Investigating the spectral characteristic and humification degree of dissolved organic matter in saline-alkali soil using spectroscopic techniques

Abstract

Soil samples were collected from the areas surrounding Wuliangsuhai Lake in China. Dissolved organic matter (DOM) was extracted from the samples and characterized by fluorescence and UV-Vis spectra. Spectral properties and humification degree of DOM were studied. The results indicated that both humic- and protein-like fluorophores were present in the DOM spectra, and the former was the dominant component. The analysis of humification (HIX) and r (A, C) indices revealed that the maximum humification degree in three agricultural soils (AAF, ASC, and ASW) was presented in the second soil layer (20–40 cm). However, the humification degree of the two Halophytes soils (SSE and GKF) decreased with increasing depth. One index, I_344/270, showed that humification degree increased gradually with an increasing proportion of humic-like acid. There was a significant positive correlation between humification degree (HIX) and aromaticity (SUVA₂₅₄), indicating that a higher aromaticity corresponded to a higher humification degree. Land use was an important factor responsible for the major difference of cation exchange capacity (CEC) in different soils, which led to a higher CEC value in the second soil layer for the three agricultural soils. CEC values and humification degree had the same trend for all five soils. The correlation analysis showed that there was a significant positive correlation between HIX and CEC, and a negative correlation between the r (A, C) index and CEC, indicating that humification degree increases gradually with increasing CEC values.

Keywords

dissolved organic matter / fluorescence spectroscopy / humification degree / cation exchange capacity

Introduction

Desertification is a subtle, outspread, and continuous process, driven by a number of causes, including short- and long-term climatic variations and human activities. Desertification involves reduction in vegetation cover and species diversity, a loss of soil structure, a decrease in soil fertility, an altered hydrological cycle, and reduced crop yields and livestock production ( Cilenti et al., 2005), and most of it takes place far away from desert margins. Among all the desertification processes, salinization has become the one of greatest concern. And while many countries are making efforts to utilize and control saline-alkali soil, improving such soil is extremely difficult. There are more than 2666.7 km² of saline-alkali soil and low-yielding fields in China, many of which can reasonably be developed and utilized. The improvement and control of saline-alkali soil has become increasingly important, as the amount of arable land is decreasing due to urbanization, desertification, and salt-alkalization.

Soil organic matter, even at very low levels, is well known to be the foundation for critical soil properties such as fertility, permeability, and structural stability. Consequently, the percentage of organic matter in soil is considered one of the most important indicators for evaluating the extent of salinization processes ( Cilenti et al., 2005). Cation exchange capacity (CEC) is the most important chemical property of soil, and is determined by the surface properties of soil colloid, which is composed of organic and inorganic base exchanges. The former is mainly humic acid, which has a particularly high CEC, and the latter is mainly clay minerals. There is a significant correlation between the content of organic matter and CEC: the higher the organic matter content, the greater the CEC value. The CEC value of organic soils also increases with greater degrees of humification ( Stevenson, 1994).

Dissolved organic matter (DOM) is considered to be the most active and important component of soil organic matter ( McDowell, 2003), and it includes more activity points than solid organic matter ( Temminghoff et al., 1997). DOM has been defined as the water-soluble organic material that can pass through a 0.45-mm membrane filter, it consists of a complex mixture of molecules of relatively low molecular weight bearing both polar and non-polar sites ( Zsolnay, 1996). Spreading organic materials over an area of land introduces large amounts of DOM into the soil. DOM has the ability to form stable complexes with heavy metals and can facilitate their movement into the groundwater ( Knoth de Zarruk et al., 2007). Humic substances (HS) comprise the largest proportion of DOM, and their physico-chemical characteristics depend on their sources and biogeochemical pathways ( Thurman, 1985). Soil HS are composed of lignin-derived materials and other plant residues, and these tend to be subject to relatively long humification processes ( Hur et al., 2009). Many previous studies have revealed that the humification degree of HS is associated with the extent of the interaction between HS and hydrophobic organic matter (HOC) ( Chin et al., 1997; Hur and Schlautman, 2003; Hur and Kim, 2009).

