Aridification Inhibits the Release of Dissolved Organic Carbon from Alpine Soils in Southwest China

Abstract

The alpine peatlands in western Sichuan Province are currently experiencing aridification. To understand the effects of aridification on the characteristics of organic carbon release from alpine soils, the soil in the northwest Sichuan Plateau was investigated. Soil columns were incubated under different moisture conditions in situ and in the laboratory, and ultraviolet-visible absorption spectroscopy and three-dimensional fluorescence spectroscopy were used to assess the soil dissolved organic carbon (DOC) levels. The results revealed that (1) the cumulative release of DOC from alpine soil in the northwest Sichuan Plateau decreased with decreasing moisture content. The cumulative release of soil DOC in the laboratory (0–5 cm soil reached 1.93 ± 0.43 g/kg) was greater than that from soil incubated in situ (0–5 cm soil reached 1.40 ± 0.13 g/kg); (2) the cumulative release of DOC in 0–5 cm soil exhibited the greatest response to changes in water content, and the cumulative release of DOC from the 0–5 cm soil layer (1.40 ± 0.13 g/kg) was greater than that from the 5–15 cm soil layer (1.25 ± 0.03 g/kg); and (3) UV-visible absorption spectra and 3D fluorescence spectral characteristics indicated that aridification increases the content of chromophoric dissolved organic matter (CDOM) components with strong hydrophobicity, especially tyrosine components (surface soil increased 39.59~63.31%), in alpine soil DOC. This increase in hydrophobic CDOM components enhances the aromaticity and degree of humification of DOC. Our results revealed that drought inhibits the release of soil DOC, which is unfavorable for the sequestration of organic carbon in alpine soils, potentially resulting in the loss of soil carbon pools and further degradation of alpine ecosystem functions.

1. Introduction

The organic carbon pool of alpine soil is among the most critical carbon reservoirs within terrestrial ecosystems and plays an exceedingly vital role in the soil carbon cycle. In particular, in the alpine region of southwest China, the alpine environment promotes accumulation of large amounts of soil organic matter [1]. Alpine soils have a stronger and more rapid response to climate change and are an initiating and sensitive zone for global climate change [2,3,4]. Over the past 50 years, the high-altitude areas of the Qinghai-Tibet Plateau have experienced significant warming, and the heating rate has been higher than the global average heating rate [5,6]; the projected increase in average annual temperature by the end of the 21st century is 2.8–4.9 °C [7]. In the context of global environmental change, the aridification of alpine soils, especially alpine wetlands and meadows, is becoming increasingly serious as a result of environmental changes leading to warming, persistent drying, and changes in the pattern of alternating precipitation and seasonal freezing and thawing [8,9,10]. Aridification of alpine soils results in a series of changes, such as alterations in the activity and dynamics of SOC, as well as the transformation of SOC sources and sinks [10,11].

Soil dissolved organic carbon (DOC) is an important component of the soil organic carbon pool; although it generally accounts for only 1% to 5% of the total soil organic carbon, DOC has the most vigorous metabolic activity of the chemical substances in soil organic matter and plays a crucial role in the accumulation, decomposition, and transformation of soil organic matter [12]. DOC usually refers to water-soluble organic mixtures (including organic acids, carbohydrates, humic substances, amino sugars, polyphenols, etc.) that can pass through a 0.45 filter membrane and contain a variety of functional groups [13,14]. Soil DOC is replenished and controlled mainly by carbon inputs from plant litter, root secretions, soil organic matter, and microbial biomass [15,16] The concentration of DOC in soil water at any given time represents a balance between generation processes (e.g., microbial decomposition, root exudation, litter leaching, and desorption) and removal processes (e.g., flushing, microbial consumption, and chemisorption) [17]. Climatic conditions (i.e., changes in temperature and precipitation) are important factors influencing changes in DOC [18,19,20]. However, studies have shown that temperature changes have not yet caused fundamental changes in alpine soil temperature, but soil aridification, soil degradation, and meadow desertification have caused soil moisture degradation and thus affected the release of DOC from alpine soils [21,22,23].

