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Review

Carbonate Mineral Dissolution and Its Carbon Sink Effect in Chinese Loess

by
Mingyu Shao
1,2,
Muhammad Adnan
3,4,
Liankai Zhang
5,6,*,
Pengyu Liu
1,2,
Jianhua Cao
1,2 and
Xiaoqun Qin
1,2
1
Institute of Karst Geology, CAGS/Key Laboratory of Karst Ecosystem and Rocky Desertification, Ministry of Natural Resources, Guilin 541004, China
2
Ministry of Natural Resources, Guangxi Key Laboratory of Karst Dynamics, Guilin 541004, China
3
State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China
4
University of Chinese Academy of Sciences, Beijing 100049, China
5
Kunming Comprehensive Survey Center of Natural Resources, China Geological Survey, Kunming 650111, China
6
Technology Innovation Center for Natural Ecosystem Carbon Sink, Ministry of Natural Resources, Kunming 650111, China
*
Author to whom correspondence should be addressed.
Land 2023, 12(1), 133; https://doi.org/10.3390/land12010133
Submission received: 9 November 2022 / Revised: 28 December 2022 / Accepted: 28 December 2022 / Published: 31 December 2022
(This article belongs to the Special Issue New Insights in Soil Quality and Management in Karst Ecosystem)
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Review Reports Versions Notes

Abstract

:
The relationship between the source and sink of atmospheric CO2 has always been a widely discussed issue in global climate change research. Recent studies revealed that the chemical weathering of carbonate rocks contributed to 1/3 (~0.5 Pg C/yr) of the missing carbon sinks (MCS) globally, and there are still 2/3 of MCS (~0.5 Pg C/yr) that need to be explored. As one of the main overburdened parts of the earth, loess is one of the important driving factors for atmospheric CO2 consumption. Here, we elaborated on the dissolution process and the carbon sink effect from carbonate and silicate minerals in loess. The relationship between carbonate dissolution and carbon source/sink is elucidated, and the mechanism of carbon sink formation from secondary carbonates in loess is clarified. Additionally, the commonly used methods for the identification of primary and secondary carbonates are summarized, and the methods for the study of loess carbon sinks and the influencing factors of loess carbon sinks are also revealed. Based on the research results and progress interpretations, the prospects of loess carbon sinks are discussed to provide a scientific basis for further research on loess carbon sinks.

