Origin and Factors Effecting Dissolved Organic Matter in Karst Spring, Southwest China

doi:10.21203/rs.3.rs-644094/v1

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Origin and Factors Effecting Dissolved Organic Matter in Karst Spring, Southwest China

https://doi.org/10.21203/rs.3.rs-644094/v1

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The dissolved biomarker composition of water samples was detected and quantified through gas chromatography-mass spectrometry to investigate the source of organic matter and its influencing factors in a subtropical monsoon karst spring. All water samples were collected in Chongqing, southwest China, in May, June, and July 2015. An analysis of the carbon number distribution characteristics and molecular characteristic parameters indicated that the dissolved organic matter in water samples mainly came from microorganism. The average biomarker contents in the samples were controlled by multi-factor coupling and showed an increasing trend in May, June, and July; this trend was consistent with temperature and inverse with rainfall. Human activities, such as urbanization and land use type, affected the fatty acid composition of the spring water. In forested land, the main fatty acids in water were saturated normal fatty acids with carbon numbers of 14,16, and 18, respectively. The C16:1ω9 (c) fatty acid, denoting the origin of algae, was found in high abundance in the water samples of the Mudu River Basin, which contains agricultural land far from urban areas. C18:1 ω9 (t), a biomarker of Gram-negative bacteria, was detected in large quantities in the cultivated land of the Laolongdong watershed.

Environmental Engineering

Karst spring

Dissolved organic matter

Urbanization

Land-use type

Biomarker

N-alkane

Fatty acid

Organic matter consists of a complex series of mixtures that are widely distributed in sedimentary strata , soil (Zhou et al. 2021), water (Pan et al. 2017, Valle et al. 2020), atmosphere (Kim et al. 2013), and organisms. These organisms actively participate in the global material cycle and energy transfer processes. Organic matter is a vital piece of evidence of biological participation in this process (Amundson et al. 2007, Lin 2010). Endogenous organic matter, synthesized by aquatic organisms by immobilizing inorganic carbon, can form a stable carbon sink after burial. This organic matter is considered an important omission sink in studies of the carbon cycle (Liu et al. 2010, Liu et al. 2018). As a component of the global carbon cycle, dissolved organic matter (DOM) plays a crucial role in a variety of physical, chemical, and biological processes in aquatic ecosystems (Hansell &Carlson 2002) . Because of the complexity of its composition, research on DOM is relatively rare. However, n-alkanes, fatty acids, and other biomarkers are known as biological fingerprints because of their extensive existence in the environment, high stability, and the ability to indicate the source of organic matter (Cranwell et al. 1987, Meyers 1994, Wang et al. 2016).

According to 2019 statistics, the total population of the world has exceeded 7.713 billion. The growing number of human beings has brought about an increase in total resource consumption and reduced the planet’s environmental carrying capacity (Grimm et al. 2008). In the process of pursuing economic development, unreasonable human behavior directly or indirectly affects the environment. The three wastes (waste gas, wastewater, and industrial residue) from human activities (Hays et al. 2005, Li et al. 2019, Ren et al. 2018) lead to a series of problems, such as environmental pollution (Pruden et al. 2006) and global warming.

The karst mountains in southwest China, which are the most widespread among the three major karst concentrated distribution areas in the world, account for approximately 83.5% of the bare karst area in China (Feng et al. 2016). Water in this area is often used as the main drinking water source for residents (Li et al. 2020). Many sinkholes, tunnels, shafts, and grikes have been developed in the lithosphere, and the overlying soil layer has a strong permeability and discontinuous coverage (Sun et al. 2020). The karst water environment is fragile, sensitive, and vulnerable to human activities (Buckerfield et al. 2020). Human activities have a profound impact on the earth's environment (Wu et al. 2018), and changes in the global environment will also harm human beings (Alexander et al. 2006, Liao et al. 2019, Westra et al. 2013). In this study, we reveal the source of DOM in the karst water environment and the main factors affecting its origin, verify the influence of human activities on the karst water environment, and provide technical support for related studies on the carbon cycle and water resource security in karst areas by analyzing the component characteristics of biomarkers in the samples.

Study area

As one of the important karst areas in China, Chongqing is an important strategic fulcrum of the nation’s western development and an important joint point of "One Belt and One Road" (Bao et al. 2019) and the "Yangtze River Economic Belt" (Lim &Horesh 2017, Zhang et al. 2010) Chongqing has a humid subtropical monsoon climate, characterized by warm winters, hot summers, minimal frost and snow, heavy fog, and less than normal sunshine. The average annual rainfall is 1125.3 mm, which is abundant but not evenly distributed. Rainfall is mainly concentrated from April to October. However, in July and August, owing to high pressure systems, the precipitation in this region is relatively low. The average annual temperature is 18.7 °C, the vegetation is mainly subtropical evergreen broad-leaved forest, and the soil is typical zonal soil (yellow soil).

Population

According to statistics (STATISTICS 2020), the total population of the Chongqing area in 1952 was 17.83 million. Since the establishment of Chongqing municipality in 1997, the permanent population has increased from 30.23 million to 34.04 million in 2018. The overall urbanization rate of the city ranged from 29.25% in 1996 to 65.5% in 2018.

The population of Nan'an District, one of the main urban areas, increased from 0.69 million to 0.89 million in 2007, and the urbanization rate has remained at around 95%. The population of Nanchuan County increased from 0.23 in 2007 to 0.38 in 2018. The city has developed rapidly, with the urbanization rate rising rapidly from 32.3% to 62.35%. The city size of Chongqing has expanded rapidly, and a city scene has formed with a developed industry and dense population. Because of the unique geological structural characteristics of the karst region, groundwater, as an important water resource in Chongqing, is easily affected by industry, agriculture, and residents. This leads to a prominent juxtaposition between human activities and water resources. Reports (He et al. 2016) indicate that underground rivers in Chongqing have become seriously polluted, and some of them effectively act as urban sewage pipes.