Hydrological conditions play important roles in organic carbon accumulation and organic carbon dynamics in alpine soils, controlling the mobilization, transformation, and leaching of soil DOC [24,25]. Researchers have shown that aridification of alpine soils on the Tibetan Plateau increases ecosystem respiration (especially soil respiration and microbial respiration) and decreases soil organic carbon (SOC) and soil microbial biomass C and N [10,26]. In addition, aridification affects the soil DOC content. When the soil is in arid conditions, the low soil water content affects DOC desorption and dissolution [14]. Moreover, the diffusion of dissolved organic carbon is impeded, and microbial and enzyme activities are reduced, which affects the input processes of soil DOC (decomposition and transformation of SOC and plant litter) [27,28]. Peng et al. [29] reported that drought reduced the DOC content of forest soils by 9.24%, and this effect increased with increasing drought intensity and average annual temperature.

Soils in the alpine region of southwest China have experienced persistent aridification in recent decades, but in situ studies on the effects of aridification on SOC release are lacking. To further explore the influence of water-related changes on DOC release from alpine soil in the context of global environmental changes, in this study we collected undisturbed soil columns of alpine soil for short-term laboratory incubation at different depths (0–5 cm, 5–10 cm, 10–15 cm) under two different moisture contents, namely field capacity (FC) and saturated water content (SWC). Moreover, in situ incubation experiments were conducted under three natural moisture gradients. The objective of this study was to determine the response of DOC in alpine soils to moisture conditions and soil depth, which could provide reference data for the study of the dynamics of the organic carbon pool in alpine soils in the context of global environmental change.

2. Materials and Methods

2.1. Site Description

The alpine sampling location (31°30′34″ N, 102°20′32″ E, 3870 MASL) is located in Majiagou, Xiaojin County, on the northwest plateau of Sichuan. Majiagou trends towards the southwest, and the terrain is higher in the northwest and lower in the southeast. There are several snow peaks higher than 5000 m above sea level in the ditch, and modern glaciers have developed at the end of the ditch, forming unique polar landform characteristics. According to China Meteorological Administration (CMA) data, the area has an average annual temperature of approximately 11 °C and an annual precipitation of 656.8 mm [30], and the main vegetation types are alpine wetlands and alpine shrubs. The soil-water environment in the research area has a natural gradient distribution in the soil at the edge of the alpine lake (Figure 1). The gradients are marked A, B, and C according to the height of the slope from low to high: A corresponds to the edge of the alpine lake, where the water level is high and the soil is basically saturated year round; B corresponds to the alpine grassland in the middle of the slope, where the soil is relatively moist; and C corresponds to a higher slope position, where the soil is relatively dry and the vegetation is mainly alpine shrub grassland (Figure S1).

2.2. Experimental Design

Controlled laboratory incubation experiments: A customized soil core sampler made of stainless steel (7 cm in diameter and 15 cm in length) was used to collect a total of 36 undisturbed soil columns from 0–15 cm in position A. The topsoil, plants, and litter were removed, and the soil columns were divided into 0–5 cm, 5–10 cm, and 10–15 cm layers for 80-day controlled incubation experiments in the laboratory in June 2019. Before incubation, we measured the gravimetric water content (GWC), field capacity (FC), and saturated water content (SWC) of the fresh soil. The physicochemical properties at position A are shown in

Table 1. Physicochemical properties of the soil samples.

Position		Bulk Density (g/cm3)	pH	ORP (mV)	DOC (g/kg)	TOC (g/kg)	TN (g/kg)	GWC (%)
A	0–5 cm	0.80 ± 0.12	5.25 ± 0.02	76 ± 7	0.20 ± 0.00	90.27 ± 10.91	7.82 ± 0.85	89.71 ± 10.92
	5–10 cm	0.78 ± 0.04	5.14 ± 0.04	113 ± 5	0.17 ± 0.00	65.67 ± 0.59	6.01 ± 0.14	78.04 ± 4.52
	10–15 cm	1.00 ± 0.15	5.25 ± 0.05	147 ± 9	0.18 ± 0.00	53.94 ± 1.86	4.72 ± 0.22	69.15 ± 4.44
B	0–5 cm	0.90 ± 0.01	5.37 ± 0.01	251 ± 13	0.25 ± 0.00	75.33 ± 5.28	6.99 ± 0.70	81.87 ± 13.36
	5–10 cm	0.73 ± 0.05	5.49 ± 0.01	265 ± 12	0.28 ± 0.00	57.54 ± 3.51	5.22 ± 0.28	72.43 ± 3.59
	10–15 cm	0.93 ± 0.04	5.00 ± 0.04	297 ± 9	0.23 ± 0.00	70.40 ± 3.63	6.61 ± 0.31	66.88 ± 1.05
C	0–5 cm	0.94 ± 0.01	5.61 ± 0.02	333 ± 3	0.27 ± 0.00	97.39 ± 10.08	9.06 ± 1.04	76.78 ± 13.36
	5–10 cm	0.83 ± 0.02	5.41 ± 0.03	316 ± 14	0.19 ± 0.00	59.36 ± 1.49	5.56 ± 0.18	69.85 ± 6.09
	10–15 cm	0.75 ± 0.02	5.17 ± 0.03	311 ± 13	0.20 ± 0.00	64.50 ± 0.85	5.94 ± 0.22	59.09 ± 5.15

