UV–visible spectroscopic properties of dissolved fulvic acidsextracted from salined fluvo-aquic soils in the Hetao IrrigationDistrict, China

Jinyuan JiangA,B,C, Huibin YuB, Beidou XiB, Fansheng MengB, Yuexi ZhouB,and Hongliang LiuA,B

ASchool of Environment, Beijing Normal University, Beijing 100875, China.BChinese Research Academy of Environmental Sciences, Beijing 100012, China.CCorresponding author. Email: jiangjy@craes.org.cn

Abstract. The elemental composition and the UV–visible spectroscopic properties of dissolved fulvic acids (FA)extracted from salined fluvo-aquic soils in the Hetao Irrigation District were examined to evaluate the humification degreeof FA and salinisation processes of soils. Composite soil samples of different depths (0–20, 20–40, 40–60, 60–80 cm) werecollected from four native halophyte soils [communities Salicornia europaea (CSE), Suaeda glauca (CSG), Kalidiumfoliatum (CKF), Sophora alopecuroides (CSA)] and two furrow-irrigated fields [corn (CFD), wheat (WFD)] along a salineimpact gradient. Seven humification indices (HIXs: C/N, SUVA, E2/E3, E2/E4, S275–295, S350–400, and S250–400), deducedfrom elemental analysis and UV–visible spectroscopy of FAs, were used to evaluate the humification degree. Thehumification degree of FA from the WFD soil was the highest, followed by CFD, CSA, CKF, CSG, and CSE. There weresignificant relationships between the seven HIXs (P < 0.05). The exchangeable sodium percentage (ESP) exhibited closecorrelations with the seven HIXs, and the humification degree increased with decreasing ESP. The HIX can not onlyindicate the humification degree of soil organic matter but also characterise the salinisation processes of soils.

Additional keywords: exchangeable sodium percentages, humus, humification, saline soils, ultraviolet–visiblespectroscopy.

Received 27 August 2011, accepted 18 October 2011, published online 28 December 2011

Introduction

Desertification has been defined by the Food and AgricultureOrganisation (FAO) and the United Nations EnvironmentProgram (UNEP) (1982) as: ‘a comprehensive expression ofeconomic and social processes as well as those natural orinduced ones which alter the equilibrium of soil, vegetation,air and water, in the areas subject to edaphic or climatic aridity.Continued deterioration leads to a decrease of the biologicalpotential of the land, deterioration of living conditions and anincrease of desert landscapes.’ Desertification usually takesplace far away from desert margins and is a subtle, dispersed,and continuous process that is affected by several factors,including short- and long-term climatic variations and humanactivities (Cilenti et al. 2005; Yu et al. 2010, 2011a)

Salinisation, one of the most common forms ofdesertification, is a process of soil degradation. It iswidespread in arid and semi-arid regions and has become amajor concern throughout the world (Liang et al. 2005; Yuanet al. 2007). Salinisation frequently occurs as a consequenceof over-irrigation caused by the improper management ofirrigation facilities, poor internal soil drainage conditions, orirrigation water of unsuitable quality. As the watertable rises,

salts dissolved in the groundwater reach, and accumulate at, thesoil surface through capillary movement.

Humus is a heterogeneous mixture of a variety of organiccompounds consisting of aromatic, aliphatic, phenolic, andquinolic functional groups with different molecular sizes andproperties (Aiken et al. 1985; Suffet and MacCarthy 1989). It isknown to play important roles in the interaction and transportof many toxic organic or inorganic chemicals and in nutrientcycling throughout the environment (Chorover et al. 1999;Burgos et al. 2000; Chen et al. 2003), despite being just asmall fraction of the soil composition. Humification is relatedto the qualitative and quantitative changes in humus, and isinvolved in the transformation of macromorphologicallyidentifiable matter into amorphous compounds (Zech et al.1997). Soil salinity may greatly affect a series of microbiallymediated processes (formation and transformation of humus)(Martin and Haider 1971). Therefore, humus is considered tobe one of the most important indicators when evaluating theextent of salinisation processes in soils. Chemical, biological,microbiological, spectroscopic, and chromatographic methodshave been proposed for evaluating the humification degreeof humus (Smernik et al. 2000; Hulugalle and Scott 2008;

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Soil Research, 2011, 49, 670–679http://dx.doi.org/10.1071/SR11213

Yu et al. 2010). It is necessary to develop fast, inexpensive, andconsistent methods for determining the humification degree ofsoil organic matter.