Land-use type

The Laolongdong underground river is located in Huangjiaoya Town, Nanan District, Chongqing City, which has a relatively high degree of urbanization. The total length of the underground river is approximately 6 km, and the drainage area is approximately 12.6 km², of which self-established residences in village land constitute approximately 6.5 km², the quarry area spans approximately 2.5 km^2, the agricultural land constitutes approximately 2 km², and the rest comprises forests and the construction site. There is an open-air sewer with the same flow direction as the Laolongdong underground river located in the drainage basin. The sewer began in Huangjiaoya town and receives a large amount of domestic sewage from villages and schools along its path.

Zhaojiayuanzi (ZJYZ) Spring belongs to the Laolongdong underground river basin; this spring is exposed in the Triassic strata cracks and overflows to the surface. Xiajiazui (XJZ) Spring is located in a karst depression and it is close to the open sewer. Surrounding the two sites are several vegetable plots and cornfields, and spring water is mainly used for irrigation and cleaning.

Lan huagou (LHG) Spring and Hougou (HG) Spring belong to the Mudu River Basin, located in Nanping Town, Nanchuan District, Chongqing. Nanchuan District, where the basin is located, is a suburban county far from Chongqing’s main city center. The LHG spring is mainly surrounded by agricultural land. During the sampling period, rice, vegetables, and corn were the main crops in this area. The land use of the HG spring water is mainly agriculture, grasslands were also found near the sampling sites. The soil was cultivated using a crop rotation of maize and rape. The northernmost part of the HG contains Nanshan Botanical Park, which collects plant species from subtropical low mountains in China and carries out plant preservation, collection, and cultivation.

Baishuwan (BSW) Spring is located in Dongsheng Commune, Nanchuan District, Chongqing City. The spring’s water flows mainly to Baizhi Creek. The spring area, containing low mountain landforms, constitutes forested land that was converted from grain plots 20 years ago; as a result, no agricultural activities are present.

Sampling process

In May, June, and July 2015, we collected water samples at -10 cm from the water’s surface at the four sampling sites using brown glass bottles with a 4 L capacity and helical cover with polytetrafluoroethylene lining. During sampling, bubbles were avoided in the bottles. When the water sample was filled to a meniscus on the mouth of the bottle, 1 mL (5 g ·L^− 1) of HgCl₂ was added to inhibit the growth of microorganisms in the water. After screwing the cap of the bottle, the bottle was turned over and inspected to ensure that there was no error. We then sent the sample to the laboratory to be measured as soon as possible. Samples were numbered according to the format of the sampling site plus the sampling month. Consider the sample of ZJYZ in May as an example: the sample was named ZJYZ₅.

Experiments and materials

The n-hexane, dichloromethane, and methanol used in the analysis were all chromatographically pure and purchased from Fisher Company in the United States. A glass fiber membrane (Whatman GF/F, aperture 0.7 µm) was burned for 6 h in a muffle furnace at 450°C. Anhydrous sodium sulfate (analytically pure) was burned for 8 h in a muffle furnace at 550°C, and then cooled and sealed in a dryer for later use. After 72 h of extraction with dichloromethane, the absorbent cotton was air-dried, sealed, and further dried for later use. Hydrochloric acid was extracted six times using dichloromethane for backup. HEXADECANOIC-D31 ACID, a type of surrogate standard, was purchased from Dr. Ehrnstorfer (Germany). The internal standard N-TETRACOSANE-D50 (C24D50) was purchased from SUPELCO, USA.

Sample pre-treatment

Four L of a water sample, filtered by the glass fiber filter membrane with an effective pore diameter of 0.7 µm, was placed into a brown bottle and acidified with purified hydrochloric acid to pH < 2. The water samples were extracted using an automatic phase extraction system (SPE-DEX 4790, purchased from Horizon, USA). A 47 mm diameter C18 membrane purchased from 3M was used for the extraction process. The extracting solution was collected in a 50 mL specimen bottle and then saponified and methylated. The n-alkanes and fatty acids in the solution were extracted using n-hexane. The extract solution was dehydrated and concentrated and blown to 0.2 mL with nitrogen. Next, 2 µL of internal standard was added and then placed in a refrigerator at -26°C for testing.

Lipid biomarkers

The molecular characteristics of n-alkanes, such as the peak type of carbon number distribution, the carbon number at which the n-alkane distribution maximizes (Cmax), and the carbon preference index (CPI), can be used to comprehensively determine the main source of organic matter (Table 1) (Bray &Evans 1961, Marzi et al. 1993, Rogge et al. 2007).

CPI = 1/2 (∑odd_(C25−33)/ ∑even_(C25−32)+ ∑odd_(C25−33)/ ∑even_(C26−34)) (a)

Because samples often do not contain all the alkanes from C24 through C34, the weight percentages of a smaller number of compounds are often used in computing CPI. As a result, other ranges of n-alkanes may be used to determine whether there is an odd-to-even predominance (OEP) (Scalan &Smith 1970).

OEP=(C25 + 6C27 + C29)/4(C26 + C28) (b)

Table 1

Examples of hydrocarbon biomarkers
Source	Distrabution	CPI	Cmax	References
terrestrial vascular plants	23 to 35		27, 29, or 31	(Cranwell 1983, Rogge et al. 2007)
submerged, floating, and man-sized emergent aquatic plants	21 to 25	3.49 to 17.25	23 or 25	(Cranwell 1983, Eglinton &Calvin 1967, Ficken et al. 2000)
Algae	14 to C32	close to 1	15, 17, or 19.	(Cranwell 1983)
bacteria	C12 to C24	< 1		(Grimalt &Albaiges 1987, Nishimura &Baker 1986)
Petroleum fuel	nC12 to nC33	close to 1	22	(Fowler 1995, Hakimi et al. 2012)

Detection of lipids

The samples were analyzed using GC-MS (7890A/5975C, Agilent, Inc., US). The chromatographic column was an HP-5MS capillary column with dimensions of 30.0 m×0.32 mm×0.25 µm. The carrier gas was high-purity helium, the flow rate was 1 mL·min^− 1, and the inlet temperature was 280°C. The heating procedure was as follows: the initial temperature was 50°C, which was maintained for 1 min. The temperature was then increased to 200°C at 20°C ·min^− 1, then further raised to 290°C at a rate of 10°C ·min^− 1, and the latter temperature was maintained for 15 min. A splitless injection with an injected quantity of 1 µl was performed. The electronic ionization source was 70 eV and the ion source temperature was 230°C. All scan data were collected and quantitatively analyzed using the internal standard method. The target compounds were characterized using the NIST2011 standard.