Notes: Value = mean ± SE, ORP = oxidation-reduction potential, DOC = dissolved organic carbon, TOC = total organic carbon, TN = total nitrogen, GWC = gravimetric water content.

. The soil moisture was then adjusted to FC (the FC values of the 0–5 cm, 5–10 cm, and 10–15 cm soil layers at position A were 93.97%, 85.33%, and 76.31%, respectively) and SWC (the SWC values of the 0–5 cm, 5–10 cm, and 10–15 cm soil layers at position A were 102.74%, 89.98%, and 80.56%, respectively) with distilled water, and eighteen replicates for each treatment were prepared. After sectioning, the undisturbed soil columns were placed in beakers, the soil water content was adjusted with ultrapure water, and, after water equilibrium, the samples were incubated in the dark at 4 °C in a constant-temperature incubator to simulate the environmental conditions of soils in alpine regions. Samples were taken at 0–5 cm, 5–10 cm, and 10–15 cm on the 0th, 5th, 10th, 20th, 40th and 80th days, respectively, three replicates were collected at each depth for each time point, and all collected samples were stored at 4 °C for analysis.

Four hundred eighty–day in situ incubation experiments: A custom-made soil core sampler made of stainless steel (7 cm in diameter and 15 cm in length) was used to collect 0~15 cm soil columns from Positions A, B, and C in June 2019. The physicochemical properties at Positions A, B, and C are shown in Table 1. The soil columns from position A were placed in pits created by the removal of soil columns from position A (A-A), position B (A-B), and position C (A-C) for the 480-day exchange incubation experiments to simulate the short-term process of soil drying. Twelve groups of replicates were set up for each position, for a total of 36 soil columns. Soil columns were collected on the 0th, 120th, 360th, and 480th days for analysis. Three replicate samples were collected from each position, and the soil column samples were sealed with parafilm and stored at 4 °C. The mean annual air temperature is 11 °C, and the mean maximum (August) and minimum (January) air temperatures are 26 °C and −8.0 °C, respectively. The annual precipitation is 786.7 mm.

2.3. Analysis Methods

Soil physical and chemical indicators. The pH of all the soil samples was determined via a pH meter (PHS-25, Shanghai Rex, Shanghai, China). The soil oxidation reduction potential (ORP) was determined via a redox potential meter (TR-901, Shanghai Rex). The soil bulk density was determined via the cutting-ring method. The soil OWC, FC, and SWC were determined via the drying method. First, the soil samples were collected via steel cylinders and weighed ($m_1$). The steel cylinders were saturated in ultrapure water for 8 h to a constant weight ($m_2$). Second, the infiltrated soil samples were placed on dry sand for 48 h, and then weighed, and the weights were recorded ($m_3$). All the undisturbed soil samples, along with steel cylinders, were subsequently weighed ($m_4$) after being oven dried (105 °C) until a constant weight was reached (>24 h). Finally, the steel cylinders were cleaned, dried, and weighed ($m_0$). The details of the method were given by Zhang et al. [31] and Yi et al. [32].

$$OWC={\displaystyle \frac{m_1-m_4}{m_4-m_0}}\times 100\%$$

(1)

$$FC={\displaystyle \frac{m_3-m_4}{m_4-m_0}}\times 100\%$$

(2)

$$SWC={\displaystyle \frac{m_2-m_4}{m_4-m_0}}\times 100\%$$

(3)