1. Introduction

The concentration of CO2 in the atmosphere increased from 295 ppmv before the industrial revolution to 417 ppmv in 2022 [1], increasing the global average temperature by 1.09 °C [2]. A key issue in global climate change research is the budget imbalance of the atmospheric carbon cycle [3,4,5,6,7]. Recent studies showed that the residual amount of the terrestrial carbon sink is 1.8~1.9 Pg C/a (1 Pg = 1015 g) [8,9], in which the carbonate weathering sink contributes to ~0.5 Pg C/a, accounting for 1/3 of missing carbon sinks (MCS) globally [10,11]. It can be calculated that there is still above 0.5 Pg of carbon that is missed in terrestrial ecosystems. Therefore, balancing the global carbon budget becomes an important task for the next stage of global carbon cycle research.
The process of carbonate weathering responds promptly to climate changes due to its rapid weathering kinetics [12,13]. It can reach reaction equilibrium within 3 h under experimental conditions [14], and its dissolution rate is ~15 times faster than that of silicate rocks. On a short timescale (less than a millennium), carbonate weathering is the main weathering carbon sink on the planet, accounting for 94% of the total rock-weathering carbon sink [10,15]. Coupled with the carbonate rock weathering, the biological carbon pumping (BCP) effect is the main contributor to long-term stable carbon sinks (a more than ten-thousand-year timescale) in the terrestrial carbon cycle [11,16]. Organic carbon transformed from the inorganic stage under BCP is stored in lakes, rivers, or oceans and then transformed into stable carbon [17,18].
On a global scale, about 6~10% of the land surface is covered by loess [19,20,21], with the largest area of loess being found in China (640,000 km2 and a thickness >250 m). Because of this, doing loess research in China has several natural advantages. The overall weight of Chinese loess is 1.89~9.47 × 104 Pg [20,22]. Carbon in loess includes the pool of organic carbon and inorganic carbon. A soil’s organic carbon and cation exchange capacity are the most important factors determining chemical deposits [23]. An inorganic carbon pool is about 850 PgC and 2~5 times higher than an organic carbon pool [24]. Due to the alkaline character of loess, recent research has demonstrated that soil with more water and a decrease in temperature can increase the abiotic fixation of CO2 [25]. Soil organic carbon (SOC) stability is a fundamental issue in understanding the evolution of the soil carbon pool, which is influenced by changes in the ecological environment [26]. In addition, studies have shown that with the development of social urbanization, agricultural carbon emissions and carbon absorption will be influenced by the adaptive alterations of farmed land’s input structure and planting structure [27]. Under continual erosion, soil carbon storage on the slopes of the Loess Plateau reached a new equilibrium state, and erosion resulted in a net sink of atmospheric CO2 on the side of degraded hillsides [28]. In addition, the inorganic carbon pool has amounted to 2~8 times the carbon emissions of fossil fuels since the industrial revolution [29]. Chinese loess has a high content of carbonate minerals (10~20%) [30,31,32,33]. According to provenance research, loess carbonate minerals are divided into primary carbonate minerals (PCM) and secondary carbonate minerals (SCM) [34]. PCM are eolian sedimentation from a faraway source area and disposed of with loess dust in a detrital form. SCM are also called pedogenic carbonates, which are generated during accumulation and soil development. Parent materials are the sedimentation of CaCO3 after the chemical weathering of primary CaCO3 and calcium-bearing silicate minerals. Its formation environment is arid to semi-humid climatic conditions with a soil pH > 7 and annual precipitation <750 mm [35]. Our model is based on the calcium and carbon cycle in loess, showing the dissolution of carbonate and silicate minerals, as shown in Figure 1. During PCM dissolution, atmospheric/soil CO2 will be absorbed, and then the carbonate minerals will crystallize and deposit as SCM. In this process, the same amount of CO2 is consumed and released. Therefore, it is a carbon shift but not a carbon sink, as reported by [36]. The silicate minerals will absorb 2 mol CO2 during the dissolution process, one will be fixed in the SCM, and one will be released into the soil or atmosphere. According to current carbon cycle theory, this is a net carbon sink process [37].
In these two processes, the dissolution rate of PCM in loess is too low to be neglected in the carbon cycle of loess [30]. The dissolution rate of silicate minerals is much more than one million orders of magnitude when compared with carbonate minerals [38,39]. Considering the short timescales that humans are concerned with (e.g., <1000 years), the amount of carbon sinks generated by both processes is weak [10,40,41].
Therefore, as the main carbon pool in loess, SCM may be an important contributor to the carbon sink in loess during its dissolution. Based on [42], it is suggested that the dissolution of the SCM in loess can consume atmospheric CO2 and the CO2 generated by SOC decomposition, which would, to a certain extent, influence atmospheric CO2 regulation and the global carbon cycle. The CO2 in the deep layer (under 3 m) of Chinese loess displayed a high concentration of 1434~2459 ppm, which is 3.8~6.5 times the atmospheric CO2 [43,44]. It participates in SCM dissolution and generates dissolved inorganic carbon (DIC), which will enter the groundwater. The DIC is a carbon sink not only on a short timescale (less than a thousand years) but also on a long-term scale (more than 100,000 years), according to [10,11]. In addition, loess is mostly a loamy soil type with a high silt content and lighter bulk density (1.0~1.3 g/cm3), with high infiltration performance. The infiltration and leakage rates can reach 0.5~1.35 mm/min and 15~28.5 mm/min, respectively [45]. The high water infiltration rate allows most of the precipitation to be stored in deep soil, promoting the dissolution of SCM. However, the dissolution process of inorganic carbonate and its carbon sink effect is rarely discussed in the literature.
The Loess Plateau is one of the most ecologically fragile systems on Earth, with serious soil and water loss, low vegetation coverage, a loose structure of the soil, and broken terrain [20,46]. The Chinese government has adopted various measures to recover this delicate environment, such as the Natural Forest Protection Project (NFPP), the Grain to Green Project (GTGP), and the Ecological Construction Program (ECP). These projects have optimized the land utilization structure causing land-use changes in the loess region, reduced arable land, and increased natural vegetation, especially in forest areas. It also improved ecological quality and carbon sequestration capacity, as [47] reported. According to [48], it was confirmed that ~260 million tons of carbon were stored from 2000 to 2010, profiting from the gradual vegetation improvement in the Chinese Loess Plateau.
Like the karst carbon sink [49], loess is also an essential part of the global carbon cycle due to its high content and rapid dissolution of carbonate minerals.
The weathering of carbonate minerals in loess causes the surrounding water body to have similar geochemical characteristics to the karst aquatic ecosystem, i.e., high DIC, high Ca2+, and high pH (Table 1); a notable example is the Yellow River Basin in China, where drought and high CaCO3 content lead to 90% of bicarbonate being formed by carbonate weathering [50,51]. This indicates that in the carbonate-rich loess region, the contribution of carbonate minerals to the total river weathering load dominates [50,51]. The latest research shows that when the annual average rainfall is lower than 1200 mm, the soil carbonate content is >5%, and the weathering load of the watershed is mainly due to the weathering contribution of soil carbonate [16]. Meanwhile, compared with carbonate rocks, minerals in loess are dispersed, finely grained, and have a large specific surface area. For this reason, their carbon sink effect is remarkable. Therefore, the global area characterized by carbonate weathering control (carbonate + non-carbonate-rich soils) expanded to 33.8% of the surface [16]. However, loess carbon cycle processes still need to be further studied. Three main scientific problems need to be solved urgently: (1) How strong is the dissolution in loess? (2) What is the migration process of carbon between air–soil/minerals–water? (3) How can we estimate the carbon sink fluxes model in the loess watershed? In this paper, the relationship between carbonate mineral dissolution and the carbon cycle, the identification methods of PCM and SCM, the study methods of the loess carbon cycle, and the accuracy of the loess carbon sink are overviewed. The future research direction on the loess carbon cycle has also been prospected. This paper aims to provide a systematic summary and an outlook on the future of the loess carbon cycle research.