Temporal variations in n-alkanes and fatty acids

We counted and analyzed the samples for n-alkanes and fatty acids (Table 2). The TAL samples ranged from 83.76 ng·L^− 1 to 1596 ng·L^− 1, with an average value of 329 ng·L^− 1. The minimum value of TFA in all samples was 562 ng·L^− 1, with a maximum value of 32091 ng·L^− 1 and a mean of 7530 ng·L^− 1. The coefficients of variation of TAL and TFA were 1.23 and 1.24, respectively, which indicated that there were differences in organic matter input amount among samples. The average TAL and TFA in May, June, and July were 209 ng·L^− 1, 223 ng·L^− 1, and 518 ng·L^− 1 and 10003 ng·L^− 1, 3862 ng·L^− 1, and 8726 ng·L^− 1, respectively. During the three-month study period, the TAL and TFA for the sites showed an increasing trend.

Table 2

Total concentration of n-alkane and fatty acid for each sample (ng·L^− 1).
		ZJYZ	XJZ	HQ	LHG	BSW	AVERAGE
n alkane	May	148	4308	189	119	159	209
	Jun.	85.99	95.49	114	722	99	223
	Jul.	533	83.76	273	1596	106	518
	av	255	203	192	812	122	317
	C.V	0.95	0.97	0.41	0.91	0.27
fatty acid	May	2601	32091	1635	2983	10709	10004
	Jun.	865	1783	4982	9182	2496	3862
	Jul.	2110	562	14793	25436	727	8726
	av	1859	11479	7137	12534	4644	7530
	C.V	0.48	1.56	0.96	0.93	1.15

A change in weather conditions may be one of the main reasons for the change in dom content in spring water. The perennial average temperatures in the three months were 21.63°C, 24.07°C, and 27.71°C, respectively (Fig. 2). The increase in the average concentration was consistent with the increasing trend of the temperature. The enhancement of biological activity caused by the increase in temperature within a certain range may led to an increase in organic matter content in water (Xu et al. 2016). Therefore, temperature increases may be one of the factors driving the increase in organic matter content. The variation trend of the composition content for a specific sampling site can be divided into two categories. ZJYZ, LHG, and HG (Table. 2) showed the same trend as the general, whereas the trends of XJZ and BSW stabilized after a sharp decrease in June, which indicated that other factors may also control the organic matter input of the water body.

Rainfall has an important effect on organic matter content in water (McDonough et al. 2020). The sharp increase in precipitation may have contributed to the decrease of biomarkers in XJZ and BSW in June. The perennial average rainfall in May, June, and July was 156 mm, 185 mm, and 166 mm, respectively (Fig. 2a). The precipitation of Chongqing in June 2015 totaled up to 225 mm (Fig. 2a, bright yellow part). This was the largest rainfall amount in 2015, accounting for 30% more than the totals of the other years. In contrast, the precipitation in May and July was close to the multi-year average values of 146 mm and 171 mm (Fig. 2a), respectively. Therefore, the increased rainfall diluted DOM, leading to a drop in lipids in the samples in June. Tiefenbacher and colleagues (2021) provide evidence that subsurface runoff is an important pathway for DOM leaching by Rainfall simulation experiments. The decrease in organic matter content in some samples in June may have been related to the explosion of rainfall. Rainfall gradually becomes increasingly common with the arrival of the monsoon. In antecedent precipitation, runoff generation and rainwater confluence may increase the content of organic matter in the water body by scouring and leaching the surface and subsurface soil. Subsequently, with the increase in rainfall, the DOM content decreased gradually owing to dilution.

The lipid contents of many samples increased in July, which was probably caused by the increased microbial population. Temperature and rainfall synergistically control the content of DOM in spring water. The increased temperature is the key factor in the increased number of organisms, and the sudden increase in rainfall reduces the DOM content in some months due to dilution.

Characteristics of n-alkane and fatty acid components

A total of 20 n-alkanes were detected in the samples, and their carbon numbers were distributed between 14 and 35. To determine the variation in the n-alkane ingredients, we divided all n-alkanes into three types: short-chain n-alkanes with carbon numbers ranging from 14 to 20 (nC14-20), medium-chain n-alkanes (nC21-25), and long-chain n-alkanes (nC26-35).

The component distribution of short-chain n-alkanes (Fig. 3a) ranged from 54.6–79.1%, with an average percentage value of 67.9% and a coefficient of variation of 0.29. medium-chain n-alkanes varied from 5.6–17.8%, with a mean ratio of 11.1% and a coefficient of variation of 0.91. Long-chain n-alkanes ranged from 14.5–29.4%, with an average value of 21.0% and a coefficient of variation of 0.74. The n-alkane components in each spring site obviously changed with time, and the content percentage of short-chain n-alkanes was dominant. The percentage of medium-chain and long-chain alkanes varied enormously, and they successively became the main structure in the samples from ZJYZ, LHG, and HG in certain months. For ZJYZ₇, LHG₆, and HG₅, the contents of short-chain n-alkanes were 10.2%, 42.9%, and 46.7%, respectively; the medium-chain amounts were 30.2%, 32.2%, and 0.95%, respectively; long-chain amounts comprised 59.6%, 24.9%, and 52.3%, respectively. This indicated that the dissolved organic carbon for these samples may originate differently from the other samples.