Extraction and analysis of soil DOC solutions. The soil DOC was extracted using Jones’ [33] method. Fresh soil samples (10 g) were put into a 100 mL centrifuge tube, and 2 mol/L KCl solution was added at a solid–liquid ratio of 1:5. After complete shaking, the samples were extracted by shaking at 200 r/min at 10 °C for 1 h. Subsequently, the suspension was centrifuged at 8000× g (8459 r/min) at 4 °C for 10 min. The supernatant was filtered through a 0.45 μm microporous membrane (PVDF, Millipore, Burlington, MA, USA). The filtrate was the DOC solution. An elemental total organic carbon analyzer (Vario DOC Elementar, Germany) was used to measure the concentration of organic carbon in the soil DOC solution and calculate the cumulative release of soil DOC.

The calculation formula for soil DOC release E(i) is as follows:

$$E\left(i\right)=C\left(i\right)V/1000m$$

(4)

where C(i) is the concentration of organic carbon in the DOC solution at sampling time i (mg/L), V is the volume of the DOC solution (mL), and m is the soil mass (g).

The calculation formula for the cumulative release of soil DOC ε(n) is:

$$\epsilon \left(n\right)={\sum }_{i=1}^{n}E\left(i\right)$$

(5)

Spectral characterization of soil DOC. A simultaneous absorbance and three-dimensional fluorescence spectrometer (Aqualog^®, HORIBA, Shanghai, China) was used for determination of soil DOC (filtered through a 0.45 μm microporous membrane) via ultraviolet-visible absorption spectroscopy and three-dimensional fluorescence spectroscopy. The instrument was automatically calibrated by scanning the spectrum. The scanning wavelength range was 240~550 nm for the excitation wavelength (${\mathsf{\lambda }}_{{\mathrm{E}}_{\mathrm{x}}}$) and 214~621 nm for the emission wavelength (${\mathsf{\lambda }}_{{\mathrm{E}}_{\mathrm{m}}}$); the interval was 3.0 nm, and the slit width was 2.5 nm. Millipore ultrapure water was used as the blank. After the measurements, blanks were subtracted from the system, and Raman and Rayleigh scattering were removed.

Data processing and analysis. The characteristic parameters a(355), SUVA₂₅₄, SUVA₂₆₀, and SR were calculated from the soil DOC ultraviolet-visible absorption spectrum data. The relative concentration of CDOM in soil is typically quantified using the absorption coefficient a(355) (m⁻¹) at 355 nm determined by nonlinear regression analysis. SUVA₂₅₄ and SUVA₂₆₀ are the ratios of the absorption coefficients of the ultraviolet spectrum at 254 nm and 260 nm, respectively, to the mass concentration of DOC (mg/L). The specific calculations are shown in

Table 2. Basic information of spectral characteristic parameters.

Spectral Parameters	Computational Formula	Formula Parameters
a(355)	$a\left(\mathsf{\lambda }\right)=2.303D\left(\mathsf{\lambda }\right)/l$	a(λ) is the absorption coefficient (m−1), D(λ) is the absorbance at a wavelength of λ, l is the optical path (m)
SUVA254	${SUVA}_{254}=a\left(254\right)/DOC$	DOC is dissolved organic carbon (mg/L)
SUVA260	${SUVA}_{260}=a\left(254\right)/DOC$	/
SR	$S_R=S_{275-295}/S_{350-400}$	$S_{275-295}$ and $S_{350-400}$ are the slope of the absorption spectra in the 275~295 nm and 350~400 nm bands, respectively.

. The larger the SUVA₂₅₄ is, the higher the aromaticity of the DOC, and the greater the degree of humification. The larger the SUVA₂₆₀ is, the greater the content of hydrophobic organic components in the DOC. The SR is calculated by dividing the 275~295 nm band absorption spectrum slope by the 350~400 nm band absorption spectrum slope, which reflects changes in the source composition and structure of soil CDOM [34]. The larger the SR is, the lower the molecular weight of DOC; a low molecular weight indicates that DOC is mainly decomposed by endogenous microorganisms [35].

