Spectroscopic Indices Reveal Spatiotemporal Variations of Dissolved Organic Matter in Subtropical Karst Cave Drip Water

Abstract

The migration and transformation of dissolved organic matter (DOM) in highly heterogeneous and permeable karst aquifers are of great significance to the regional carbon cycle but are rarely explored, especially in response to extreme climate events. In this study, we utilized spectroscopic indices to investigate variations in DOM concentration, composition, and origin in Mahuang Cave, Southwest China, from August 2020 to October 2022. The findings indicate that meteorological conditions, hydrological pathways, and in–situ metabolism primarily control DOM dynamics in karst cave drip water. However, due to the complex cave fractures and stratigraphy, the DOM composition at the four monitoring sites—classified into slow and fast seepage—showed different responses to meteorological events. Therefore, surface reworking must be fully considered when assessing the environmental information recorded by cave sediments.

1. Introduction

Dissolved organic matter (DOM) is a complex mixture of degradation by–products and freshly produced compounds, comprising carbohydrates, hydrocarbons, polyhydroxy phenols, fatty acids, amino acids, and quinones. It is a highly heterogeneous combination of a large number of organic molecules with diverse characteristics and components [1,2,3,4]. Due to its intricate structure and diverse functional groups [5], DOM plays a crucial role in both terrestrial and aquatic environments, influencing carbon and nitrogen cycles, pollutant distribution, and nutrient retention and loss [6,7]. It also represents the most active fraction in regional and global carbon cycles [8,9], significantly impacting various ecological processes, such as the biogeochemical cycling of carbon [10]. Numerous studies have shown that the chemical characteristics of DOM are regulated by its original source and the biogeochemical processes it undergoes during transportation [11]. Climate conditions, hydrogeochemical processes, and biochemical metabolic processes play important roles in constraining DOM dynamics in ecosystems [12,13]. DOM is sensitive to global changes, and this sensitivity, in turn, can affect biogeochemical processes [14]. Thus, variations in DOM concentration, structure, and composition during geochemical processes can provide valuable insights into processes such as the regional carbon cycle in the context of climate change.

Optical spectroscopy is a common method for interpreting the chemical components of DOM. UV–Vis spectroscopy, known for its molecular–level insights, is widely used to analyze DOM structures [15,16,17]. Specific absorbance wavelengths are used to identify DOM composition: 254 nm for aromatic fractions, 285 nm for fulvic fractions, and 350 nm for lignin fractions [18,19,20]. Absorption ratios, calculated from the absorption coefficients at two wavelengths, are frequently employed to trace DOM sources and determine its quality. For example, the E2/E3 ratio (a₂₅₀/a₃₆₅) reflects DOM molecular size and inversely correlates with the degree of humification, while the E2/E4 ratio (a₂₆₅/a₄₆₅) estimates the relative composition of autochthonous versus terrestrial DOM [5,21,22,23]. Additionally, spectral slope coefficients are crucial for characterizing DOM structure. Bricaud et al. [24] used spectral slopes to distinguish DOM sources, and Helms et al. [25] calculated slopes between 275–295 nm and 350–400 nm by converting the UV absorbance spectrum to natural logarithms. The spectral slope ratio (S_R) inversely correlates with DOM molecular weight and illustrates the sources and signatures of DOM, making spectral slopes and absorption ratios valuable tools for investigating DOM geochemistry.