Ultraviolet (UV)–visible spectroscopy can represent a usefultool to evaluate humification degree with accuracy and rapidity.It is already used to quantify the dissolved organic matter ofsoil and water and to evaluate the quality of humus (Kalbitz et al.1999; Morán Vieyra et al. 2009). However, few investigationshave examined the structural components of humus extractedfrom saline soils and characterised humification degree andsalinisation processes using the UV–visible spectroscopicmethod.

The UV–visible spectroscopic properties of fulvic acid (FA)extracted from native halophyte soils and furrow-irrigatedfields in the Hetao Irrigation District were analysed in thisstudy. Humification indices (HIXs) deduced from theUV–visible spectroscopy were used to evaluate humificationdegree of the FA. The HIXs can substitute for exchangeablesodium percentages (ESPs) when evaluating the salinisationprocesses of soils.

Materials and methods

Study areas

The Hetao Irrigation District is the largest irrigation districtdiverting from the main stream of the Yellow River in China,located at the southern part of Bayannur City in the InnerMongolia Autonomous Region (Fig. 1). It is an arid regionwith an average annual rainfall of 93.8mm and a middletemperate zone with an annual mean temperature of 6.58C,where there would be no agriculture without irrigation. Thetotal area of the agricultural district is ~1.4Mha, the designedirrigation area is ~740 000 ha, and the actual area under irrigationamounts to ~400 000 ha. The major crops are grains, sugarbeet, rapeseed (for vegetable oil), and vegetables. There is acomprehensive irrigation system (Fig. 1), with a trunkirrigation canal, 12 main irrigation canals, and an extensive

network of branched irrigation canals. Water is drawn fromthe Sanshenggong Dam in Dengkou County, with whichfarmlands are irrigated through the three-band canals. Almostall of the return water from the branch drains and main drains isdischarged through the trunk drain and lifted into WuliangsuhaiLake before being discharged into the Yellow River. However,the irrigation system suffers from water logging and salinityproblems due to the high salinity of the farmland and poormanagement.

Wuliangsuhai Lake, the biggest freshwater lake in InnerMongolia and the Yellow River basin, is in the eastern HetaoIrrigation District (408030–408270N, 1088430–1088570E) (Fig. 1).The altitude of the yearly average water level is 1018.5m, witha total volume of water of 250–300Mm3. It is one of the mostimportant aquatic wetlands of northern Asia. Many species offish and birds as well as reeds, grasses, and herbaceous weedsare abundant.

Sample collection and physico-chemical determination

Four native halophyte communities [communities Salicorniaeuropaea (CSE), Suaeda glauca (CSG), Kalidium foliatum(CKF), Sophora alopecuroides (CSA)] and two furrow-irrigated grain fields [corn (CFD), wheat (WFD)] in theHetao Irrigation District were selected for the study on 9June 2009 (Fig. 1). The soil was a salined fluvo-aquic soil.A 3 by 2m2 plot was pegged out at each sampling site, and 20soil samples from four soil horizons (0–20, 20–40, 40–60, and60–80 cm) at each site were taken randomly from an area witha 2-m radius in the marked plots using a tube sampler (Rietzand Haynes 2003; Dodla et al. 2008). Each soil sample wasthoroughly mixed, homogenised, and sieved (<2mm) aftercarefully removing organic materials and roots. The soilsamples were immediately placed into protective packages topreserve freshness and were transferred on ice to the laboratory.

A portion of each soil sample was air-dried and sieved(diameter 100mm), and the remainder was stored frozen or

Comm. Salicornia europaea (CSE)

Comm. Suaeda glauca (CSG)

Comm. Kalidium foliatum (CKF)

Comm. Sophora alopecuroides (CSA)

Corn Farmland (CFD)

Wheat Farmland (WFD)

LegendTrunk canal

Main canal

Trunk drain

Main drain

Branch drain50 km

Branch canal

Fig. 1. Location of the Hetao Irrigation District in Inner Mongolia, China.

Spectroscopic properties of dissolved fulvic acids Soil Research 671

directly analysed. Selected physico-chemical parameters of thesoils are presented in Table 1. Soil pH and electrical conductivity(EC) were determined using moist field soil samples, whereasother properties were determined using air-dried samples. SoilpH was measured in a 1 : 2.5 soil : water slurry using a pH meter(Sartorius), and EC was determined in a 1 : 1 soil : water solutionusing an electronic conductivity meter (FE30, Mettler Toledo,Greifensee, Switzerland). Total nitrogen (TN) was determinedusing an elementary analyzer (Vario EL, Elementar, Hanau,Germany). Total organic carbon (TOC) was measured using aShimadzu V-CPH analyser (Shimadzu, Kyoto, Japan).