For fluorescence regional integration (FRI), the method of Chen et al. [36] was used to divide the three-dimensional fluorescence spectra of soil DOM into five characteristic peak regions, characterized as follows (

Table 3. Characteristics of five common fluorescent peaks in three-dimensional fluorescence.

Fluorescence Peak	Fluorescence Peak Excitation and Emission Wavelength Range
Peak I	${\mathsf{\lambda }}_{{\mathrm{E}}_{\mathrm{x}}}$= 200–250 nm, ${\mathsf{\lambda }}_{{\mathrm{E}}_{\mathrm{m}}}$= 280–330 nm
Peak II	${\mathsf{\lambda }}_{{\mathrm{E}}_{\mathrm{x}}}$= 200–250 nm, ${\mathsf{\lambda }}_{{\mathrm{E}}_{\mathrm{m}}}$= 330–380 nm
Peak III	${\mathsf{\lambda }}_{{\mathrm{E}}_{\mathrm{x}}}$= 200–250 nm, ${\mathsf{\lambda }}_{{\mathrm{E}}_{\mathrm{m}}}$= 380–550 nm
Peak IV	${\mathsf{\lambda }}_{{\mathrm{E}}_{\mathrm{x}}}$= 250–450 nm, ${\mathsf{\lambda }}_{{\mathrm{E}}_{\mathrm{m}}}$= 280–330 nm
Peak V	${\mathsf{\lambda }}_{{\mathrm{E}}_{\mathrm{x}}}$= 250–450 nm, ${\mathsf{\lambda }}_{{\mathrm{E}}_{\mathrm{m}}}$= 380–550 nm

Notes: Peak I, associated with aromatic proteins like tyrosine; Peak II, linked to aromatic proteins and biochemical oxygen demand (BOD₅); Peak III, resembling fulvic acid; Peak IV, indicative of microbial byproducts such as tryptophan; and Peak V, akin to humic substances, specifically macromolecular humic acids. ${\mathsf{\lambda }}_{{\mathrm{E}}_{\mathrm{x}}}$ is the excitation wavelength, and ${\mathsf{\lambda }}_{{\mathrm{E}}_{\mathrm{m}}}$ is the emission wavelength.

): Peak I, associated with aromatic proteins such as tyrosine; Peak II, linked to aromatic proteins and biochemical oxygen demand (BOD₅); Peak III, resembling fulvic acid; Peak IV, indicative of microbial byproducts such as tryptophan; and Peak V, akin to humic substances, specifically macromolecular humic acids. Origin Pro 9.0 was used to integrate a specific fluorescence region (ϕi), which represents the cumulative fluorescence intensity of organic matter with similar properties to reflect the relative amount of organics with a defined structure in that region.

$${\mathrm{\varnothing }}_{\mathrm{i}}={\int }_{{\mathrm{E}}_{\mathrm{x}}}{\int }_{{\mathrm{E}}_{\mathrm{m}}}\mathrm{I}\left({\mathsf{\lambda }}_{{\mathrm{E}}_{\mathrm{x}}}{\mathsf{\lambda }}_{{\mathrm{E}}_{\mathrm{m}}}\right)\mathrm{d}{\mathsf{\lambda }}_{{\mathrm{E}}_{\mathrm{x}}}\mathrm{d}{\mathsf{\lambda }}_{{\mathrm{E}}_{\mathrm{m}}}$$

(6)

where $\mathrm{I}\left({\mathsf{\lambda }}_{{\mathrm{E}}_{\mathrm{x}}}{\mathsf{\lambda }}_{{\mathrm{E}}_{\mathrm{m}}}\right)$ is the excitation wavelength range and emission wavelength range for the specific fluorescence peak.

Data integration was performed in Excel, data processing and analytical applications (ANOVA) were performed in SPSS 22.0 and Origin Pro 9.0, and graphs were produced in Origin Pro 9.0. In addition, a distribution map of the study area on the northwestern Plateau of Sichuan with the approximate locations of the sampling sites was produced in ArcMap 10.8.