2. Materials and Methods

2.1. Identification Methods of PCM and SCM

2.1.1. Stable Isotope

Isotope techniques have been used as geochemical indicators to trace the origin of PCM and SCM. The most commonly used two elements for stable isotope analysis are strontium (Sr) and carbon (C), all being major structural components in carbonate minerals [60,61,62,63,64,65].
PCM in loess come from paleo-marine carbonate formation in the loess source region area (Wen, 1989). The source of Sr in SCM is much more complicated. It includes the Sr dissolved from PCM and the Sr released from calcium-aluminosilicate mineral weathering. The latter has a higher 87Sr/86Sr ratio, which causes the Sr isotope composition to be relatively heavy in SCM. The 87Sr/86Sr ratio in loess depends on the composition proportion of PCM and SCM. The authors in [61] reported that the PCM have a high Sr content and a low 87Sr/86Sr ratio, while SCM always present a low Sr content and a high 87Sr/86Sr ratio. Based on these two end-member components, the following formula (1) can be established to quantify the proportion of PCM and SCM in loess.
(87Sr/86Sr)M = (87Sr/86Sr)S·(1 − X) + (87Sr/86Sr)P·X
where (87Sr/86Sr)M, (87Sr/86Sr)S, (87Sr/86Sr)P are the Sr isotopic compositions of mixed carbonate, SCM, and PCM, and X is the proportion of PCM in loess.
δ13C isotope was used by [65] to identify PCM and SCM. PCM mainly come from the Gobi desert and paleo-marine carbonate formations, and their δ13C value is about 0‰ [66,67]. Nevertheless, the carbon provenance in SCM is a multi-source of organic matter mineralization, plant root respiration, and atmospheric CO2 dissolution [68,69]. Formula (2) can be used to estimate the proportion of PCM and SCM [65]:
δ13Cl = Fp·δ13Cp + Fs·δ13Cs
where Fp and Fs are the proportion of PCM and SCM in loess in which Fp + Fs = 1; δ13Cl is the δ13C of loess (‰); δ13Cp and δ13Cs are δ13C of PCM and SCM.
δ13Cs can be calculated according to Formula (3) [35]:
δ13Cs = Δδ13C + δ13CSOM
Then, Formula (2) can be expressed as:
δ13Cl = Fp·δ13Cp + (1 − Fs)·(δ13CSOM + Δδ13C)
where the δ13CSOM is the carbon isotope of SOC, and Δδ13C is the deviation of δ13C between SCM and SOC. According to [35], it is believed that under different temperature conditions, the Δδ13C of homologous soil is between 13.5‰ and 16.5‰. Therefore, the proportion of PCM (Fp) and SCM (Fs) in Formula (2) can be calculated.