Fluorescence spectroscopy has been widely used to evaluate the structural and compositional changes of DOM ( Kalbitz et al., 1999; Baker, 2001; Chen et al., 2003; He et al., 2011a). Fluorescent organic matter is typically composed of condensed aromatic rings and/or unsaturated aliphatic carbon chains. Humification occurs as natural processes that alter the molecular structure of DOM, and generally increases its aromaticity and reduces its availability for microbial utilization ( Zsolnay et al., 1999). Several humification indices (HIX) have been proposed to estimate the degree of the humification of DOM based upon the general observation that the emission spectra of fluorescent organic matter tends to shift to longer wavelengths with the condensation of the constituent molecules ( Zsolnay et al., 1999; Milori et al., 2002; Patel-Sorrentino et al., 2002; He et al., 2011b).

The goals of this study were: (i) to illustrate the utility of fluorescence for obtaining information on a large fraction of dissolved organic matter; (ii) to assess the humification degree of saline-alkali soil using fluorescence spectroscopy; and (iii) to investigate the changes of CEC in different soil types and to reveal the relationship between CEC and humification degree.

Materials and methods

Study area and sample collection

Wuliangsuhai Lake (108°43'‒108°57'E, 40°27'‒40°03'N) is located in the eastern Hetao Irrigation District in the Inner Mongolian Autonomous Region of China. It is the largest fresh-water lake in the Yellow River Basin, covering 293 km² with an average total water volume of 250 to 300 million m³. There is a large agricultural irrigation and drainage network in the upstream area of Wuliangsuhai Lake, which is the primary origin of water supply for the region. It receives not only the drainage of agriculture, but also the upstream industrial wastewater and domestic sewage, the former accounting for 96%, the remainder, 4%. Serious soil salinization and soil erosion is occurring in the region.

For our study, five soil samples were collected from the areas surrounding Wuliangsuhai Lake, identified as: (i) AAF, an abandoned farmland area; (ii) ASC, agricultural soil planted with corn; (iii) ASW, agricultural soil planted with wheat; (iv) SSE, a soil containing vegetation known as Salicornia europaea; and (v) GKF, a deliberately grown Kalidium foliatum community. Soil samples were taken at three depths (0–20 cm, 20–40 cm, 40–60 cm, identified as M₂₀, M₄₀, M₆₀, respectively), at each of the five locations. Soil samples were taken randomly from an area of 2 m radius from marked plots, using a tube sampler. Each soil sample was thoroughly mixed and homogenized after carefully removing the organic materials and roots.

Materials and chemical analysis

All chemicals used for this study were AR grade. Soluble cations (Na⁺, K⁺, Ca²⁺, Mg²⁺) were measured using an atomic absorption spectrophotometer (AA-6300, Shimadzu), and electrical conductivity (EC) was measured in a 1:1 soil-to-water ratio, using an electronic conductivity meter (FE30, Mettler Toledo). Soil pH was measured in a 1:2.5 (w/v) soil-to-water solution using a pH meter (Sartorius). CEC was measured using the sodium acetate-flame photometric method because of the saline-alkali properties of the soil ( Lu, 1999). The solutions, including 1 mol·L^–1 sodium acetate, 95% ethanol and 1 mol·L^–1 ammonium acetate, were prepared for leaching the soils. Sodium acetate (pH=8.2) was used to treat soils and saturate them with sodium. Excess sodium acetate was washed away with 95% ethanol (NH₄⁺, ammonium acetate, can be exchanged for Na⁺). Each step was repeated at least three times. Centrifugation speed was 4000 r·min^–1 The concentration of Na⁺ was analyzed using the atomic absorption spectrophotometer indicated above. The CEC was calculated as:

(1) \mathrm{CEC},\mathrm{c}\mathrm{m}\mathrm{o}\mathrm{l}·\mathrm{k}{\mathrm{g}}^{-\mathrm{1}}(+)={\displaystyle \frac{\rho \times V}{m\times \mathrm{23.0}\times {\mathrm{10}}^{\mathrm{3}}}}\times \mathrm{100.}

DOM extraction from the five samples was conducted by mixing one part of solid sample with two parts of deionized water and continuously shaking it for 24 h. Extracts were centrifuged for 10 min. at 7000 r·min^–1 at 4°C and filtered through a 0.45 mm a glass fiber filter (GF/F). The solutions, including 0.1 mol/L of NaOH, and HCl, were prepared using deionized water for adjusting pH values. The pH values of the DOM samples were measured using a Sartorius PB-10 pH meter. TOC concentrations were measured using a total organic carbon analyzer (multi N/C 2100, Analytikjena, Germany). Reported TOC values are averages based on triplicate analysis (coefficient of variation<3%).