Karst landforms, which account for 15% of the global land surface, exhibit unique geological, hydrologic, and biological conditions [26,27]. The weathering of carbonate rock consumes atmospheric CO₂, while calcite deposition releases CO₂, indicating that carbonate minerals play a significant role in regulating regional and global carbon cycles through dissolution and deposition [28,29,30,31,32,33,34,35,36,37]. These processes of mineral dissolution and precipitation are sensitive to precipitation, temperature, and runoff, making the response of karst regions to global climate change particularly noteworthy. Previous studies have revealed that inorganic carbon responds rapidly to extreme precipitation and drought events, and δ¹³C_DIC values are sensitive to changes in local hydrological conditions, especially regional precipitation variations caused by meteorological events [38]. This highlights the importance of studying the effects of climate change on the carbon cycle in karst areas. As another crucial aspect of the carbon cycle, the transformation of organic carbon has its own particularities. First, the composition and structure of natural DOM are key regulators of its microbial biodegradation and photodegradation, which, in turn, continuously and dynamically modify its molecular signatures [39,40,41]. This process can be especially significant in karst aquifers, characterized by heterogeneous hydrological processes due to varying residence times. Second, the mineralization of DOM can explain CO₂ emissions to the atmosphere, with low molecular weight DOM preferentially decomposed to produce biogenic CO₂ [15,42,43]. The dual structure of the karst zone means that underground CO₂, formed by the decay of organic matter, infiltrates into deeper unsaturated zones, contributing to pCO₂ levels [44]. This CO₂ can participate in carbonate dissolution, affecting the regional carbon cycle and carbon sink [34]. Given the high sensitivity of karst regions to hydrological variations, changes in rainfall patterns induced by climate change will significantly impact carbon cycling processes, particularly the coupling between inorganic and organic carbon [45].

Karst caves act as sinks for materials from overlying soil, making them essential for analyzing carbon transmission and transformation processes [46,47]. Cave drip water, which originates from atmospheric precipitation, serves as an important medium connecting aboveground and underground environments, carrying information about the external environment into the cave through downward seepage. Several studies have identified meteorological conditions as crucial regulators of DOM variations in cave water [47,48,49]. Rainfall stimulates soil erosion, causing organic matter to migrate underground, a process that is also influenced by subsurface reworking [50]. However, the non–homogeneity and high permeability of karst aquifers result in heterogeneous hydrological processes during infiltration, leading to uncertainties and varied responses of cave waters to external environmental changes, including climate change [38]. While it is evident that a deep understanding of the predominant factors affecting DOM vertical migration is still lacking, a comprehensive study of DOM dynamics—especially structural variations—can provide insights into its composition, origin, and fate, thereby enhancing our understanding of the carbon cycle in karst systems. This study aims to (i) examine the temporal and spatial variations of DOM concentrations and components, (ii) trace the drivers and origins of DOM using optical properties, and (iii) distinguish the spectral characteristics of cave drip water in response to regional extreme climate events.

2. Materials and Methods

2.1. Study Area

Mahuang Cave, one of the primary branches of the Shuanghe Cave System (28°08′00″–26°20′00″ N; 107°02′30″–107°25′00″ E), is located in Guizhou Province, Southwest China, with an entrance at an altitude of 720 m (Figure 1). The cave is covered by a bedrock layer, including an overlying soil layer, with an estimated average thickness of 100 m. Geologically, the Shuanghe Cave system lies within the broad and gentle box anticlinal wing of northern Guizhou Province. Tectonic stresses in various directions have formed NE, NW, and SN fold fault zones, creating a relatively uplifted triangular block that encloses the cave area. The geology mainly consists of dolomite, lime dolomite, chert, and argillaceous dolomite from the Middle to Upper Cambrian Loushanguan Formation (ϵ_2–3ls) and Lower Ordovician Tongzi Formation (O₁t) [51]. Secondary calcium carbonate deposits, such as stalagmites, stone curtains, and helictites, are well–developed along the cave passage. The study area’s meteorology is characterized by a subtropical monsoon climate with simultaneous rain and heat, featuring an average annual rainfall of 1076 mm and an average annual temperature of 16.1 °C. The rainy season spans from May to October, while the dry season lasts from November to April. Precipitation is the sole recharge source for the aquifer, which discharges through the cave system. The overlying vegetation primarily consists of evergreen broadleaf forests and shrubs.