Exchangeable cations were extracted with 1 M ammoniumacetate (1 : 4 soil : extractant ratio for 2 h), and Ca2+, Mg2+, K+,and Na+ in the extracts were analysed by atomic absorptionspectrophotometry (AA-6300, Shimadzu). Exchangeablecation concentrations were calculated as ammonium acetate-extractable cations minus soluble (saturation-extract) cations. Asall soils had pH(water) values >8.0, the exchange acidity was notmeasured. The ESP was calculated as (Rietz and Haynes 2003):

ESP ¼ ½Naþ�½Naþ þ Kþ þ Ca2þ þMg2þ� � 100% ð1Þ

Extraction of dissolved fulvic acids from saline soils

Extraction and purification of dissolved FAs were carried outaccording to the methodology proposed by the InternationalHumic Substances Society (Swift 1996). Briefly, 400 g of air-dried, sieved (diameter 100mm) soil was added to 600mL of0.1 M Na2P2O7 with 0.1 M NaOH (pH 13) and was stirred over aperiod of 24 h at room temperature. The extract was centrifuged

with a Sigma centrifuge (4000G, 15min), and the supernatantwas then filtrated through 0.45mm. The FA fraction wasseparated by acid precipitation (pH 2) with 0.5 M H2SO4.After separation of the solution from the precipitate, thesolution was filtered with activated carbon. The activatedcarbon was cleaned with 0.2 M NaOH, and the eluate was theFA fraction. The eluate was neutralised to pH 7 using 0.5 M HCl.The powdered form of FA was obtained after electrodialysis andfreeze-drying, and it was stored in a vacuum desiccator.

Characterisation methods

Elemental analysis (C, H, O, and N) was performed in triplicateon freeze-dried samples of FA (0.5–1mg) using an elementaryanalyzer (Vario EL, Elementar). The ash content was determinedafter ashing at 7508C for 4 h and the oxygen content wascalculated by determining the difference (O = 100 –

(ash +C+N+H)) (Kalbitz et al. 1999). Samples of 2.5mg offreeze-dried FA were redissolved in 125mL of 0.05 M NaHCO3,and the pHwas adjusted to 7.0 (�0.05). UV–visible spectra wereobtained by using a 1-cm quartz cuvette at 258C on a ShimadzuUV-1700 spectrophotometer equipped with UV Probe 2.01 fordata processing in the wavelength range 200–600 nm.

Results and discussion

Physico-chemical properties

In the Hetao Irrigation District, different physico-chemicalproperties were shown among the native soils and furrow-irrigated fields, as well as within each soil profile(Table 1). The soil EC of the top layer was much higher than

Table 1. Physico-chemical properties of soils in the Hetao Irrigation District (6 sites� 4 soil layers = 24 soil samples and each sample in replicate)Communities: CSE, Salicornia europaea; CSG, Suaeda glauca; CKF,Kalidium foliatum; CSA, Sophora alopecuroides. Fields: CFD, corn fields; WFD, wheat

fields

Samples Depth pH EC TOC TN Na+ Ca2+ Mg2+ K+ ESP(cm) (mS/cm) (g/kg) (mmol/kg) (%)

CSE 0–20 8.76 10.90 6.13 1.54 117.14 130.13 10.25 3.92 44.8120–40 8.51 4.48 5.22 1.44 101.55 124.77 9.61 2.75 42.5540–60 8.51 3.31 4.66 1.55 89.06 117.77 9.32 1.92 40.8460–80 8.41 3.13 6.66 1.97 93.12 119.36 9.27 2.42 41.54

CSG 0–20 8.08 8.31 7.52 2.07 55.31 104.51 8.60 5.64 31.7820–40 8.19 3.90 9.72 2.26 86.79 107.19 9.07 4.98 41.7240–60 8.29 2.81 6.53 2.08 66.17 101.48 9.07 4.61 36.4960–80 8.36 3.46 6.62 1.63 55.01 99.54 8.85 1.08 33.44

CKF 0–20 8.74 21.10 9.27 2.26 102.94 137.58 10.41 11.51 39.2220–40 8.57 6.26 9.70 2.30 108.71 113.28 9.94 11.13 44.7340–60 8.44 3.80 3.60 1.34 47.19 105.28 8.72 5.02 28.3960–80 8.43 3.57 2.64 1.25 30.92 100.17 8.38 3.11 21.69