2.1.2. Thermo-Gravimetric Analysis (TGA) and X-ray Diffraction (XRD) Technology Used in Loess Mineral

TGA is a thermal analysis method used to heat samples in a controlled gas environment to measure the relationship between sample mass changes and temperature. It is used widely to study different minerals’ thermodynamic stability [70,71,72,73]. The more commonly used instrument for TGA testing is the TGA-701 (Leco Instrumente GmBH, Moenchengladbachm, Germany). The temperature range of this instrument is set from 20 to 1000 °C, and the ramp rate is 10 °C/min. The TGA-701 thermogravimetric test system has 5 stages of temperature rise and gas exchange procedure, and the first stage is 20~105 °C and the gas environment is compressed air; the second stage is 105~330 °C and the gas environment is oxygen; the third stage is 330~550 °C and the gas environment is compressed air; the fourth stage is 550~615 °C and the gas environment is nitrogen; and the fifth stage is 615~1000 °C and the gas environment is nitrogen. The gas flow rate was 7.0 L/min. Each full and fractionated sample was weighed at 1~2 g and placed in a porcelain crucible for measurement [74].
XRD is a physical method to analyze the structure and phase of minerals using the diffraction effect produced when X-rays interact with mineral crystals. Minerals are mostly in a crystalline state. Since the wavelength of X-rays is close to the atomic distance in mineral crystals, X-rays are diffracted into diffraction patterns with different intensities and angles when interacting with mineral crystals so that the crystal structure and phase of the mineral can be determined. The test instrument for XRD of carbonate minerals was an X’Pert Pro MPD powder X-ray diffractometer from PANalytical, Netherlands. A quantitative amount of the sample powder was taken and pressed vertically into a 1 cm × 2 cm sample slot to ensure that the sample surface was flat and to avoid forming a merit orientation as much as possible. The test conditions of the X-ray diffractometer were CuKα target (n = 1.54056 Å), a working tube voltage and tube current of 40 kV and 40 mV, respectively, 0.01° step, 1.50 s step, and a scanning angle of 20°~35° (2θ) [74]. The TGA combined with the XRD method is technically mature and an effective tool for identifying carbonate minerals in loess [75,76,77]. For example, Liang et al. successfully distinguished between PCM and SCM in two loess profiles, Jingyuan and Gulang, on the northwestern Chinese Loess Plateau using the above method [74,77].

2.1.3. Element Pairings

The pairing partner correlations show significant variability of carbonate minerals in loess. The electron-microprobe is a new in situ provenance identification method in loess to distinguish PCM and SCM. The author of [78] tested the Mn in loess with an electron-microprobe and concluded that Mn is deposited from Mn2+ released from PCM dissolution in loess under oxidizing conditions. As pure minerals, the secondary calcite has no Mn inside. Mn-bearing carbonate in loess mainly comes from PCM. Therefore, the ratios of Mn and Ca or Mg can reflect the provenance of carbonate minerals. The authors of [34] used the element pairings of Mn/Ca and Mg/Ca to identify the PCM and SCM. They concluded that the SCM in loess are characterized by low Mn/Ca and Mg/Ca, while the two-element pairings are high in PCM. Additionally, these element pairings are sensitive to changes in the loess environment. Carbonate’s Mn/Ca and Mg/Ca ratios can determine the purity of secondary carbonate rocks for paleo-environment reconstruction.