A total of 39 fatty acids were detected, and the carbon number distribution ranged from 10 to 28. Based on their structures, the fatty acids were divided into four types: saturated straight-chain fatty acids (SSFA), saturated branched-chain fatty acids (BRFA), monounsaturated fatty acids (MUFA), and polyunsaturated fatty acids (PUFA). The mass percentages of SSFA, BRFA, MUFA, and PUFA varied from 32.9%-94.9%, 0.2%-6.32%, 1.5%-48.2%, and 0.6%-18.6%, respectively, with mean ratios of 77.2%, 2.91%, 15.5%, and 3.63%, respectively (Fig. 3b). SSFA and BRFA showed relatively low mean coefficient of variations of 0.23 and 0.64, whereas MUFA and PUFA demonstrated higher means 1.02 and 1.67. This indicated that the primary fatty acid in the samples was SSFA, followed by MUFA, and BRFA and PUFA were lowest. The considerable fluctuations in the mass percentages of unsaturated fatty acids revealed that the dominant species in the study area showed remarkable differences in each month.

n-alkanes trace the source of organic matter

For the carbon distribution(Fig. 4), YJZ₇, LHG₆, and HG₅ were different from the other samples. HG₅ (Fig. 4a) and LHG₆ (Fig. 4b) had a bimodal distribution, whereas the others were unimodal. For the unimodal type, ZJYZ₇ (Fig. 4c)was dominated by long-chain n-alkanes, whereas the others were dominated by short-chain n-alkanes (Fig. 4d).

In this study, we used carbon number 20 as the boundary to compare the relative amounts of n-alkanes with low carbon numbers and high carbon numbers. The ratio of ∑C₂₀₊/∑C₂₀₋was 8.82, and CPI ₍₂₀₊₎ was 1.86 (Fig. 5b), with C_max 23 and C_max 29(Fig. 4c), which indicated that the DOM in ZJYZ₇ were mainly derived from a mixture of aquatic plants and woody plants.

The ∑C₂₀₊/∑C₂₀₋ values of HG₅ and LHG₆ were 1.33 and 1.14 (Fig. 5b), respectively, indicating approximately equal inputs of the low and high normal alkanes. The CPI ₍₂₀₊₎ of HG₅ and LHG₆ were 3.18 and 1.76 (Fig. 5b) with C_max 31 (Fig. 4a) and C_max 23 (Fig. 4b), respectively. The CPI ₍₂₀₋₎ was 0.22 and 0.61 (Fig. 5a), and C_max 16 (Fig. 4b and Fig. 4b). The carbon shape of an n-alkane is bimodal and the latter peak shows an obvious odd-carbon preponderance, the n-alkane in the sample is considered mixed input of microorganisms and vascular plants. LHG₆ was fed by a mixture of aquatic plants and microorganisms, whereas HG₅ was imported by a mixture of herbaceous plants and microorganisms.

Additional indicators are needed to determine the source of organic matter in these samples because their CPI ₍₂₀₊₎ values do not range from 3.49 to 17.25, which has been demonstrated as a characteristic of vascular plants (Bray &Evans 1961). Both petroleum and bacterial-derived n-alkanes can have a CPI value of less than 1.

Studies have indicated that the ratio of total n-alkane content to nC16 (Tnalk/C16) is less than 15 in the samples contaminated by petroleum(Fowler 1995, Hakimi et al. 2012), whereas the ratio of biogenic samples is more than 50. The OEP, representing the maturity of organic matter, is 1.0 to 1.2 in mature crude oil (PETERS &A., Scalan &Smith 1970). The CPI₍₂₀₊₎ of ZJYZ₇ and LHG₆ are 1.86 and 1.76 respectively, but in other indexes such as Cmax (Fig. 4), the Tnalk/C16 and OEP (Fig. 5b) values do not conform to the characteristics of n-alkanes in petroleum.

The CPI value of bacteria is usually less than 1, implying an even carbon preference in its n-alkane (Grimalt &Albaiges 1987, Kumar et al. 2020, Nishimura &Baker 1986). However, the carbon number of n-alkanes from bacteria is usually below 22, with a Cmax of 16 or 18. Moreover, although n-alkanes have been reported to contain a high carbon number in some bacteria species, but their carbon number distribution also shows an even carbon preference (Grimalt &Albaiges 1987). The molecular characteristics of the three samples didn’t match the molecular characteristics of n-alkanes derived directly from petroleum and bacteria.

The ∑C₂₀₋/∑C₂₀₊ ratio in the other samples ranged from 1.58 to 14.97, with an average value of 4.50. The main carbon numbers of the samples were 16. The average value of CPI₂₀₋, which ranged from 0.07 to 0.75, was 0.24, which indicated an even carbon number distribution. The molecular characteristic indicators indicate that the DOM in this part of the samples was mainly derived from microorganisms.

Fatty acids determine the microbial origin of dissolved organic matter

Fatty acids are important biological compounds that are widely present in the environment. Although palmitic acid and stearic acid are prevalent in the environment, fatty acids from diverse sources specifically in composition (Bhandari et al. 2021, Pondell &Canuel 2020, Wakeham &Canuel 2006). To determine the source of microorganisms in this research, we analyzed the molecular characteristic parameters of fatty acids in the samples. Saturated fatty acids are the main component of all samples (Fig. 3b), and fatty acids with 16 carbons are the predominant fatty acids in each sample (Fig. 7).

The values of ∑FAC₂₂₋/∑FAC₂₂₊ for the SFA varied from 22.17 to 1426 (Fig. 6a ), with a mean of 332. SSFAs are common in water and may originate from vascular plants and microorganisms such as algae and bacteria (Volkman et al. 1998); in general, fatty acids with a carbon number greater than 22 originate from terrestrial plants, and aquatic organisms and microorganisms produce fatty acids with carbon numbers less than 22 (Volkman et al. 2008). The CPI value of SSFA changed from 0.02 to 0.11, with a mean of 0.06, which demonstrated an even predominance. The ∑FAC₂₂₋/∑FAC₂₂₊ and CPI_{10 − 30} indicated that the organic matter of microbial origin contributed greatly to the samples (Fig. 6a ). The value of ∑FAC₂₂₋/∑FAC₂₂₊ for ZJYZ₇ was 22.17, which was much lower than that of the other samples. The ∑FAC₂₂₋/∑FAC₂₂₊ values of XJZ₆, XJZ₇, Hg₅, Hg₆, LHg₅, and BSW₇ were all lower than those in other months at the same sampling site. This is consistent with the above results of the n-alkanes. The TAR_FA ratio reflects the relative contribution of terrigenous to aquatic fatty acids ranging from 0.0003 to 0.0241, with an average value of only 0.0045; this indicated that the organic matter input mainly originated from endogenous organisms (Meyers 1997).