CSA 0–20 8.72 7.74 6.48 1.73 70.45 105.73 9.83 10.56 35.8420–40 8.20 2.87 4.54 1.90 48.29 112.17 9.36 8.64 27.0640–60 8.21 3.01 4.24 1.56 44.41 105.83 9.11 2.96 27.3660–80 8.22 3.38 3.04 1.43 41.25 104.15 9.01 2.82 26.24

CDF 0–20 8.12 2.29 8.61 2.72 44.36 117.88 9.42 9.09 24.5420–40 8.10 2.25 7.98 2.83 50.61 118.99 9.57 6.34 27.2840–60 8.08 2.26 6.34 2.43 50.08 118.44 9.53 1.91 27.8360–80 8.08 2.25 3.90 2.18 53.27 116.48 9.23 1.60 29.50

WFD 0–20 7.94 2.27 11.59 2.63 20.61 123.31 9.00 2.86 13.2320–40 7.94 2.32 10.57 3.37 23.09 122.70 8.95 2.94 14.6440–60 7.96 2.30 7.48 2.41 17.43 123.93 9.04 2.76 11.3860–80 7.95 2.35 6.70 2.22 34.16 129.10 9.43 4.08 19.32

672 Soil Research J. Jiang et al.

that of the other three layers within the four native halophytesoil profiles, but it varied only slightly within the two furrow-irrigated soil profiles. Soil pH was similar in the nativehalophyte soils but it was lower in the furrow-irrigated fields.The TOC varied appreciably within the soil profiles, andthere was no trend with depth, except that TOC decreasedwith depth in the furrow-irrigated fields. These variationscould be attributed to different interactions betweenvegetation and sediment deposition during the developmentof these halophyte community soils (Dodla et al. 2008). TheTN in the furrow-irrigated fields was much higher than in thenative halophyte soils. Fertilisation not only resulted in anenrichment of nitrogen, it also improved microbiologicalactivities and stimulated decomposition of organic debris(Table 2). Exchangeable Ca2+ was approximately similar inthe four soil profiles. Trends of exchangeable Mg2+ and K+

were similar to that of exchangeable Ca2+. The CSE soil profilehad the highest average value of exchangeable Na+, followedby CKF, CSG, CSA, CFD, and WFD. Mean ESP was highest inthe CSE soil, followed by CSG, CKF, CSA, CFD, and WFD,and there was obvious variation in ESP within a given profile.

Elemental analysis of fulvic acid

The element content and atomic ratios of the FA (Table 2) weretypical of this humic fraction according to the C/N ratio (Senesiet al. 1996). The measured O/C ratios were in the upper range ofFA (Senesi et al. 1996; McKnight et al. 1997; Peuravuori andPihlaja 1997). This was possibly related to the young age of the

dissolved humic substances (Geyer et al. 1998). The FAfrom the furrow-irrigated soils contained more nitrogen andexhibited a lower C/N ratio than the FA from the nativehalophyte soils. Long-term tillage resulted in deeplydecomposed plant residues and soil organic matter frommicrobiological activities, and degraded molecules becamereorganised to form more condensed compounds, richer inaromatic components. Furthermore, fertilisation stimulatedresidue decomposition, which included a relative enrichmentof nitrogen, while nitrogen addition further reduced the C/Nratio (Yu et al. 2011b).

The C/N ratio indicated the aromaticity and the level ofdecomposition of organic material (Stevenson 1994; Rosa et al.2005). Humification was linked to enrichment in aromaticstructures and the content of lignin-derived phenols byenhanced humification levels (Guggenberger et al. 1994;Zech et al. 1997). Therefore, the C/N ratio may be bettersuited as a humification index (HIX) to evaluate thehumification degree of soil organic matter, and humificationdegree increased with decreasing C/N ratio (Stevenson 1994).The C/N ratio of FA from WFD soils showed the lowest mean,followed by CFD, CSA, CKF, CSG, and CSE (Table 2). Thisindicates that the humification degree of FA from the WFD soilswas the highest, followed by CFD, CSA, CKF, CSG, and CSE.