2.2. Methods for Loess Carbon Sink Research

2.2.1. Galy Model and Hydro-Chemical Method

The Galy model is a method that uses element ratios of surface water and groundwater to attribute the different weathering end members [79,80,81,82]. It simplifies the end members and their contributions to easily quantify the proportion of different sources [79]. It is expressed in Formula (5). Based on this theory, the Galy model can quantify the contributions of DIC from carbonate and silicate in the loess carbon sink estimation. Several researchers [80,81,82,83] calculated the karst carbon sink in the catchment of the Wujiang River, Xijiang River, Hong (Red) River basin, and Yangzte River, respectively, in China.
[D]riv = [D]atm + [D]hum + [D]eva + [D]car + [D]sil
where the subscript riv stands for the river, atm stands for the atmospheric input source, hum stands for the anthropogenic input source, eva stands for evaporative salt mineral source, the car stands for carbonate mineral source, and sil stands for silicate mineral source.
Hydro-chemical measurement is a common method in carbon sink estimation proposed in 1959 by the author of [84]. By determining DIC (including HCO3−, CO2, and CO32−) at the outlet of the catchment, the total amount of atmospheric CO2 consumption by carbonate or silicate rocks can be calculated. The preconditions for this method are a clear watershed boundary, definitive runoff quantity, and an accurate DIC concentration. Figure 2 shows its basic schematic diagram. The hydro-chemical model is simplified as follows:
R = Z(t) × Q(t) (X + 0.5 Y)/A
F = Z(t) × Q(t) (X + 0.5 Y)
Where R is the atmospheric CO2 consumption per unit area (carbon sink intensity); F is the gross bulk of carbon sink in the catchment (carbon sink flux); Z is the average in situ concentration of DIC in the outlet of the watershed; Q is the average runoff of the watershed; A is the area; and X and Y are the contribution proportions of the silicate minerals and carbonate minerals. The sum of X and Y in Formulas (6) and (7) is 100%, namely, X + Y = 1. The X and Y values are quantified by the Galy model as presented in [79]. The study of carbon transfer in river water using the hydrochemical runoff approach has become more significant nationally and internationally.
Currently, major river basins such as the Amazon River [58,86,87,88,89], Congo River [90,91], Orinoco River [92], Mackenzie River [93,94], and the Yangtze River [95] evaluated watershed rocks. As a result of the CO2 flues consumed by chemical weathering, it is possible to determine the worldwide CO2 fluxes consumed by terrestrial chemical weathering [59,96].