Spectral analysis

Excitation-emission matrices (EEM) spectra were measured in 1 cm quartz fluorescence cells at a temperature of 25°C, using a fluorescence spectrofluorometer (F-7000, Hitachi, Japan) equipped with a 150-W Xenon arc lamp as the light source. The slit widths were 5 nm for excitation and emission monochromators, and the scan speed was set at 1200 nm•min^–1. EEM spectra were obtained by scanning at an emission (Em) wavelength range from 280 to 550 nm in 5-nm steps, while the excitation (Ex) wavelength was increased from 200 to 450 nm in 5-nm steps. EEM contour maps were obtained in which each different fluorophore was characterized by an Ex/Em wavelength pair. The fluorescence response to a blank solution (Milli-Q water) was subtracted from the spectra of each sample ( McKnight et al., 2001; Chen et al., 2003).

Synchronous fluorescence spectra were recorded from samples prepared at a consistent concentration with a constant offset (Δl=30 nm) between excitation and emission wavelengths and 5 nm slit widths, as proposed by Hur et al. (2009). The scan speed and response were set to 240 nm•min^–1 and 0.5 s respectively. The excitation wavelength was fixed at 254 nm and the emission wavelength ranged from 300 to 480 nm. The bandwidths of the excitation and emission slits were set at 5 nm.

Ultra-violet and visible measurements were carried out with 1 cm quartz UV-visible cells at room temperature (~25°C), using a Shimadzu UV-visible double beam spectrophotometer (UV-1700, Shimadzu, Japan). UV-Vis absorption spectra of water samples were obtained at wavelength 200 to 600 nm. Two optical parameters were determined: 1) the specific UV absorbance at 254 nm (SUVA₂₅₄) and 2) the specific UV absorbance at 280 nm (SUVA₂₈₀).

Statistical analyses

Statistical analyses were performed with SPSS 16.0 software (Statistical Program for Social Sciences). Regression and correlation analyses were used to examine the relationships between variables using SPSS. Significance levels are reported as non-significant (NS) (p>0.05), significant (0.05>p>0.01), or highly significant (p<0.01). The linear model was validated with analysis of variance (ANOVA).

Results and discussion

Physico-chemical properties of soil

Because the Wuliangsuhai region depends on irrigation for both agricultures and basic water supplies, improper methods of irrigation and salt drainage have resulted in soil salinization, which has become a major issue in the region. Salinity and/or sodicity are common problems under irrigated agriculture, especially under high evaporative demand ( Sumner, 1995). Poor irrigation and drainage management are normally the main causes of salinization ( Rietz and Haynes, 2003). Table 1 lists the characteristics of the soils studied. All were alkaline soils with a pH of 7.94–8.80, indicating serious soil salinization. Soil electrical conductivity (EC) showed great variance, from a maximum of 13.11 mS·m^-1 in the SSE₂₀ soil profile to a minimum of 2.25 mS·m^-1 in the ASC₄₀ soil profile. The halophyte (Salicornia europae and Kalidium foliatum) soils had higher EC values, which decreased gradually with increasing soil depth. However, the three agricultural soils showed lower EC values (2.25–3.90 mS·m^–1) than did the two halophyte soils. From Table 1, it can be seen that these soils contain higher levels of SO₄^2– and Cl^–, particularly those soils that came from areas occupied by papermaking enterprises or fertilizer plants, or were downstream of farmland drainage. Because of the differences in land use, the contents of NO₃-N and NH₃-N also showed obvious differences. The highest content of NO₃-N was in the SSE₂₀ soil. The content of NO₃-N in the AAF, SSE, and GKF soils decreased gradually with increasing soil depth.