2.2. Fieldwork

Daily air temperature and precipitation data were obtained from the Zheng’an meteorological station. Four drip water sampling sites (MH0, MH2, MH3, and MH4) were selected from different locations along the cave passageways in Mahuang Cave (Figure 2). Samples were collected monthly from August 2020 to October 2022. Discharge rates were manually measured using measuring cylinders [34,50,51]. Water temperature was measured in situ with a TW Multi 3630 portable multiparameter water quality probe (WTW, Munich, Germany) [26,42,52]. For DOM measurements, water samples were filtered through 0.45 μm polyether sulfone (PES) filters into 125 mL deionized water–cleaned fluorinated high–density polyethylene bottles. The samples were then kept at 4 °C in the dark and transported to the laboratory within 2 days using ice packs [39,53,54].

2.3. Laboratory Analysis

Ultraviolet–visible absorption spectra (200–800 nm) were obtained using a UV–Vis spectrophotometer (Cary 300, Agilent, USA) with a 1–cm path length cell. The wavelength scan rate and slit width were set to 210 nm min⁻¹ and 1 nm, respectively. Concentrations of DOM were measured using a C/N Analyzer (Multi N/C 3100, Jena, Germany) according to the manufacturer’s instructions. HCl (guaranteed reagent) was added to remove inorganic carbon prior to measurement. The analyzer injected the sample into a combustion chamber where organic carbon was oxidized to CO₂. The CO₂ was then detected by an NDIR detector, and the DOC concentration was quantified based on the calibration curve. Milli–Q water (Millipore, USA) was used as the blank. All measurements had a relative standard deviation (RSD) of less than 5%.

2.4. Data Calculation

The specific ultraviolet absorbances at 254 nm (a₂₅₄), 285 nm (a₂₈₅), and 350 nm (a₃₅₀) were measured to indicate the aromatic, fluvic, and lignin fraction of DOM, respectively [3,39,55]. Absorption coefficients a₂₅₀, a₂₆₅, a₃₆₅, and a₄₆₅ were used to calculate the E2/E3 and E2/E4 ratios. The spectra slope ratio (S_R) was calculated by a nonlinear regression method with two spectral slopes, 275–295 nm and 350–400 nm [39].

$$a_{\mathsf{\lambda }}=2.303\times {Abs}_{\mathsf{\lambda }}/L$$

(1)

$$S_R={\displaystyle \frac{S_{275-295}}{S_{350-400}}}$$

(2)

where $a_{\mathsf{\lambda }}$ is the absorption coefficient at λ nm, and L is the path length of the quartz cuvette.

The experimental data was statistically analyzed using SPSS 23.0. The Mann–Whitney U test was used to analyze temporal variations, while the Kruskal–Wallis test was employed for spatial variations of selected parameters. Multiple linear regressions were carried to identify the relationships between DOC concentrations and DOM absorption coefficients [42]. All charts were conducted with Origin 2022 and GraphPad Prism 9.

3. Results

3.1. Meteorological Conditions and Physical Parameters of Cave Drip Water

The monthly average atmospheric temperatures outside Mahuang Cave ranged from 2.99 °C to 28.26 °C, with a mean of 16.15 °C over the monitoring period (Figure 3). The highest temperatures were recorded in July and August, while the lowest were observed from December to February. Precipitation ranged from 9.3 mm to 194.7 mm per month, averaging 89.3 mm, with over 70% occurring during the wet season (May to October), characteristic of the subtropical monsoon climate. Notably, low rainfall was recorded in August 2020 (28.8 mm) and August 2022 (32.2 mm), which may be linked to the triple La Niña event.

Water temperatures in Mahuang Cave ranged from 6.8 °C to 18.9 °C, showing significant seasonal variation (p < 0.05) (Figure 4). Site MH0, closest to the entrance, displayed the most variability due to its sensitivity to external environmental changes. Discharge rates at each drip site also exhibited significant seasonal variations (p < 0.05) (Figure 5), with higher rates during the wet season (average 6.06 mL/s) compared to the dry season (average 2.11 mL/s). Differences in discharge rates were significant among the four sites (MH0, MH2, MH3, and MH4) (p < 0.05) (Figure 5). MH0 (with break flows occurring in August 2022 and October 2022) and MH2, which had lower average discharge rates of 0.49 mL/s and 0.08 mL/s, respectively, were located in low–flow zones. In contrast, MH3 and MH4, with higher average discharge rates of 2.63 mL/s and 13.33 mL/s, respectively, were in high–flow zones.