3. Results

3.1. Release Characteristics of DOC in Response to Moisture Changes at Different Soil Depths

To determine the effects of climate change on the role of alpine soils as organic carbon sinks, we first aimed to determine the characteristics of soil DOC release at different soil depths during laboratory and in situ incubation. Both the laboratory and in situ results revealed that the cumulative DOC release from the alpine soils decreased with decreasing moisture content (Figure 2). After 480 days of in situ incubation, the cumulative release of DOC from the surface layer (0–5 cm) of the soil in groups A-B and A-C decreased by 17.69% and 15.82%, respectively, relative to that in group A. After 80 days of laboratory incubation, the cumulative release of DOC from the surface layer (0–5 cm) was reduced by 36.17% under FC compared with that under SWC. In addition, changes in soil depth affected DOC release from alpine soil. The study showed that the cumulative release of DOC was greater in the surface layer of soils than in subsurface soils, and the cumulative release of DOC in surface soil exhibited the greatest response to changes in water content; this may be because the surface soil contains more SOC. The SOC contents in the 0–5 cm, 5–10 cm, and 10–15 cm soil layers at position A were 90.27 g/kg, 65.67 g/kg, and 53.94 g/kg, respectively.

As the incubation period increased, the cumulative release of DOC under in situ incubation with different moisture contents gradually increased, whereas the cumulative release of DOC under laboratory incubation gradually increased and then stabilized. The cumulative release of DOC from in situ incubated soil samples (0–5 cm soil reached 1.40 ± 0.13 g/kg, 5–10 cm soil reached 1.27 ± 0.01 g/kg, and 10–15 cm soil reached 1.22 ± 0.05 g/kg) was lower than that from laboratory soil samples (0–5 cm soil reached 1.93 ± 0.43 g/kg, 5–10 cm soil reached 1.53 ± 0.19 g/kg, and 10–15 cm soil reached 1.53 ± 0.13 g/kg), indicating that, under aridification conditions, alpine soils in their natural state are able to resist the influence of moisture changes on DOC release within a certain period.

3.2. Absorption Spectral Characteristics of Soil DOC Under Different Water Contents

To further reveal the effects of moisture changes on SOC fractions, we analyzed the UV-visible absorption spectral features of soil DOC. a(355) is directly proportional to the content of CDOM in the soil. During incubation, the values of the DOC UV-visible absorption spectral parameters of the alpine soils fluctuated continuously, changing with soil depth and moisture (Figure 3). Both the laboratory and in situ results revealed that a(355), SUVA₂₅₄, and SUVA₂₆₀ were greater in surface soil than in subsurface soil, indicating that the CDOM content, the hydrophobic organic fraction content, and the degrees of DOC aromaticity and humification in the alpine surface soils were slightly greater than those in the subsurface soils. Moreover, a(355), SUVA₂₅₄, and SUVA₂₆₀ were greater in soils with low moisture contents than in soils with high moisture contents under laboratory incubation. However, a(355), SUVA₂₅₄, and SUVA₂₆₀ were greater at position A-B than at positions A-A and A-C during in situ incubation. During in situ incubation, the subsurface soil (5–15 cm) SR fluctuated between 5.3 and 8.39, and the differences in the SR were small under different moisture conditions (p > 0.05), indicating that the DOC in alpine soil under different water conditions was mainly small molecule organic matter derived mainly from the autogenous decomposition of endogenous soil microorganisms. These results revealed that, against the background of global environmental changes, aridification will not change the source of DOC in alpine soil, but it will increase the amount of CDOM in DOC in alpine soil, especially the content of hydrophobic organic components, and increase the aromaticity and degree of humification of DOC.

3.3. Three-Dimensional Fluorescence Regional Integration (FRI) of DOC Under Different Water Contents

The three-dimensional fluorescence spectrum of DOC in alpine soil showed five fluorescence peaks, and three-dimensional fluorescence region integration was performed (Figure 4). Among the five fluorescence peaks of alpine soils with different water contents, the Peak II and Peak III components had the highest FRIs, indicating that aromatic proteins and fulvic acids are the main components of DOC in alpine soil. The FRIs of the Peak I, Peak II, and Peak IV components in the surface soil were greater than those in the subsurface soil, and the FRIs of the Peak III and Peak V components were less than those in the subsurface soil. These findings indicate that the content of tyrosine and tryptophan components in surface soil DOC is greater than that in the subsurface layer and that the content of humus components such as fulvic acid and humic acid is lower than that in the subsurface layer. The FRI volume of each fluorescence peak fluctuated over time but showed an overall decreasing trend.