CEC is defined as a measure of the ability of a clay or a soil to adsorb cations in such a form that they can be readily desorbed by competing ions ( Bache, 1976). Cation exchange is a common phenomenon in soil, and it is the most important chemical property of soil. Soil can provide and maintain nutrient elements, and decontaminate soil by cation exchange. CEC is the total of the exchangeable cations that a soil can hold at a specified pH value. Soil components known to contribute to CEC are clay and organic matter, and to a lesser extent, silt ( Manrique et al., 1991). CEC is the basic character of soil and the main factor that influences fertility. It becomes an important parameter for assessing the capacity of soil for retaining nutrients and performing a buffering action. Table 2 lists the CEC values of the five soils. In the AAF soil, the highest CEC value appeared in the AAF₄₀ soil layer (20.88 cmol·kg^–1). The CEC values of three soil layers varied in the order AAF₄₀>AAF₆₀>AAF₂₀. In the ASC and AW soils, the highest CEC values also appeared in the second layer, but the trend of CEC varied in the order ASC₄₀>ASC₂₀>ASC₆₀ and ASW₄₀>ASW₂₀>ASW₆₀. However, in the SSE and GKF soils, the highest CEC value appeared in the first (upper) soil layer, and the CEC values decreased gradually with increasing soil depth. In general, salinization leads to relatively low CEC values in all soils.

Excitation-emission matrix spectra

All the DOM, from all five soils, had similar EEM spectra. Two defined peaks could be observed in the EEM spectra (Fig. 1). One peak appeared at the wavelength Ex/Em=(245‒280)/(400‒450) nm, and this was identified as Peak A for this study. It was previously reported that this peak is associated with a fluorescent fulvic-like acid substance ( Wu et al., 2001; Yamashita and Tanoue, 2003). The other peak (Peak C) was found near the wavelengths Ex/Em= (310‒350)/(405‒450) nm, which is related to a humic-like acid substance ( Coble, 1996). Peaks A and C showed a significant positive relationship (r=1.00, p<0.001), indicating that they probably contained a similar structure.

r (A, C) is the ratio of fluorescence intensity between Peak A and Peak C, and is proposed as a good indicator of organic matter maturation and structure, and it has been used to compare some samples in the same pH (Patel-Sorrentino et al., 2002). A high r (A, C) value corresponds to a low humification degree. Table 2 shows the values of r (A, C) for the 15 sub-samples from the five soils. Coble (1996) stated that the r (A, C) values were 1.08, 0.77, and 1.26 for rivers, pore waters, and Lake CuiCui, respectively. However, the r (A, C) values of soils in the Wuliangsuhai region ranged 2.08 to 2.46, indicating that there is a relatively low humification degree. Due to the invasion from urbanization, desertification, and salt-alkalization, the content of organic matter is decreasing gradually, and the humification degree is also relatively low. The lowest r (A, C) values for AAF and ASC soils were found in the second soil profile, indicating a relatively high humification degree. However, the r (A, C) value of ASW₄₀ soil was the highest of all, which explained its lowest humification degree. Agricultural cultivation has resulted in the mixture of soils and caused the r (A, C) values to have no obvious trend. The SSE and GKF soils do have an obvious trend: the r (A, C) values increased with increasing soil depth. Thus, the humification degree decreased with increasing soil layers. These results suggest that land use influences the humification degree of soils.