3.2. DOM Concentrations and Optical Properties in Cave Drip Water

During the monitoring period, DOM concentrations in Mahuang Cave drip water ranged from 0.34 mg/L to 3.23 mg/L, with higher average values observed during the wet season. This finding is consistent with results observed in Marengo Cave (USA) [49] and the Longchuan River (Southwest China) [15,42]. The hydrological processes greatly enhanced soil flushing and transport, leading to increased terrestrially derived DOM input into the cave and contributing to fluctuations in DOM concentrations. However, no statistically significant temporal variation in DOC concentrations was observed (p > 0.05).

The proportions of the aromatic fraction (wet season: 40.40–51.98%; dry season: 40.39–48.52%) and the lignin fraction (wet season: 6.54–21.51%; dry season: 8.22–21.27%) showed significant seasonal differences (Figure 4), while the fulvic fraction remained relatively stable (wet season: 37.87–42.97%; dry season: 37.03–42.86%). The a₂₅₄ values, indicative of the aromatic component, ranged from 7.32 m⁻¹ to 16.46 m⁻¹, with higher values in the wet season (11.11 ± 1.67 m⁻¹) compared to the dry season (10.25 ± 1.64 m⁻¹) (

Table 1. Summary of DOC concentrations and non–normalized absorption coefficients in Mahuang Cave during monitoring period.

	Wet Season						Dry Season
	n	Max	Min	Mean	Std. Dev	n	Max	Min	Mean	Std. Dev
DOC (mg/L)	46	3.52	0.45	1.46	0.72	35	4.50	0.34	1.27	0.82
a254 (m−1)	46	16.46	8.54	11.11	1.67	35	13.57	7.32	10.25	1.64
a285 (m−1)	46	14.47	7.13	9.85	1.52	35	12.74	6.91	9.40	1.38
a350 (m−1)	46	7.28	0.74	3.44	1.70	35	6.82	0.74	4.00	1.53
E2/E3	46	3.81	1.75	2.54	0.48	35	2.95	1.63	2.21	0.38
E2/E4	46	3.67	0.93	6.01	2.10	35	5.22	1.04	3.05	0.89
SR	46	10.73	0.01	1.37	2.28	35	10.64	0.08	1.14	1.89

), showing significant seasonality (p < 0.05). Increased rainfall led to higher terrestrial DOM inputs, resulting in greater aromatic component abundance in the wet season [42]. The a₂₈₅ values were also higher in the wet season (9.85 ± 1.52 m⁻¹) compared to the dry season (9.40 ± 1.38 m⁻¹), although this difference was not statistically significant (p > 0.05). These findings are consistent with previous studies conducted in karst river systems [42] and karst caves [56]. In contrast, a₃₅₀ values were higher in the dry season (4.00 ± 1.53 m⁻¹) (p < 0.05), indicating greater lignin input during this period. This observation slightly differs with data from the Longchuan River, which showed higher a₃₅₀ values in the post–wet and drought periods [15]. The longer water residence time of downward seepage compared to surface water could account for this discrepancy, as it results in time lags in the response of drip water to terrestrial inputs. The E2/E3 values (ranging from 1.63 to 3.81) showed significant seasonality (p < 0.05) (Figure 5), with higher levels in the wet season, suggesting smaller molecular sizes and lower DOM humification. The E2/E4 ratio, an indicator of autochthonous versus terrestrial sources, exhibited significantly higher values in the wet season (6.10 ± 2.01 m⁻¹) compared to the dry season (3.05 ± 0.89 m⁻¹) (p < 0.05). Higher temperatures and abundant rainfall favored microbial activity, resulting in increased autochthonous DOM levels. The spectral slope ratio (S_R), which is negatively correlated with DOM molecular weight, also showed significant temporal variations (p < 0.05), with lower molecular weights (average 1.14) in the wet season compared to the dry season (average 1.37). This suggests that eco–hydrological processes modulate DOM dynamics [42,57].