A bacterial fatty acid (BFA) input is usually represented by a combination of branched fatty acids, trans-fatty acids, odd-carbon number fatty acids, and nC14:1, nC16:1, nC18:1, etc. unsaturated fatty acids (Kaneda 1991, Mostofian et al. 2019, Parkes et al. 1993, Sushchik et al. 2007, Volkman 1986, Wiedmaier-Czerny et al. 2021, Zou et al. 2004) .

The relative percentage content of fatty acid origin from bacterial to total fatty acid is expressed as BFA%. The BFA % was the lowest in June and the highest in May (Fig. 6b ), contrary to the variation trend of rainfall. The BFA % of the samples ranged from 3.79–50.27%, with an average of 15.51%. This relative content is much higher than those reported in other studies (Kopprio et al. 2018, Mannino &Harvey 1999). The high proportion of fatty acids in bacterial origin indicated that among all microbial components, bacteria contributed the largest amount of organic matter to the water.

Analyzing the biomarker compounds of the samples revealed that the DOM in epikarst spring water is mainly comprised of endogenous organic matter, although higher plants and emergent plants have been identified in some samples. Microorganisms are the main sources of dissolved organic carbon, among which bacteria make up a considerable proportion. Water-rock interaction in karst underground rivers tends to increase inorganic carbon content in karst water more than that of other types of natural water (Liu et al. 2021, Zhao et al. 2020). The field investigation revealed that a tremendous quantity of chemical fertilizers has been spread on the farmland in the study area and that the water is high phosphorus, nitrogen-enriched. A water environment with a high amount of inorganic nutrients supports more organisms. This may be the reason why DOM derived from microorganisms dominates in karst underground rivers.

Effects of human activities on the composition and content of biomarkers

Structure and composition changes variation

The increase in temperature and precipitation are the main factors driving the changes in n-alkane and fatty acid contents. Other factors, such as human disturbance, may also disturb the sources of organic matter.

There were conspicuous differences in the contents of n-alkane components in spring water with different land-use types ( Fig. 3). For the spring water samples from BSW, which are less affected by human activities, the content of short-chain n-alkanes was the highest in all dissolved n-alkanes. The average content of short-chain alkanes accounted for 76% of the total content. The coefficient of variation was only 0.03, which indicated that the origin of organic matter was highly stable in spring water. The content of short-chain n-alkanes in agricultural land ranged from 10.18–93.74%, with an average value of only 65.88%. The coefficient of variation was 0.9, which indicated that the source of this short-chain n-alkane in agricultural land changed considerably compared to forest land.

The relative contents of different n-alkanes have different changing trends based on land-use patterns (Fig. 3a). Agricultural land, for instance, gradually decreased in short-chain n-alkane content over time. N-alkanes with long chains were relatively stable. The content of high-carbon n-alkanes is relatively small indicated that under this type of soil utilization, the higher plant contributes relatively minimally to the DOM in the water. In BSW, the forest land, the contribution of medium-chain n-alkanes decreased, whereas the contribution of long-chain n-alkanes increased. During the sampling period, it was evident that forest land was gradually becoming more influential in the variation of the DOM composition of the water body.

The average relative content of SSFA in BSW was 89.23% (Fig. 3b), and the coefficient of variation was only 0.06, which indicated that this type of fatty acid structure was the most stable in forest land. The average percentage of SSFA in agricultural land was 74.02%, with a coefficient of variation of 0.25. For those fatty acids that vary greatly with climatic conditions, the forest land is dominated by monounsaturated fatty acids, and agricultural land is dominated by polyunsaturated fatty acids.

Principal Component variation

To reveal the impact of human activities on DOM in karst water, fatty acids that are sensitive to environmental changes were analyzed as an example. We calculated the relative contents of 39 types of fatty acids in all samples, and then counted and listed all fatty acids with a relative content of more than 3%, as shown in Fig. 6. According to the analysis, there were seven types of fatty acids, and their relative contents in the samples ranged from a dominant 78.26–87.44%.

The fatty acids in the samples were dominated by n16 and n18, and the minor content of ingredients was different. As shown in (Fig. 3b), the fatty acid composition of the BSW spring water was obviously different from that in other agricultural lands. Unsaturated fatty acids in BSW did not reach 3%, but at the other sampling sites, this metric ranged from 6.33–25.42%. The average value of the sum of nC16, nC18, and nC14 in BSW was 82.45% ( Fig. 7). The percentage of other agricultural land varied from 52.31–74.23%. The total average value was only 64.48%, which was far less than that of the BSW. This shows that human activities strongly disturbed the source of organic matter in the spring water.

Except for BSW, the land use of the sampling sites is agricultural. We discovered that the main fatty acid contents of samples that belonged to different sampling areas were also different. The average content of nC16:1ω9 in the Mudu River Basin is relatively rich (Fig. 3c & Fig. 3d ), and this fatty acid is abundant in microalgae and is considered to be an input biomarker for eukaryotes (Connelly et al. 2014). The Laolongdong watershed contains a large number of C18:1ω9 (t) (Fig. 3a & Fig. 3b), a trans-fatty acid that is thought to originate mainly from Gram-negative bacteria (Canuel 2001, von Wallbrunn et al. 2003). The Laolongdong drainage basin is distributed within the core section of the city, which is near large-scale residential areas and restaurants. Because of the thoroughly developed karst cracks and pipelines in this area, domestic sewage is easily discharged into the karst river system.