Fluorescence emission spectra

Emission fluorescence can be used to obtain estimates of DOM humification. The humification index (HIX) proposed by Zsolnay (2003) is calculated from the ratio of two integrated sections of an emission scan (the sum from lem 435–480 nm divided by the sum from lem 300–345 nm) collected with excitation at 254 nm. Low HIX values (<10) correspond to relatively non-humified DOM derived from biomass ( Hunt and Ohno, 2007; Ohno et al., 2007; Birdwell and Engel, 2010). HIX values generally increase as biomass decomposes ( Hunt and Ohno, 2007) and as the DOM is adsorbed from solution by minerals ( Ohno et al., 2007; He et al., 2011b). Humic substances isolated from soils, surface waters, and coal show ranges of HIX values: 10–30, 20–50, and>50, respectively ( Birdwell and Engel, 2010). However, most of the HIX values in Table 2 are less than 10, which can be viewed as a nominal cutoff below which DOM is not significantly humified ( Zsolnay et al., 1999; Huguet et al., 2009). It can be seen from Table 2 that the calculated HIX values of SSE and GKF soils showed the same trend: the humification degree decreased with increasing depth of the soil layer. The first soil layer showed a higher HIX value, indicating a stronger humification degree. The HIX values of ASC and ASW had a similar result: that the highest values were from the ASC₄₀ and ASW₄₀ samples. Compared with the AAF₄₀ soil, the HIX value of the AAF₂₀ soil showed a significant decrease, possibly because the AAF soil was eroded and had become a saline-alkali soil due to lack of cultivation. The HIX values show that the maximum humification index of the three agricultural soils was that in the second soil layer. Agricultural soils had been chronically influenced by agricultural cultivation and irrigation water, and their humification values exhibited significant differences. Meanwhile, agricultural chemical fertilizer also affected the humification process of soil organic matter. The results of HIX are consistent with those of r (A, C). There was a significant negative correlation between the HIX and r (A, C) values (Table 3).

Synchronous fluorescence spectra

The synchronous fluorescence spectra of soil DOM samples were characterized by a similar spectral shape, but with a difference in relative intensity (Fig. 2). These spectra showed two peaks, at wavelengths of around 270 nm (Peak I) and around 344 nm (Peak II). Peak I is usually attributed to the presence of proteinaceous materials, probably derived from recent biological activity ( Senesi et al., 1991). The signal of Peak II, according to Miano and Senesi (1992), could be caused by the presence of humic substances. There is a significantly positive relationship between Peak II and Peak A (r=0.98, p<0.001), and also between Peak II and Peak C (r=0.99, p<0.001). These synchronous fluorescence spectra are consistent with previous investigations by other researchers ( De Souza Sierra et al., 1994; Lombardi and Jardim, 1999). The protein is a newborn substance, while the humic substance is related to an old one. Mopper and Schultz (1993) suggested that the fluorescence of protein-like acid and humic-like acid represent newborn and old dissolved organic matter respectively. The fluorescence intensity ratio between Peaks II and I (I₃₄₄/₂₇₀) has been used as an indicator of the ratio between old and newborn dissolved organic matter. As shown in Table 2, the I₃₄₄/₂₇₀ values of the SSE and GKF soils show the same trend. The first soil layers exhibited the highest I₃₄₄/₂₇₀ values (1.02 and 1.47 respectively), and it appears that initially more humic-like fluorophores were present, possibly due to microbial degradation in the soils ( Fuzzi et al., 1997). The I₃₄₄/₂₇₀ index decreased with increasing depth of the soil layer. The largest I₃₄₄/₂₇₀ value among the three agricultural soils was presented in the second soil layer. This trend is consistent with the result of HIX. The same result can be obtained from correlation analysis between HIX and I₃₄₄/₂₇₀ (Table 3). A significant positive correlation between the I₃₄₄/₂₇₀ index and the HIX index suggested that humification degree increases with an increasing proportion of humic-like acid. Table 3 shows that there is a significant negative correlation (r= –0.77, p =0.0008) between the r (A, C) index and the I₃₄₄/₂₇₀ index, indicating that r (A, C) values decrease with an increasing proportion of humic-like acid.

UV-visible spectra

UV₂₅₄ represents the aromatic character of humic and fulvic acids. The UV absorbance at 254 nm measured in inverse meters (m^–1) divided by the organic carbon concentration measured in milligrammes per liter (mg•L^–1) is defined as the SUVA₂₅₄ index ( Leenheer and Croué, 2003; Weishaar et al., 2003). This index is an ‘average’ absorptivity and is used as an indicator for the aromaticity of the DOC ( Weishaar et al., 2003; Schnitzler et al., 2007). McKnight et al. (1994) proposed that the UV-sensitive fraction was mostly hydrophobic or aromatic in nature. Specific UV absorbance at 280 nm (SUVA₂₈₀) is positively correlated to DOM aromaticity ( Chin et al., 1994).