2.2.2. Limestone Tablet Method (LTM)

To evaluate the weathering strength and process of carbonate rock, LTM is a commonly used method [30]. Tablets with precise size and weight were made from the carbonate rocks from the study area. The tablets were put over one hydrological year in various soil strata, as shown in Figure 3. The limestone tablets were taken from pure carbonate rocks of the Devonian Rongxian Formation in Guilin and made into uniform size test pieces. Their average thickness was 4.1 ± 0.19 mm, and the average diameter was 40.0 ± 0.15 mm. The detailed procedures of the tests steps are as follows:
①
First, process the rock sample to standard dimensions.
②
Rinse the test piece with deionized water, then dry it in a regular oven at 105 °C for 12 h.
③
Weigh, with an accuracy of 0.0001 g.
For one hydrological year, limestone tablets were left in the field after removing the test piece and steps ② and ③ were completed again.
To determine the strength of the karst carbon sink, we used the following Formula (8), which considers the dissolution rate.
R = DR × I × Mr(CO2)/Mr(CaCO3)
Among them, R is the carbon sink strength; DR is the dissolution rate of the test piece (mg·cm−2·a−1); I is the carbonate rock purity of the dissolution test piece used in this study, and its value is 0.97; Mr(CO2) is the relative molecular mass of CO2, with a value of 44; and Mr(CaCO3) is the relative molecular mass of CaCO3, with a value of 100.
The erosion quality of the test piece was calculated by likening the weight loss before the pre-and post-field test. In conjunction with the test piece’s surface area, Equation (9) [97] was utilized to estimate the quantity of corrosion per unit area (erosion rate) of the test piece.
DR = (W1 − W2) × T/365/S
W1 and W2 are the initial mass of the test piece and the mass after retrieval (mg), respectively; (W1 − W2) is the absolute corrosion amount of the test piece (mg); T is the time that the test piece was buried (days), and S is the surface area of the limestone tablet (cm2).
The weathering rate and CO2 consumption of carbonate rocks was then calculated according to their mass variations. CO2 consumption or carbon sink is depicted following the Formula (10):
CO2 + CaCO3 + H2O = 2HCO3− + Ca2+
The LTM is easily prepared, has a flexible operation, and has short-time data acquired. Moreover, it is the closest method to reflecting the natural process [29]. In the project “Karstification and Carbon Cycle” (IGCP379), the LTM was adopted to reckon the global karst carbon sink intensity and carbon sink flux [98]. In the work of [99], the LTM was used to estimate the atmospheric CO2 sinks of 18 sites in China.
Land 12 00133 g003 550
Figure 3. A picture and schematic diagram of the placement of tablets. The blue ellipse represents the test tablets. Three parallel specimens are normally placed in the same layer, and multiple sets are put in different layers. The dotted line indicates the tablets that are set perpendicular. The solid line is the string that threaded the tablets together. Reprinted with permission from Ref. [100], Copyright 2020 Springer.
Figure 3. A picture and schematic diagram of the placement of tablets. The blue ellipse represents the test tablets. Three parallel specimens are normally placed in the same layer, and multiple sets are put in different layers. The dotted line indicates the tablets that are set perpendicular. The solid line is the string that threaded the tablets together. Reprinted with permission from Ref. [100], Copyright 2020 Springer.
Land 12 00133 g003
However, the LTM is sensitive to environmental characteristics. The regional differences in climate, hydrology, lithology, soil physical-chemical properties, and geological structure always lead to different carbonate dissolution processes. Ref. [37] pointed out that due to the secondary deposition of carbonate, the carbon sink intensity is lower than its real value, and sometimes it shows a negative result. Likewise, research has revealed that the northern loess region has the opposite characteristics to those observed in the karst region due to the influence of high inorganic carbon soil content, which also causes the dissolution rate to be low or even negative [100]. This is also the primary reason the hydrochemical runoff approach yields a carbon sink 1/15 times larger than the one obtained using the dissolution test piece method. Therefore, test sites with high inorganic carbon soil should be avoided when using the dissolution test piece method to study the carbon cycle and carbon sink in the loess area,. More studies also showed the environmental distribution by soil carbonate deposition in arid and semi-arid and loess areas [100,101,102,103].