Spatially, a₂₈₅ values and discharge rates exhibited notable differences among drip sites (p < 0.05) (Figure 6), indicating that variations in the fulvic fraction could be partly attributed to abiotic factors. Fulvic acid, being a relatively labile DOM fraction originating from soil, reflects these spatial variations. The differences also suggest that the hydrological type of the drip sites regulates DOM dynamics.

4. Discussion

4.1. Drivers and Origins of DOM in Cave Drip Water

The physical parameters of cave water exhibited significant correlations with meteorological conditions, indicating that cave drip water responds strongly to external environmental changes (Figure 7). This suggests that the physicochemical properties of cave drip water are influenced by meteorological factors. Both a₂₅₄ and a₂₈₅ demonstrated strong positive correlations with water temperature and discharge rates, implying that the aromatic and fulvic fractions, which are terrestrial signals originating from soil, are influenced by both meteorology and hydrology. These correlations likely result from water–soil erosion induced by rock weathering. The lignin fraction showed higher values during the dry season, corresponding to lower E2/E3 values in the same period. The close co–variation of E2/E3 and a₃₅₀ indicated that the molecular weight of DOM in cave drip water is primarily constrained by lignin. During late autumn, significant amounts of lignin from decomposed plant debris are flushed into the cave system with slow downward seepage, consistent with the negative correlation between meteorological conditions and a₃₅₀ (Figure 7). This pattern is similar to findings from the Longchuan River, Southwest China [15], where riverine carbon exhibited a significant positive correlation with lignin components during this period. E2/E4 values showed a negative correlation with a₃₅₀, suggesting that when lignin input is dominant, the proportion of autochthonous sources in cave water is relatively lower. As expected, E2/E4 values exhibited a significant positive correlation with both water and air temperatures. Enhanced microbial activity during warmer periods results in a higher proportion of autochthonous sources. Overall, DOM in cave drip water is regulated by the dynamics of organic input from overlying soil, which is controlled by meteorological conditions.

Aquatic DOC concentrations were higher during the wet season, likely due to increased terrigenous organic carbon inputs from rainfall. Similarly, a₂₅₄ and a₂₈₅ exhibited higher values in the wet season, while a₃₅₀ was higher in the dry season. These discrepancies in non–normalized absorption coefficients suggest that the origins of DOM vary under different hydro–meteorological conditions in the cave system. Multiple linear regressions confirmed the relationships between DOC concentrations and DOM non–normalized absorption coefficients [42,58,59]:

$${[DOC}_{wet}]=0.825a_{254}-0.471a_{285}-0.090a_{350}-0.853(R^2=0.751,p<0.05)$$

(3)

$${[DOC}_{dry}]=0.517a_{254}-0.908a_{285}+0.103a_{350}+0.666(R^2=0.344,p>0.05)$$

(4)

As Equation (3) shows, DOC levels had negative correlations with a₂₅₄, indicating that fulvic acid, a relatively labile fraction of DOM, is preferentially degraded by microorganisms, leading to the accumulation of refractory fractions (measured by a₂₅₄) during the wet season [26,60]. With decreasing rainfall and weakened terrigenous organic carbon inputs, the incomplete degradation of refractory DOM—such as the aromatic fraction—from high–molecular–weight to low–molecular–weight compounds becomes more evident [47,52] (Equation (4)). However, the relationship between DOC concentrations and DOM non–normalized absorption coefficients did not achieve statistical significance, possibly due to the “overlapping” of DOM characteristics [56,61]. Given the high heterogeneity of the karst aquifer, the hydrological processes are more complex during the dry season. Firstly, during this period, pipes and fissures may be relatively open and well–ventilated [62,63], facilitating the degassing of CO₂, which partially originates from DOM mineralization, thereby accelerating the consumption of bioavailable DOM. Secondly, enhanced absorption by minerals in the bedrock during infiltration processes, due to the longer residence time, could further alter DOM compositions [64]. Consequently, DOM migration and transformation behavior in the karst aquifer appears to be more elusive during the dry season compared to the wet season.