An open-air sewer with the same run of the Laolongdong underground river is located in the drainage basin, and the distance from the spring to the sewer directly affects the fatty acid composition and content in the spring water. The content of C18:1ω9 (t) in the water of XJZ Spring was close to 20% (Fig. 3b). Compared with ZJYZ Spring, XJZ Spring is closer to the sewer in geographical location. In addition, XJZ also contains high concentrations of 12:0 fatty acids and 18:2 fatty acids, which are considered markers of fungal origin (Baath 2003, Olsson 1999). The strong difference in the contents of the main fatty acid ingredients in epikarst spring from the unaffected area to the farming areas and then to the urban core areas reflects the gradual deepening of human activities in the fragile karst environment.

DOM in epikarst spring water is mainly comprised of endogenous organic. Microorganisms are the main sources of dissolved organic carbon, among which bacteria make up a considerable proportion. The increases in biological activity driven by temperature and rainfall are the main factors explaining the increase in DOM content in the karst water body, whereas rainfall is the key factor restricting the organic matter content in the whole region. Human activities (such as urbanization rate or different land-use types) affect the content and composition of DOM in surface karst springs. In contrast to previous research, the CPI values of n-alkanes from higher plant input in the karst water environment were low, which was related to the unique reduction environment in the karst area. Owing to the development of karst fissures, domestic sewage can easily enter the groundwater system and affect the microbial components in the groundwater system. Therefore, attention should be paid to anti-pollution education and the construction of sewage facilities in karst areas to effectively reduce the adverse effects of human activities on the environment.

Ethics approval and consent to participate Not applicable

Consent for publication Not applicable

Availability of data and materials All data generated or analysed during this study are included in this published article.

Competing interests The authors declare that they have no competing interests.

Funding This work was supported by The Strategic Priority Research Program of the Chinese Academy of Sciences Grant (XDA230402).

Authors' contributions Wang Lachun and Shen Licheng conceptualized and supervised the study. Sun Yuchuan designed and supervised the experimental operation. Liao Yu collected samples, conducted laboratory analysis, and interpreted the experimental data. Zhang Mei assisted in the sampling and preprocessing of samples. Liao Yu drafted the manuscript, and all the authors reviewed and revised it. All authors approved the final manuscript.

Acknowledgments The author is very grateful to Professor Shen Licheng , Jiang Yongjun and Sun Yuchuan for their guidance on the experiment and thesis, and to Wang Zunbo and Zhang Mei for their assistance in sampling and experimental work.

Alexander LV et al. (2006): Global observed changes in daily climate extremes of temperature and precipitation. Journal of Geophysical Research-Atmospheres 111

Amundson R, Richter DD, Humphreys GS, Jobbagy EG, Gaillardet J (2007): Coupling between biota and earth materials in the Critical Zone. Elements 3, 327-332

Baath E (2003): The use of neutral lipid fatty acids to indicate the physiological conditions of soil fungi. Microbial Ecology 45, 373-383

Bao HXH, Li L, Lizieri C (2019): City profile: Chongqing (1997-2017). Cities 94, 161-171

Bhandari R, Routh J, Sharma S, Joshi R (2021): Contrasting lipid biomarkers in mountain rivers in the Nepal Himalayas: Organic matter characteristics and contribution to the fluvial carbon pool. Geoscience Frontiers, 101231

Bray EE, Evans ED (1961): Distribution of n-paraffins as a clue to recognition of source beds. Geochimica et Cosmochimica Acta 22, 2-15

Buckerfield SJ, Quilliam RS, Bussiere L, Waldron S, Naylor LA, Li S, Oliver DM (2020): Chronic urban hotspots and agricultural drainage drive microbial pollution of karst water resources in rural developing regions. Science of the Total Environment 744

Canuel EA (2001): Relations between river flow, primary production and fatty acid composition of particulate organic matter in San Francisco and Chesapeake Bays: a multivariate approach. Organic Geochemistry 32, 563-583

Connelly TL, Deibel D, Parrish CC (2014): Trophic interactions in the benthic boundary layer of the Beaufort Sea shelf, Arctic Ocean: Combining bulk stable isotope and fatty acid signatures. Progress in Oceanography 120, 79-92

Cranwell PA (1983): LIPIDS OF AQUATIC SEDIMENTS AND SEDIMENTING PARTICULATES. Progress in Lipid Research 21, 271-308

Cranwell PA, Eglinton G, Robinson N (1987): LIPIDS OF AQUATIC ORGANISMS AS POTENTIAL CONTRIBUTORS TO LACUSTRINE SEDIMENTS .2. Organic Geochemistry 11, 513-527

Eglinton G, Calvin M (1967): CHEMICAL FOSSILS. Scientific American 216, 32-&

Feng T, Chen H, Polyakov VO, Wang K, Zhang X, Zhang W (2016): Soil erosion rates in two karst peak-cluster depression basins of northwest Guangxi, China: Comparison of the RUSLE model with (CS)-C-137 measurements. Geomorphology 253, 217-224

Ficken KJ, Li B, Swain DL, Eglinton G (2000): An n-alkane proxy for the sedimentary input of submerged/floating freshwater aquatic macrophytes. Organic Geochemistry 31, 745-749

Fowler MG (1995): An introduction to organic geochemistry: By S. D. Killops and V. J. Killops. J. Wiley and Sons Inc., New York. Organic Geochemistry 23, 709

Grimalt J, Albaiges J (1987): SOURCES AND OCCURRENCE OF C-12-C-22 NORMAL-ALKANE DISTRIBUTIONS WITH EVEN CARBON-NUMBER PREFERENCE IN SEDIMENTARY ENVIRONMENTS. Geochimica Et Cosmochimica Acta 51, 1379-1384

Grimm NB, Faeth SH, Golubiewski NE, Redman CL, Wu JG, Bai XM, Briggs JM (2008): Global change and the ecology of cities. Science 319, 756-760

Hakimi MH, Abdullah WH, Shalaby MR (2012): Molecular composition and organic petrographic characterization of Madbi source rocks from the Kharir Oilfield of the Masila Basin (Yemen): palaeoenvironmental and maturity interpretation. Arabian Journal of Geosciences 5, 817-831

Hansell DA, Carlson CA (2002): Biogeochemistry of Marine Dissolved Organic Matter. Biogeochemistry of marine dissolved organic matter /