3. The Influencing Factors of Loess Carbon Sink

3.1. Temporal and Spatial Distribution Characteristics of Soil CO2 in the Loess Region and Its Influencing Factors

Comprehensive environmental factors influence the loess carbon cycle. Ref [104] analyzed the hydrothermal factors on the daily and seasonal changes in soil respiration in pea farmland on the Loess Plateau by using dynamic closed gas chamber infrared CO2 analysis (IRGA). The dissolution of SCM in the soil was the main driving force for the negative flux of soil respiration (carbon sink), and low temperatures in winter aggravated it. Ref. [105] carried out research on farmland and adjacent grassland, showing that changes in hydrothermal factors caused significant differences in soil CO2 emission fluxes in these two ecosystems. The correlation between soil temperature and CO2 content was higher than soil moisture in agricultural land, fallow grassland, and jujube woodland in hilly loess regions [106]. There was a significant correlation between soil respiration CO2 and plant growth under different land-use patterns in loess [107]. Under the four planting coverings of Platycladus orientalis, Caragana korshinskii Kom., Hippophae rhamnoides Linn., and Pinus tabuliformis Carr., the relationship between soil moisture content and soil temperature of different vegetation was significantly different which resulted in the difference in soil CO2 flux [108]. Additionally, soil CO2 emission flux (take alfalfa as an example) increased with planting years [109]. CO2 levels are affected by various factors, including land management practices, plant cover, and weather conditions (rainfall, air temperature, soil temperature, soil moisture, etc.), so further research on the interaction between the above factors needs more study.
To clarify the influence of soil CO2 on the dissolution process of loess minerals, the authors of [110] calculated the amount of soil CO2 fixed during the formation of soil SCM according to the stoichiometric equilibrium. It was found that the average amount of CO2 fixed by soil at different layers was 27.9 g CO2/kg. The fixed amount of CO2 in this process is very weak because CO2 will be released again during the formation of SCM, as shown in Figure 1, and only a very small amount of DIC from SCM comes into the water as a carbon sink.
According to the balance of CO2 before and after the carbonate mineral dissolution, the authors of [111] calculated the karst CO2 uptake in a typical small loess watershed (Bahe River watershed in the loess plateau of China). Its basic principle is to subtract the free CO2 in groundwater after carbonate weathering from atmospheric precipitation, as shown in the Formula (11).
ETC = RTC − STC
Among them:
  • ETC is the amount of CO2 absorbed during carbonate weathering.
  • RTC is the total amount of CO2 in atmospheric precipitation.
  • STC is the total amount of CO2 lost in groundwater.
The study found that the pH and HCO3− content in groundwater is close to the value of the limestone area. Specifically, the carbonate mineral in loess also experienced a strong weathering process. The erosion process of the loess absorbs about 82% of the total free CO2 in the rainwater, and 18% is left in groundwater. Meanwhile, the pH value and HCO3− content in groundwater increased compared with rainwater, indicating CaCO3 dissolution. As a result, the carbon sink intensity of the Bahe loess watershed can be calculated as 2.18 t CO2/km2·a (a is the unit of the year). Thus, the CO2 sink flux in the whole loess plateau of China is 1.37 million t/a.
However, this model does not consider the soil CO2 that would be dissolved in seepage and participate in mineral erosion. Ref. [43] stated that the CO2 concentration in loess is 10 times more than in the atmosphere. The high partial pressure of CO2 in loess would promote carbonate erosion and enhance the carbon sink flux.
This shows that researchers have acknowledged the significance of loess carbonate to the global carbon cycle, conducted some related investigations, and obtained some findings. On the other hand, most of these investigations examine the soil without discussing the carbonate dissolving process and its reaction mechanism, and the related research techniques need to be improved.

3.2. Mineral Chemical Weathering Rate, CO2 Consumption, and Its Influencing Factors Watershed of the Loess Area

We gathered and evaluated rock weathering rate data estimated by the Galy model and the hydro-chemical approach in a number of watersheds to better understand the factors that affect rock (mineral) weathering rate and to evaluate the extent to which weathering rates vary across the loess region (Table 2). The findings demonstrate that the weathering rate of rocks (minerals) is affected by factors including the composition of the rocks and minerals they are composed of, as well as environmental factors such as rainfall and air temperature. The rate at which silicate rocks (minerals) and carbonate rocks are weathered is positively correlated with precipitation and temperature. The correlation coefficients between rainfall and silicate rock (mineral) weathering rate and carbonate rock (mineral) weathering rate are R2 = 0.7697 and R2 = 0.8914, respectively, while the temperature is related to the silicate rock (mineral) weathering rate and carbon. The correlation coefficients of salt rock (mineral) weathering rates are R2 = 0.3776 and R2 = 0.6623, respectively (Figure 4). The chemical weathering rate has a strong positive relationship with precipitation, as shown by the fact that it increases with both rainfall and temperature [112]; this is in agreement with the findings that the influence of rainfall was greater than that of temperature. It is consistent with the correlations, and it is evident that rainfall and temperature have different effects on the weathering rates of silicate rocks (minerals) and carbonate rocks (minerals) in contrast to carbonate rocks (minerals). It is more influenced by rainfall and temperature and is more sensitive to weathering than silicate rocks (minerals), suggesting the effect of the properties of the rocks and minerals themselves on the rate of weathering of rocks (minerals) in the catchment. In summary, the chemical weathering rate and carbon sink capacity of a watershed depend not only on the lithological features of the watershed but also on the rainfall and temperature of the watershed. The loess area (Qingliangsi River) is relative to the average annual rainfall of the Nenjiang River and Songhua River, so it has the same order of magnitude of rock (mineral) weathering rate, while the Yangtze River, Wujiang, and other basins have higher annual average rainfall and annual average temperature than the Qingliangsi River; therefore, the rock chemical weathering rate and CO2 consumption rate in the study area are much lower than those of these watersheds. Although the total amount of ions in the Qingliangsi River water body is significantly higher than the global average ion content, the weathering rate of its loess minerals is lower than the average chemical weathering rate of the global rocks. The primary causes may have two points: silicate minerals account for the vast majority. As stated above, silicate minerals have strong weathering opposition, and their weathering rate is much lower than that of carbonate minerals; on the other hand, Qingliangsi is located in the Loess Plateau, where evaporation is strong, and the annual average evaporation amount is 4.9 times the average annual rainfall, which is the primary cause of the weak chemical weathering in the Qingliangsi watershed. Nevertheless, regarding the spatial heterogeneity of rock weathering rates, more research sites are needed to see comparable work, particularly in the loess area.

