley-Interscience, New York p 8. Zsolnay A., Baigar E., Jimenez M., Steinweg B., Saccomandi F. Differentiating with fluorescence spectroscopy the sources of dissolved organic matter in soils subjected to drying// Chemosphere V. 38. PP