As shown in Table 2, the SSE and GKF soils exhibited high levels of SUVA₂₅₄ and SUVA₂₈₀ values, which decreased with increasing soil depths. Higher SUVA₂₅₄ and SUVA₂₈₀ values indicate higher aromaticity. The relatively low value showed that there was a relatively low aromaticity in the ASW soils. However, the ASC soil has also higher SUVA₂₅₄ and SUVA₂₈₀ values, and the highest value was in the AAF₄₀ layer. Nishijima and Speitel (2004) suggested that the absorbance values of organic materials, at 254 nm, represent unsaturated compounds with a carbon-carbon bond, including aromatic compounds, which are difficult to decompose. The decrease in absorbance indicated the conversion from humus to non-humus. Therefore, a higher aromaticity mirrors a higher humification degree. A direct correlation has been reported between HIX and DOM aromaticity (Kalbitz et al., 2003). A consistent result can also be obtained from Table 3 that aromaticity indices (SUV₂₅₄ and SUVA₂₈₀) were positively correlated with the HIX. Similarly, the r (A, C) was negatively correlated with SUV₂₅₄ SUVA₂₈₀.

The relationship between humification degree and CEC

Previous studies have shown that Peaks A, C, and II are all associated with fluorescent humic substances. Soil humic substances can react with soil inorganic matter by adsorption and complexation, stabilizing soil nutrients and improving microbial activity and soil structure. Air permeability and water-throughput capacity were enhanced effectively in clay soil by using fulvic-acid and humic-acid fertilizers. Correlation analysis showed that Peaks A, C, and II had significant positive correlations with the soil CEC (Fig. 3). However, the organic base exchanges are mainly with humic acid, which has a particularly high CEC. Thus, soil CEC and humic substance have a close relationship. An increasing fluorescence intensity of humic substances indicates an increasing content of CEC. To some extent, changes in the fluorescence intensity of humic substances can be used as an indicator of CEC content. Table 1 showed that most of the CEC values in the SSE and GKF soils were significantly higher than those in the three agricultural soils, indicating a relatively high content of organic matter. The use of fulvic acid or humic acid in saline-alkali soil can improve the CEC of the soil and promote the formation of soil aggregate structures ( Holtzclaw and Sposito, 1979), which prevent the increase of salinity and form a buffer layer for keeping it out. Moreover, the acidic groups of fulvic acid can also neutralize soil alkalinity, speed up the maturation process, and recover arability. It has been suggested that the application of microbial organic fertilizer can improve soil fertility better than can inorganic fertilizer, especially in saline-alkali soil.

CEC was influenced by the texture and viscosity of soil, the properties and structure of soil colloid, and the pH value. Further, the degree of complexity of the soil structure can impact the soil humification degree. In general, the structure of simple substances produces fluorescence at a short wavelength range, while a complex structure and high humification of substances emits fluorescence at a longer wavelength range ( Huang et al., 2006; He et al., 2011a). This study investigated the relationship between humification degree and CEC. The result of Table 3 shows that there is a significant positive correlation between HIX and CEC, and there is a significant negative correlation between the r (A, C) index and CEC, and the relationship between humification degree and CEC is better distinguished by the r (A, C) index than by the HIX (p = 0.0028 versus p = 0.0084 based on the t test of the means for

\alpha

= 0.05 using a two-tailed t distribution). A significant positive correlation between CEC and I₃₄₄/₂₇₀ can be found in Table 3, indicating that the proportion of humic-like acid gradually increases with increasing CEC values, and revealing that soil organic matter shows a good relationship with the content of CEC, as does the aromaticity degree: a high CEC value has a relatively high aromaticity degree.

Conclusions

This study illustrates that fluorescence spectra can be used to explain the structure and humification degree of soil DOM. The combination of the methods used clearly shows that long-term intensive land use results in a changing humification degree of soil DOM in different soil layers. The maximum humification index of the three agricultural soils was present in the second soil layer, and the humification degree of the two Halophytes soils decreased with increasing soil layers. The contents of fluorescent organic matter and CEC showed a significant correlation: the higher the organic matter content, the greater the CEC values. The results also show that there is a significant relationship between humification degree and CEC: the humification degree of soils increases markedly with greater CEC values. However, whether CEC can influence the humification degree of soil needs further study.

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