The karst cave water recharge features exhibited notable spatial variations among drip sites (Figure 6) due to the influence of complex cave fractures controlled by lithology. The response of DOM in drip water to inputs from overlying soil depends on the flow path through the soil zone and bedrock layer, leading to potential discrepancies in DOM fingerprints in cave water. The four drip sites could be classified into two types: slow flow (MH0 and MH2) and fast flow (MH3 and MH4), characterized by seepage through relatively low permeability zones and fast flow through fractures and open shafts, respectively [34].

$${[DOC-slow}_{wet}]=0.743a_{254}-0.720a_{285}-0.088a_{350}-0.102(R^2=0.732,p<0.05)$$

(5)

$${[DOC-slow}_{dry}]=0.292a_{254}-0.615a_{285}+0.165a_{350}+0.439(R^2=0.182,p>0.05)$$

(6)

$${[DOC-fast}_{wet}]=0.915a_{254}-0.744a_{285}-0.162a_{350}+2.091(R^2=0.506,p<0.05)$$

(7)

$${[DOC-fast}_{dry}]=1.773a_{254}-1.611a_{285}-0.060a_{350}+0.991(R^2=0.667,p>0.05)$$

(8)

According to these equations, DOC is more accurately represented by non–normalized absorption coefficients during the wet season. In both slow and fast flow systems, fulvic fractions, due to their labile nature, are preferentially utilized by microorganisms. However, the behavior of the aromatic component differs. In slow flow systems, the contribution of the aromatic fraction to total DOC concentration decreases during the dry season, possibly due to gradual degradation by microorganisms [50]. Reduced precipitation leads to longer water residence times in fractures and pipes, allowing for incomplete decomposition of refractory organic components by microorganisms [52]. In contrast, fast flow systems exhibit higher accumulation of the aromatic fraction during the dry season, potentially due to increased organic matter from overlying soil, induced by water–soil erosion. In these systems, the labile components are preferentially utilized as “fast food”, resulting in a higher abundance of refractory components [50]. Additionally, a storage reservoir inferred in the bedrock above fast flow drip sites [38,50] may accumulate refractory organic components and release them when a particular threshold of water input is reached, especially during periods of low rainfall. Overall, DOM dynamics in cave drip water are regulated by meteorological conditions and hydrological pathways.

4.2. Response of DOM Characteristics in Cave Drip Water to Regional Climate

Meteorology is the primary driver of DOM dynamics in the cave system, while hydrology and biogeochemistry regulate these dynamics during downward processes in the karst aquifer, influencing the spatiotemporal variations of DOM in Mahuang Cave drip water. As a typical monsoon system, the Asian monsoon strongly influences the climate of Southwest China, affecting regional atmospheric temperature and precipitation. Therefore, regional climate parameters must be fully considered when assessing the environmental record of cave sediments. Meteorological data from the monitoring period revealed low rainfall amounts (28.8 mm and 32.2 mm) accompanied by high temperatures (26.0 °C and 28.6 °C) in August 2020 and August 2022, respectively, suggesting monsoon drought events. This deviates from the typical subtropical monsoon climate, which features simultaneous high rainfall and temperatures [39]. The discrepancy may be due to the northward shift of the monsoon rain belt caused by the combined effects of the southeast and southwest monsoons [38]. Reduced rainfall also diminishes the piston effect, leading to longer residence times in fissures and pipes in the karst bedrock and lower discharge rates at drip sites.