Hays MD, Fine PM, Geron CD, Kleeman MJ, Gullett BK (2005): Open burning of agricultural biomass: Physical and chemical properties of particle-phase emissions. Atmospheric Environment 39, 6747-6764

He Q, Qiu S, Jiang Y, Wu Z, Liu Z (2016): Land-use change caused microbial pollution in a karst underground river, Chongqing, China. Environmental Earth Sciences 75

Kaneda T (1991): ISO-FATTY AND ANTEISO-FATTY ACIDS IN BACTERIA - BIOSYNTHESIS, FUNCTION, AND TAXONOMIC SIGNIFICANCE. Microbiological Reviews 55, 288-302

Kim K-H, Jahan SA, Kabir E, Brown RJC (2013): A review of airborne polycyclic aromatic hydrocarbons (PAHs) and their human health effects. Environment International 60, 71-80

Kopprio GA, Dutto MS, Garzón Cardona JE, Gärdes A, Lara RJ, Graeve M (2018): Biogeochemical markers across a pollution gradient in a Patagonian estuary: A multidimensional approach of fatty acids and stable isotopes. Marine Pollution Bulletin 137, 617-626

Kumar M, Boski T, González-Vila FJ, Jiménez-Morillo NT, González-Pérez JA (2020): Characteristics of organic matter sources from Guadiana Estuary salt marsh sediments (SW Iberian Peninsula). Continental Shelf Research 197, 104076

Li M, Zhang Q, Zheng B, Tong D, Lei Y, Liu F, Hong C, Kang S, Yan L, Zhang Y, Bo Y, Su H, Cheng Y, He K (2019): Persistent growth of anthropogenic non-methane volatile organic compound (NMVOC) emissions in China during 1990-2017: drivers, speciation and ozone formation potential. Atmospheric Chemistry and Physics 19, 8897-8913

Li S-L, Xu S, Wang T-J, Yue F-J, Peng T, Zhong J, Wang L-C, Chen J-A, Wang S-J, Chen X, Liu C-Q (2020): Effects of agricultural activities coupled with karst structures on riverine biogeochemical cycles and environmental quality in the karst region. Agriculture, Ecosystems & Environment 303, 107120

Liao X, Xu W, Zhang J, Li Y, Tian Y (2019): Global exposure to rainstorms and the contribution rates of climate change and population change. Science of the Total Environment 663, 644-653

Lim KF, Horesh N (2017): The Chongqing vs. Guangdong developmental "models' in post-Mao China: regional and historical perspectives on the dynamics of socioeconomic change. Journal of the Asia Pacific Economy 22, 372-395

Lin H (2010): Earth's Critical Zone and hydropedology: concepts, characteristics, and advances. Hydrology and Earth System Sciences 14, 25-45

Liu J, Zhong J, Chen S, Xu S, Li S-L (2021): Hydrological and biogeochemical controls on temporal variations of dissolved carbon and solutes in a karst river, South China. Environmental Sciences Europe 33

Liu Z, Dreybrodt W, Wang H (2010): A new direction in effective accounting for the atmospheric CO2 budget: Considering the combined action of carbonate dissolution, the global water cycle and photosynthetic uptake of DIC by aquatic organisms. Earth-Science Reviews 99, 162-172

Liu Z, Macpherson GL, Groves C, Martin JB, Yuan D, Zeng S (2018): Large and active CO2 uptake by coupled carbonate weathering. Earth-Science Reviews 182, 42-49

Mannino A, Harvey HR (1999): Lipid composition in particulate and dissolved organic matter in the Delaware Estuary: sources and diagenetic patterns. Geochimica et Cosmochimica Acta 63, 2219-2235

Marzi R, Torkelson BE, Olson RK (1993): A REVISED CARBON PREFERENCE INDEX. Organic Geochemistry 20, 1303-1306

McDonough LK, O’Carroll DM, Meredith K, Andersen MS, Brügger C, Huang H, Rutlidge H, Behnke MI, Spencer RGM, McKenna A, Marjo CE, Oudone P, Baker A (2020): Changes in groundwater dissolved organic matter character in a coastal sand aquifer due to rainfall recharge. Water Research 169, 115201

Meyers PA (1994): PRESERVATION OF ELEMENTAL AND ISOTOPIC SOURCE IDENTIFICATION OF SEDIMENTARY ORGANIC-MATTER. Chemical Geology 114, 289-302

Meyers PA (1997): Organic geochemical proxies of paleoceanographic, paleolimnologic, and paleoclimatic processes. Org. Geochem. 27, 213-250

Mostofian B, Zhuang T, Cheng X, Nickels JD (2019): Branched-Chain Fatty Acid Content Modulates Structure, Fluidity, and Phase in Model Microbial Cell Membranes. Journal of Physical Chemistry B 123, 5814-5821

Nishimura M, Baker EW (1986): Possible origin of n-alkanes with a remarkable even-to-odd predominance in recent marine sediments. Geochimica et Cosmochimica Acta 50, 299-305

Olsson PA (1999): Signature fatty acids provide tools for determination of the distribution and interactions of mycorrhizal fungi in soil. FEMS Microbiology Ecology 29, 303-310

Pan H, Yu H, Song Y, Zhu L, Liu R, Du E (2017): Tracking fluorescent components of dissolved organic matter from soils in large-scale irrigated area. Environmental Science and Pollution Research 24, 6563-6571

Parkes RJ, Dowling NJE, White DC, Herbert RA, Gibson GR (1993): CHARACTERIZATION OF SULFATE-REDUCING BACTERIAL-POPULATIONS WITHIN MARINE AND ESTUARINE SEDIMENTS WITH DIFFERENT RATES OF SULFATE REDUCTION. Fems Microbiology Ecology 102, 235-250

PETERS, A. R BIOMARKER GUIDE, THE - VOL. 2 BIOMARKERS AND ISOTOPES IN PETROLEUM SYSTEMS AND EARTH HISTORY.