UV–visible absorption properties of FA

The UV–visible absorption spectra for all FA samples increasedapproximately exponentially with decreasing wavelength

Table 2. Elemental composition and atomic ratios of dissolved fulvic acids extracted from soils in the Hetao Irrigation District (6 sites� 4 soillayers = 24 soil samples and each soil sample in triplicate)

Communities: CSE, Salicornia europaea; CSG, Suaeda glauca; CKF,Kalidium foliatum; CSA, Sophora alopecuroides. Fields: CFD, corn fields; WFD, wheatfields

Samples Depth Ash C N H O C/N C/H C/O(cm) (%) (atomic ratio)

CSE 0–20 1.69 42.6 1.4 4.4 51.6 34.9 ± 3.4 0.8 ± 0.04 1.1 ± 0.0920–40 1.38 42.8 1.4 4.0 51.8 36.2 ± 2.6 0.9 ± 0.03 1.1 ± 0.0440–60 0.41 40.4 1.5 4.2 53.9 31.3 ± 2.1 0.8 ± 0.01 1.0 ± 0.1260–80 0.13 39.1 1.4 4.1 55.4 33.5 ± 1.9 0.8 ± 0.10 0.9 ± 0.05

CSG 0–20 1.98 42.7 1.6 3.9 51.8 31.3 ± 2.4 0.9 ± 0.01 1.1 ± 0.1420–40 1.87 42.5 1.4 4.4 51.7 34.8 ± 2.8 0.8 ± 0.02 1.1 ± 0.3140–60 0.72 40.4 1.5 4.2 53.9 31.2 ± 2.1 0.8 ± 0.03 1.0 ± 0.0460–80 0.24 42.5 1.6 4.4 51.5 30.9 ± 2.5 0.8 ± 0.05 1.1 ± 0.07

CKF 0–20 0.28 42.4 1.8 4.4 51.4 27.4 ± 2.0 0.8 ± 0.04 1.1 ± 0.0420–40 0.77 42.6 1.6 3.9 51.9 30.5 ± 1.6 0.9 ± 0.04 1.1 ± 0.0840–60 2.76 40.4 1.8 4.2 53.6 26.9 ± 1.1 0.8 ± 0.03 1.0 ± 0.2060–80 0.41 42.6 1.8 3.9 51.7 27.5 ± 2.5 0.9 ± 0.05 1.1 ± 0.34

CSA 0–20 0.97 39.1 1.9 3.6 55.4 23.5 ± 1.0 0.9 ± 0.01 0.9 ± 0.0620–40 0.51 40.5 1.8 3.7 54.0 26.1 ± 1.7 0.9 ± 0.06 1.0 ± 0.0340–60 2.01 42.6 1.9 3.9 51.6 25.6 ± 1.6 0.9 ± 0.04 1.1 ± 0.1660–80 0.05 40.3 1.8 4.2 53.7 26.1 ± 2.4 0.8 ± 0.03 1.0 ± 0.06

CFD 0–20 0.99 44.4 2.4 3.7 49.5 21.4 ± 0.9 1.0 ± 0.07 1.2 ± 0.0720–40 0.14 42.8 2.2 3.0 52.0 22.7 ± 2.2 1.2 ± 0.02 1.1 ± 0.0840–60 1.27 42.2 2.2 4.4 51.2 22.1 ± 2.5 0.8 ± 0.05 1.1 ± 0.0460–80 5.01 39.2 2.2 4.1 54.5 20.7 ± 1.4 0.8 ± 0.05 1.0 ± 0.04

WFD 0–20 0.88 44.4 2.9 3.4 49.3 17.7 ± 1.2 1.1 ± 0.10 1.2 ± 0.0620–40 2.18 42.3 2.6 3.9 51.2 19.3 ± 1.4 0.9 ± 0.04 1.1 ± 0.0540–60 1.26 42.1 2.4 3.2 52.3 20.7 ± 1.8 1.1 ± 0.08 1.1 ± 0.0660–80 1.22 42.7 2.1 3.6 51.6 23.4 ± 2.3 1.0 ± 0.06 1.1 ± 0.06

Spectroscopic properties of dissolved fulvic acids Soil Research 673

(Twardowski et al. 2004). There were five importantwavelengths in the spectra where exact absorbance valueswere measured: l= 285, 240, 250, 365, and 420 nm(Fig. 2a–e). The absorbance at 240, 250, and 285 nm couldbe due to lignin and quinone moieties, i.e. the material at the verybeginning of transformation (Zbytniewski and Buszewski2005), while the absorbance at 365 and 420 nm might reflectthe organic material at the beginning of humification(Gieguzynska et al. 1998). Absorption at 285 nm (absorptionper mass unit dissolved organic carbon), known as a specific UVabsorbance (SUVA), was normalised to the dissolved organiccarbon concentration, and this has been widely used as asurrogate measurement for the abundance of aromatic carbonin dissolved humus (Summers et al. 1987; Kalbitz et al. 1999;Weishaar et al. 2003) (Fig. 2a). SUVA showed a strong

negative correlation with the C/N ratio, indicating thatSUVA could be used as a general tracer of humification toevaluate the humification degree of FA. The humificationdegree increased with a rise in SUVA. The FA from WFDsoils contained the highest mean SUVA values, followedby CFD, CSA, CKF, CSG, and CSE (Table 3). This resultindicated that the humification degree of FA from the WFDsoils was the highest, followed by CFD, CSA, CKF, CSG, andCSE.