Under drought conditions, decreased soil microbial activity leads to reduced decomposition of macromolecular organic matter, resulting in lower seepage of small molecule organic matter into the cave [50]. This situation facilitates the accumulation of high molecular weight components. Similar results were observed in the Furong River and Pengxi River [52], where recalcitrant compounds of terrigenous DOM accumulated during dry periods. The spectroscopic indices of cave drip water showed different variations during these months. Lower E2/E3 values in the two dry Augusts (Figure 8a) suggest higher molecular weight DOM in cave drip water, due to the accumulation of refractory organic components (e.g., aromatic fractions) from incomplete decomposition (Figure 8b). In contrast, during normal Augusts, higher fulvic fraction abundance (Figure 8b), resulting from soil wash–in due to abundant rainfall, leads to lower molecular weight DOM in drip water. Autochthonous DOM sources also exhibited higher levels during the two dry Augusts, likely due to reduced rainfall. With insufficient fresh DOM input, the proportion of microbial activity–produced organic matter increases. Conversely, in August 2021, with abundant rainfall (189 mm), enhanced soil erosion increased terrestrial DOM levels. In subsequent months, E2/E3 values rose in September 2020 and October 2022 but declined in September 2021, reflecting rainfall variations. Elevated rainfall stimulates fresh organic matter input from overlying soil [65], while reduced input leads to the consumption of “fast food”, supported by the dynamics of aromatic and fulvic fractions. The ratio of DOM origins (autochthonous vs. terrestrial) co–varied with rainfall changes, indicating that DOM components in cave drip water respond well to external meteorological changes. The discrepancies in E2/E3 values between monsoon drought events and the dry period suggest that DOM had relatively higher molecular weight during dry periods. This is possibly due to elevated temperatures, which stimulate microbial activity and contribute to macromolecular DOM decomposition [39,52]. These variations in DOM fingerprint indicate that even under drought conditions, microbial activity modifies DOM differently depending on atmospheric temperatures.

4.3. Implications for Regional Climate Influence on DOM in Karst Cave Drip Water

The carbon cycle in karst cave systems is intricate due to the highly non–homogeneous nature of aquifers and the diverse hydrological processes involved. During vertical migration, various factors—including inputs from overlying soil, microbial decomposition, mineral absorption, and possible interactions with inorganic carbon—affect DOM dynamics and alter its characteristics [3,66,67,68]. This study demonstrates that DOM components in cave drip water are responsive to meteorological events. Specifically, high molecular weight DOM, such as aromatic fractions, tends to accumulate during periods of low precipitation and high atmospheric temperatures. Conversely, increased rainfall leads to higher signals of soil–water erosion. However, due to the combined effects of input, decomposition, and accumulation processes at drip sites, establishing a direct link between DOM components and meteorological events with the current monitoring methods is challenging. Furthermore, varying meteorological and hydrological conditions lead to complex subsurface responses across different pathways, introducing uncertainties in using cave sediment organic signals for paleoclimate reconstruction. Therefore, future research should include high–frequency and long–term monitoring to capture more precise responses of DOM components to meteorological changes. Additionally, exploring the potential connections between organic and inorganic carbon in karst cave systems under climate change is crucial for a more comprehensive understanding.

5. Conclusions

This study underscores the substantial impact of biochemical processes and hydrological pathways on DOM dynamics in karst cave drip water, with meteorological events playing a pivotal role in modulating these interactions. The analysis of optical properties highlights the role of karst caves as continuous systems for the transport, utilization, and storage of organic matter. Rainfall intensifies terrestrial inputs, significantly affecting DOM composition. These findings reveal the potential of using DOM optical properties as tracers to understand the dynamic interactions within karst systems. This approach not only deepens our comprehension of organic matter cycling in these unique environments but also provides a valuable tool for predicting how such systems might respond to future climatic and environmental changes. The study’s results offer a foundation for developing more refined models of DOM behavior in karst landscapes, with important implications for water quality management and ecosystem conservation.