Moreover, there are differences in the FRIs of alpine soils under different moisture conditions. In the in situ incubation experiment, the FRIs of the Peak I, Peak II, and Peak IV components in the surface soil decreased with increasing water content, indicating that aromatic proteins and tryptophan decreased with increasing water content, especially the tyrosine fractions. During in situ incubation, the FRIs of the tyrosine components in the surface soil at locations A-B and A-C increased by 51.70% and 63.31%, respectively, compared with those at location A. The FRIs of the peak III and peak V components were greatest at position A-B, indicating that fulvic and humic acids were the most abundant at position A-B. The laboratory incubation results further validated the in situ experimental results. After 80 days of incubation in the laboratory, compared with the results under SWC, the FRI volumes of the tyrosine component of the 0–5 cm, 5–10 cm, and 10–15 cm soil layers under FC increased by 39.59%, 9.31%, and 37.17%, respectively. Both the laboratory and in situ incubation results indicate that, in the context of global environmental change, aridification has led to a decrease in water content and an increase in the content of DOC fluorescent fractions, especially tyrosine-like fractions, in alpine soils.

4. Discussion

4.1. Response of Alpine DOC to Environmental Changes

High-elevation regions such as the Tibetan Plateau have high SOC density and high climate sensitivity, and climate change is a major driver of organic carbon, especially DOC, in alpine soils [17,37]. Our study revealed that aridification has led to a decrease in soil DOC release from Majiagou, Xiaojin County, on the northwest plateau of Sichuan (Figure 2). Combined with related studies from China and elsewhere, these results suggest that aridification will unavoidably cause SOC emission, and that the carbon stored in alpine soils will change from a sink to a source to be released into the atmosphere. Aridification affects soil carbon balance and SOC dynamics, thus influencing the release of DOC [38]. Aridification affects the rate of organic carbon decomposition, thereby increasing SOC mineralization and reducing DOC output [39]. Sowerby et al. [40] demonstrated that the total soil respiration rate increased by 40% after drought treatment, allowing the carbon stored in the soil to be released from sinks to the atmosphere. Sowerby [41] reported that drought treatment reduced the export of DOC by 9%, decreasing it to 20 g C m⁻² year⁻¹, and caused a shift in ecosystem carbon dynamics from a net carbon sink of 126 g C m⁻² year⁻¹ to a net carbon source of 33 g C m⁻² year⁻¹. In addition, aridification can also affect plant growth, thereby altering the input of soil dissolved organic carbon [42,43,44].

The distribution of DOC varies across the soil profile due to a variety of factors; thus, the response of DOC in alpine soils to climate change is different at different depths [45,46]. In this study, we found that the cumulative release of DOC from surface soil (0–5 cm) was greater than that from subsurface soil (5–15 cm) (Figure 2). The reason may be that surface soil is significantly affected by plant and atmospheric carbon cycles, and plant biomass input causes the surface soil of alpine soils to accumulate a large amount of organic carbon, resulting in high soil activity and high DOC release [47,48,49]. Moreover, the soil organic matter (SOM) in the 0–10 cm layer was more reactive to precipitation than was the SOM in the 10–40 cm layers, and an increase in precipitation promoted the construction of soil microbial communities, thus accelerating the turnover rate of SOM in the surface soil [1,50,51] and the release of DOC from the surface soil. With increasing soil depth, decreases in the amount of litter and roots, organic carbon inputs, complexity of DOC molecules in the subsoil [52,53], and microbial activity and extracellular enzyme activity [54] are observed.