As the light absorption molecules of dissolved humus reachhigher levels of aromaticity, the absorption spectra tend to shifttowards longer wavelengths. Therefore, the absorbance ratios of250 and 365 nm (E2/E3; see Fig. 2b, d) and of 240 and 420 nm(E2/E4, see Fig. 2c, e), which were largely independent of theconcentration of dissolved organic carbon, were correlated with

Abs

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Fig. 2. Means and standard deviations of absorbance values at the wavelengths of (a) 285 nm, (b) 240 nm,(c) 250 nm, (d) 365 nm, and (e) 420 nm of fulvic acids extracted from the soils in the Hetao Irrigation District.Communities: CSE, Salicornia europaea; CSG, Suaeda glauca; CKF, Kalidium foliatum; CSA, Sophoraalopecuroides. Fields: CFD, corn fields; WFD, wheat fields.

674 Soil Research J. Jiang et al.

the degree of condensation and the molecular weight of theorganic materials (De Haan and De Boer 1987; Piccolo et al.1992; Chin et al. 1994; Plaza et al. 2003). Ratios E2/E3 and E2/E4

can be validated through their correlations with C/N (Table 4).They were widely used for characterisation of humificationdegree, but there are some controversies in the literature.Chen et al. (1977) reported that E2/E3 and E2/E4 were mainlycontrolled by the size of humus; however, studies of Baes andBloom (1990) showed that humus did not exhibit the propertiesof light scattering, and therefore, the ratios were not controlledby molecular size. Thus, these ratios showed conflictingresults and limitations, suggesting caution with their use.Since the ratios are often used in the literature to characterisethe humification degree of soil organic matter (Helms 2006;Morán Vieyra et al. 2009; Yu et al. 2010), are made available inthis study.

The ratios E2/E3 and E2/E4 of FA from WFD soils had thehighest average values, followed by CSE, CSG, CKF, CSA,

Table 3. HIXs deduced from the UV–visible spectra of fulvic acids from soils in the Hetao Irrigation District (6 sites� 4 soil layers = 24 soil samplesand each sample with two replicates)

Communities: CSE, Salicornia europaea; CSG, Suaeda glauca; CKF,Kalidium foliatum; CSA, Sophora alopecuroides. Fields: CFD, corn fields; WFD, wheatfields

Samples Depth SUVA E2/E3 E2/E4 S275–295 S350–400 S250–400

CSE 0–20 5.88 8.40 34.00 0.020 0.021 0.02020–40 4.79 7.75 26.18 0.019 0.019 0.02040–60 4.93 9.29 44.50 0.020 0.030 0.02260–80 4.68 11.73 32.00 0.019 0.020 0.024

CSG 0–20 5.11 6.52 21.67 0.016 0.020 0.02020–40 6.20 8.55 37.17 0.019 0.023 0.01740–60 6.83 6.73 22.75 0.017 0.019 0.01760–80 6.40 5.75 16.00 0.017 0.015 0.016

CKF 0–20 5.39 7.09 31.75 0.015 0.018 0.01820–40 6.19 7.97 15.91 0.018 0.021 0.01840–60 7.45 6.67 24.43 0.015 0.022 0.01860–80 6.04 5.50 14.17 0.017 0.013 0.015

CSA 0–20 7.73 9.23 33.72 0.018 0.024 0.02120–40 8.74 5.50 33.00 0.019 0.017 0.01740–60 6.24 5.25 10.60 0.015 0.014 0.01560–80 7.56 5.67 10.60 0.015 0.015 0.013

CFD 0–20 10.74 4.20 8.83 0.016 0.014 0.01220–40 9.05 5.36 38.00 0.015 0.017 0.01740–60 8.75 8.19 28.00 0.016 0.014 0.01660–80 7.45 6.00 10.00 0.017 0.019 0.015

WFD 0–20 13.65 4.82 14.40 0.013 0.015 0.01420–40 12.48 6.25 24.33 0.013 0.020 0.01640–60 12.47 4.25 6.16 0.013 0.014 0.01460–80 9.41 6.07 18.63 0.017 0.015 0.016

Table 4. Pearson rank-order correlations between the seven HIXs deduced from elemental analysis and UV–visible spectroscopy of FA*P< 0.05 (2-tailed); **P < 0.01 (2-tailed); listwise n= 24

HIX C/N SUVA E2/E3 E2/E4 S275–295 S350–400 S250–400

C/N 1 –0.843** 0.592** 0.426* 0.751** 0.466* 0.616**SUVA 1 –0.594** –0.374 –0.684** –0.428* –0.620**E2/E3 1 0.667** 0.685** 0.692** 0.860**E2/E4 1 0.569** 0.708** 0.757**S275–295 1 0.573** 0.660**S350–400 1 0.775**S250–400 1

6

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In a

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Fig. 3. Natural log-transformed absorption coefficient correction spectra offulvic acids extracted from selected soil samples in the Hetao IrrigationDistrict.