Pondell CR, Canuel EA (2020): Sterol, fatty acid, and lignin biomarkers identify the response of organic matter accumulation in Englebright Lake, California (USA) to climate and human impacts. Organic Geochemistry 142, 103992

Pruden A, Pei RT, Storteboom H, Carlson KH (2006): Antibiotic resistance genes as emerging contaminants: Studies in northern Colorado. Environmental Science & Technology 40, 7445-7450

Ren XY, Zeng GM, Tang L, Wang JJ, Wan J, Liu YN, Yu JF, Yi H, Ye SJ, Deng R (2018): Sorption, transport and biodegradation - An insight into bioavailability of persistent organic pollutants in soil. Science of the Total Environment 610, 1154-1163

Rogge WF, Medeiros PM, Simoneit BRT (2007): Organic marker compounds in surface soils of crop fields from the San Joaquin Valley fugitive dust characterization study. Atmospheric Environment 41, 8183-8204

Scalan ES, Smith JE (1970): An improved measure of the odd-even predominance in the normal alkanes of sediment extracts and petroleum. Geochimica et Cosmochimica Acta 34, 611-620

STATISTICS CMBO (2020): CHONGQING STATISTICAL YEARBOOK 2019. China Statistics Press

Sun Y, Zhang S, Xie Z, Lan J, Li T, Yuan D, Yang H, Xing B (2020): Characteristics and ecological risk assessment of polycyclic aromatic hydrocarbons in soil seepage water in karst terrains, southwest China. Ecotoxicology and Environmental Safety 190, 110122

Sushchik NN, Gladyshev MI, Kravchuk ES, Ivanova EA, Ageev AV, Kalachova GS (2007): Seasonal dynamics of long-chain polyunsaturated fatty acids in littoral benthos in the upper Yenisei river. Aquatic Ecology 41, 349-365

Tiefenbacher A, Weigelhofer G, Klik A, Mabit L, Santner J, Wenzel W, Strauss P (2021): Antecedent soil moisture and rain intensity control pathways and quality of organic carbon exports from arable land. CATENA 202, 105297

Valle J, Harir M, Gonsior M, Enrich-Prast A, Schmitt-Kopplin P, Bastviken D, Hertkorn N (2020): Molecular differences between water column and sediment pore water SPE-DOM in ten Swedish boreal lakes. Water Research 170, 115320

Volkman JK (1986): A review of sterol markers for marine and terrigenous organic matter. Organic Geochemistry 9, 83-99

Volkman JK, Barrett SM, Blackburn SI, Mansour MP, Gelin F (1998): Microalgal biomarkers: A review of recent research developments. Organic Geochemistry 29, 1163-1179

Volkman JK, Revill AT, Holdsworth DG, Fredericks D (2008): Organic matter sources in an enclosed coastal inlet assessed using lipid biomarkers and stable isotopes. Organic Geochemistry 39, 689-710

von Wallbrunn A, Richnow HH, Neumann G, Meinhardt F, Heipieper HJ (2003): Mechanism of cis-trans isomerization of unsaturated fatty acids in Pseudomonas putida. Journal of Bacteriology 185, 1730-1733

Wakeham SG, Canuel EA (2006): Degradation and preservation of organic matter in marine sediments. In: Volkman JK (Editor), Handbook of Environmental Chemistry. Handbook of Environmental Chemistry, pp. 295-321

Wang R, Wang J, Li F, Yang S, Tan L (2016): Vertical distribution and indications of lipids biomarkers in the sediment core from East China Sea. Continental Shelf Research 122, 43-50

Westra S, Alexander LV, Zwiers FW (2013): Global Increasing Trends in Annual Maximum Daily Precipitation. Journal of Climate 26, 3904-3918

Wiedmaier-Czerny N, Schroth D, Topman-Rakover S, Brill A, Burdman S, Hayouka Z, Vetter W (2021): Detailed analysis of the fatty acid composition of six plant-pathogenic bacteria. Journal of Chromatography B 1162, 122454

Wu J, Zheng H, Zhe F, Xie W, Song J (2018): Study on the relationship between urbanization and fine particulate matter (PM2.5) concentration and its implication in China. Journal of Cleaner Production 182, 872-882

Xu B, Li J, Huang Q, Gong Q, Li L (2016): Impacts of land use patterns and typhoon-induced heavy rainfall event on dissolved organic matter properties in the South Tiaoxi River, China. Environmental Earth Sciences 75, 632

Zhang N, Gao Z, Wang X, Chen Y (2010): Modeling the impact of urbanization on the local and regional climate in Yangtze River Delta, China. Theoretical and Applied Climatology 102, 331-342

Zhao H, Xiao Q, Zhang C, Zhang Q, Wu X, Yu S, Miao Y, Wang Q (2020): Transformation of DIC into POC in a karst river system: evidence from delta C-13(DIC) and delta C-13(POC) in Lijiang, Southwest China. Environmental Earth Sciences 79

Zhou Z, Wang Y, Yang H, Liu A, Wu S, Teng H, Niu X (2021): Sedimentary record of nutrients and sources of organic matter in the Shuanglong reservoir, Dianchi watershed, China. Environmental Science and Pollution Research 28, 17763-17774

Zou L, Wang XC, Callahan J, Culp RA, Chen RF, Altabet MA, Sun MY (2004): Bacterial roles in the formation of high-molecular-weight dissolved organic matter in estuarine and coastal waters: Evidence from lipids and the compound-specific isotopic ratios. Limnology and Oceanography 49, 297-302

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Origin and Factors Effecting Dissolved Organic Matter in Karst Spring, Southwest China

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Abstract

Figures

Introduction

Study area

Population

Land-use type

Materials And Methods

Sampling process

Experiments and materials

Sample pre-treatment

Lipid biomarkers

Detection of lipids

Results

Temporal variations in n-alkanes and fatty acids

Characteristics of n-alkane and fatty acid components

Discussion

n-alkanes trace the source of organic matter

Fatty acids determine the microbial origin of dissolved organic matter

Effects of human activities on the composition and content of biomarkers

Structure and composition changes variation

Principal Component variation

Conclusions

Declarations

References

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