4.2. Influence Mechanism of Moisture on DOC

Moisture is an important factor influencing the release of DOC. The findings of this study revealed that the cumulative release of DOC from alpine soil decreased with decreasing moisture content (Figure 2). As soil moisture decreases, the soil organic matter environment changes from a reduced state to an oxidized state, and the soil microbial and aeration conditions change drastically, exacerbating decomposition of SOM and leading to a decrease in the cumulative release of organic carbon [55]. However, after 480 days of in situ incubation, the cumulative release of DOC from the 0–5 cm soil was greater in group A-C than in group A-B, which may be due to the high concentration of organic carbon in the 0–5 cm soil surrounding the soil columns at position A-C. With the aridification of soil columns at position A-C, the clay content of the soil decreased and the soil became looser, increasing the influx of DOC from the surrounding soil to the soil columns [56]. Previous studies have shown that the DOC content is related to the SOC content, pH, and aggregate stability. Tipping et al. [57] revealed that soils with high organic matter content have higher DOC. Aridification also affects soil pH, and studies have shown that degraded soils have significantly higher pH values than nondegraded soils do [58,59]. At high pH, the acidic portion of DOC is easily neutralized with other substances such as calcium and magnesium compounds, resulting in a lower soil DOC content [60,61]. A lower soil water content also accelerates the destabilization of soil aggregates, which promotes microbial decomposition of soil DOC and does not result in soil organic carbon sequestration [62,63]. In addition, soil moisture affects soil carbon and nitrogen deposition and organic carbon storage by affecting plant growth and soil microbial activities [64,65,66,67]. Studies have shown that microbial biomass generally increases in moist soils [68,69], while the rates of carbon and nitrogen mineralization are significantly and positively correlated with soil enzyme activity and microbial biomass (p < 0.01) [70].

Moisture changes affect not only the stocks of DOC but also DOC fractions [71]. From the UV–visible absorption spectra of soil DOC (Figure 3), the degree of aromaticity and humification in the alpine soil DOC increased with decreasing soil water content, suggesting that the stability of soil CDOM decreased with increasing water content, which is similar to the findings of previous studies [72,73]. The decrease in soil aromaticity and humification with increasing water content may be due to high soil moisture, which reduces the soil pore space and creates an anaerobic environment, inhibiting the growth and activity of plants and soil microorganisms and resulting in slow humification of organic matter [38,69,74]. According to the FRI results (Figure 4), the fluorescence components, especially the tyrosine components, of alpine soil DOC increased as decreasing soil water content decreased, indicating that the aromaticity and humification of alpine wetland soil DOC increased with decreasing soil moisture, which was in agreement with the spectral absorption characteristics. The spectral characteristic parameters of the surface soil at position A-B were significantly greater than those in other areas, indicating that the degree of humification at position A-B was higher than that at other locations, possibly because position A-B has stable and suitable moisture conditions and is suitable for the growth and activities of soil microorganisms, thus promoting the process of organic matter humification [72,75,76].

This study has several limitations that can be improved upon in further studies. First, this study was based on short-term incubation in situ and in the laboratory and focused on the characteristics and effects of DOC in alpine soil at a certain spatial scale over a short period. However, the response of alpine DOC to climate change is a long-term process that has lasted several decades; therefore, further studies on the long-term effects of organic carbon dynamics in alpine soils in the context of environmental change may further validate our conclusions. In addition, our study focused mainly on the changes in the cumulative release of alpine DOC and its fluorescence components, while DOC components are complex and diverse. Further study of the process and mechanism of the conversion of alpine soil organic carbon into DOC is conducive to enhancing comprehension of the environmental effects and response mechanisms of alpine organic carbon.

5. Conclusions

Global environmental changes have led to the warming and drying of soil, especially alpine soil. We gained insight into the effects of soil water content changes on SOC release through in situ and laboratory incubations of soils at different depths from alpine soils in the northwestern Sichuan Plateau. The laboratory findings provide further in situ evidence. The cumulative release of DOC from alpine soils tended to decrease with decreasing water content, the cumulative release of DOC from the 0–5 cm soil layer in groups A-B and A-C decreased by 17.69% and 15.82%, respectively, relative to that in group A, and the cumulative release of DOC decreased by 36.17% under FC compared with that under SWC. The spectral characteristics of the soil DOC indicated that the aromaticity and degree of humification of the DOC and the stability of the DOC decreased with increasing moisture. Aromatic proteins and fulvic acids are the main components of DOC in alpine soil, and the content of DOC fluorescent fractions varies with soil water content. Aridification leads to changes in the DOC content and structure of alpine soils, but the specific processes and metabolic pathways involved in DOC transformation are still unclear. An in-depth study of the dynamics of alpine DOC in the context of environmental change is highly important for predicting the source-sink effect of carbon in alpine ecosystems. Subsequently, we could incorporate more advanced methods, such as multiomics joint analysis, to conduct an in-depth investigation into the transformation process of soil DOC.