Spectroscopic properties of dissolved fulvic acids Soil Research 675

CFD, and WFD (Table 3), indicating that the humificationdegree of FA from the WFD soils was the highest, followedby CFD, CSA, CKF, CSG, and CSE. This result is consistentwith the analysis of C/N and SUVA.

Although the E2/E3 absorbance ratio relies on two fixedwavelengths only, the absorption spectral slope (S, nm–1), ameasure of the rate at which absorbance decreases withwavelength, may reflect expanded spectroscopic information

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0.022

0.018

0.020

0.016

0.014

0.012

0.022

0.018

0.020

0.016

0.014

0.012

10 20

y = 0.449x + 13.214R 2 = 0.695P = 0.0004x = (11.38, 44.81)

30 40 50 10 20 30 40 50

10 20 30 40 50

10 20 30

ESP (%)40 50

10 20 30

ESP (%)40 50

10 20 30 40 50

10 20 30 40 50

y = 0.134x + 2.691R 2 = 0.545P = 0.0004x = (11.38, 44.81)

y = 0.0002x + 0.0117R 2 = 0.620P = 0.0005x = (11.38, 44.81)

y = 0.0002x + 0.0100R 2 = 0.498P = 0.0001x = (11.38, 44.81)

y = 0.0003x + 0.0104R 2 = 0.391P = 0.0001x = (11.38, 44.81)

y = 0.594x + 5.087R 2 = 0.545P = 0.0004x = (11.38, 44.81)

y = –0.216x + 14.253R 2 = 0.707P = 0.0003x = (11.38, 44.81)

Fig. 4. Correlations of ESP with (a) C/N, (b) SUVA, (c) E2/E3, (d) E2/E4, (e) S275–295, (f) S350–400, and (g) S250–400determined by elemental composition and UV–visible spectroscopy of fulvic acids extracted from soils in the HetaoIrrigation District.

676 Soil Research J. Jiang et al.

over a broad wavelength range. ‘S’ has been proposed toestimate the molecular weight, composition, source, anddegree of condensation of dissolved organic matter (Carderet al. 1989; Twardowski et al. 2004; Spencer et al. 2007).The following expression is used to determine S:

al ¼ alref e�Sðl�lref Þ ð2Þ

where a is Napierian absorption coefficient, l is wavelength(nm), and lref is reference wavelength (nm) (Twardowski et al.2004).

Absorbance units are converted to absorption coefficients asfollows:

a ¼ 2:303Al

ð3Þ

where a is absorption coefficient, A is absorbance, and l is pathlength (m). The aims of such calculations include reductionsin the instrument baseline drift, temperature, scattering, andrefractive effects.

Higher (or steeper) slopes indicate a more rapid decreasein absorption with increasing wavelength. The ranges 275–295,350–400, and 250–400 nm were chosen because the firstderivatives of the natural-log spectra indicated that thegreatest variations in S from a variety of samples (soil,marsh, riverine, estuarine, and coastal) occurred within thenarrow bands of 275–295 nm (S275–295), 350–400 nm(S350–400), and 250–400 nm (S250–400) (Helms 2006). Thespectral slopes were determined by fitting the absorptionspectra to a single exponential decay function (Eqn 2) bynonlinear regression (Sigma Plot) and were calculated usinglinear regression of the natural log-transformed spectra (Fig. 3).

Variation between the log-transformed linear regression and thenonlinear regression was <1%.

There was a significant positive correlation between S275–295and C/N; this was also true for S350–400 and S250–400 (Table 4). Itwas concluded that the three slopes could be used as HIXs toevaluate the humification degree of FA. Decreases in the slopeindicated a higher degree of humification. The mean of S275–295was lowest within the WFD soil profile, followed by CFD, CSA,CKF, CSG, and CSE; this was also true for S350–400 and S250–400(Table 3). The results showed the highest humification degree ofFAwithin theWFD profile, followed by CFD, CSA, CKF, CSG,and CSE.

Relationship between HIX and ESP

The ESP showed a significant negative linear correlation withSUVA and positive linear correlations with C/N, E2/E3, E2/E4,S275–295, S350–400, and S250–400 (Fig. 4), indicating that thehumification degree of FA increased with decreasing ESP. Inthe differentiation of salinity and sodicity, ESP is an importantparameter. Not only can high ESP deteriorate soil chemico-physical properties, it can also suppress microbial activity(enzyme activity, carbon mineralisation, ammonification,nitrification, etc.) (Rietz and Haynes 2003; Yuan et al. 2007;Wong and Dalal 2008). Because microbial activity can promotehumus formation (Martin and Haider; 1971), ESP suppresses thehumification processes of soil organic matter.

Variations in ESP between different soil profiles wereobserved, but not between soil depths (Table 1). Furthermore,the values of HIX within a given soil profile showed obviousvariation too, and no trend with soil depth (Table 3). This was incontrast to an investigation reported by Ussiri and Johnson(2003), where increases in carbonyl, alkyl, and aromatic

100

90

80

70

60

50

40

30

Humification indices (HIXs)

HIX

Rel

ativ

e (%

); re

lativ

e m

axim

umva

lue

bein

g ad

just

ed to

100

20

10

C/N SUVA E2/E3 E2/E4 S275–295 S350–400 S250–400

Fig. 5. The C/N, SUVA, E2/E3, E2/E4, S275–295, S350–400, and S250–400 of fulvic acids from the soils shownover a relative scale (respective maximum value being adjusted to 100). Communities: CSE, Salicorniaeuropaea; CSG, Suaeda glauca; CKF, Kalidium foliatum; CSA, Sophora alopecuroides. Fields: CFD, cornfields; WFD, wheat fields. Soil layers: 1, 0–20 cm; 2, 20–40 cm; 3, 40–60 cm; 4, 60–80 cm.

Spectroscopic properties of dissolved fulvic acids Soil Research 677

carbon with soil depth within forest soil profiles were observed.These behaviours indicated that ESP had more distinct effectson humification degree of FA extracted from the saline soilsthan other factors such as source materials and transport oforganic matter. It is difficult to measure the ESP values ofsaline soil with an abundance of calcium carbonate and/orcalcium sulfate in arid and semi-arid regions. Therefore, theHIX be used as a surrogate for ESP to indicate soil salinisationprocesses.

HIX correlation and comparison of the methods used

Pearson rank-order correlations between C/N, SUVA, E2/E3, E2/E4, S275–295, S350–400, and S250–400 of FA from the saline soils areshown in Table 4. There were good correlations between theseven HIXs (P< 0.05), except for the relationship betweenSUVA and E2/E4. The S275–295 was closely correlated withthe other six HIXs (P < 0.01), as was S250–400. The HIX cancharacterise not only the aromaticity of FA but also thehumification degree.

For a distinct comparison of the methods used, the sevenHIXs were normalised according to the method of Kalbitz et al.(1999). The formula of normalisation was defined as:

HIXR ¼ HIXO � 100HIXMax

ð4Þ

where HIXO is the original value within a humification, HIXMax

is the maximum within the humification, and HIXR is the treatedvalue.

The results of the various methods are summarised in Fig. 5.The elemental analysis and UV–visible spectra were useful todistinguish between all of the FA originating from the salinesoils. However, C/N, SUVA, and S350–400 could more distinctlydifferentiate the humification degrees of FA between the tilledsoils and the native soils than the other HIXs (Fig. 5). Thesemethods were rapid, inexpensive, and accessible to a lot oflaboratories. Therefore, not only can HIXs evaluate thehumification degree of FA from saline soils, but especiallyC/N, SUVA, and S350–400 can also be indicators of thesalinisation process.

Conclusions

The combination of methods used in the present work clearlyshowed the different levels of aromaticity and humificationdegree of FA under the environmental conditions (bothanthropogenic and natural) of saline soil with a relatively lowproportion of organic matter. The HIXs, deduced from theelemental analysis and UV–visible spectra of FA, can beused as tools for evaluating the humification degree and canalso be a surrogate for ESP to indicate soil salinisation processes.The obtained results enable the monitoring of saline soilremediation; it is assumed that this methodology can beapplied to other soil types from different landscapes.

Acknowledgments

This work was financially supported by Chinese National High-techResearch and Development Projects (Program no. 2008AA10Z406) andChinese National Water Special Projects (Program no. 2008ZX07207-004).

We express our sincere thanks to the anonymous reviewers. Their insightfulcomments were helpful for improving the manuscript.

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