4. Conclusions and Further Considerations

The carbonate dissolution process of loess is an important geological carbon sink on Earth, which is of great significance for atmospheric CO2 regulation. Many scholars have recognized the vital function of loess carbon sinks in the global carbon cycle. The imperfections of current research lie in the following:
  • These studies focus on loess as a whole, not touching carbonate’s dissolution process and its reaction mechanism.
  • Research methods need to be improved, and new methods more suitable for carbon sink research in loess areas should be established as soon as possible.
  • The research on SCM dissolution, which is the main carbon cycle process in loess, is weak, and the dissolution rate, migration law, and the carbon sink flux of SCM needs to be more well known.
Even though the karst carbon cycle has gained widespread recognition in the last two decades, additional research into the process of loess holding significantly higher carbonate mineral concentrations is needed. Research methodologies and the carbon cycle process differ in loess and karst areas. Several questions need to be addressed in the future:
(1)
The dissolution rate of loess secondary carbonate and its influencing factors;
(2)
The migration process and carbon sink mechanism of carbon in the air–soil/mineral–water in the loess area;
(3)
The influence of different land-use methods on the dissolution rate and carbon sink of loess secondary carbonate;
(4)
The influence of climatic conditions on the secondary carbonate dissolution rate and carbon sink;
(5)
An estimation model of carbon sink in small loess watershed and its application in loess areas.
Long-term in situ observation is useful for carbon cycle mechanism research during the loess carbon cycle research. The law of temporal and spatial changes and the response to rainfall, climate, and human activities can be clarified through the high-resolution real-time monitoring system established for atmospheric precipitation, surface water, soil water, groundwater, and spring. In addition, we believe that diurnal-scale studies and sampling are necessary.
Moreover, as the permanent karst carbon sink model, the water-rock/mineral-gas-biology (BCP) model revealed the transformation processes of DIC to DOC (dissolved organic carbon) and long-time fixed as a stable carbon sink [10,11,37,121,122,123]. This model has not been confirmed in loess watershed. However, we believe that the loess area’s surface water ecosystem has high DIC and pH characteristics since the water chemistry characteristics are the same as the water body in the karst area. Coupled with intensive anthropogenic activities, especially agricultural activities, in the loess area, the BCP has a significant effect and may also have a high carbon sink potential.
Based on the above, we suppose that the following are needed to comprehensively estimate the carbon sink effect in the loess area:
(1)
The fate of carbon dioxide uptake by alkaline soils in the loess region is largely unknown, and further accurate assessments of the ability of abiotic uptake of carbon dioxide to contribute to carbon sequestration are required.
(2)
How the carbon sink effect of soil changes under different land-use patterns in the loess region.
(3)
What is the potential of surface water aquatic organisms to sequester carbon (i.e., BCP effect) in the loess region?
By reviewing loess carbon sink research, this paper provides new research ideas and methods for studying loess carbon sinks. This is of great significance for understanding the carbon migration and transformation process in the loess carbon pool, understanding the global carbon cycle, and finding global “missing sinks”.

Author Contributions

Conceptualization, M.S.; Investigation, M.S.; Software, M.S.; writing—original draft preparation, M.S.; writing—review and editing, M.A., L.Z., P.L., J.C. and X.Q.; supervision, funding acquisition, L.Z. and P.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (41671213) and the Geological Survey Project of China Geological Survey (DD20190502, DD20160305).

Data Availability Statement

Not applicable.

Acknowledgments

The authors wish to thank the anonymous reviewers and the editor for their comments and suggestions.

Conflicts of Interest

The authors declare no conflict of interest.

Abbreviations

MCS “Missing Carbon Sinks” BCP “Biological Carbon Pumping” NEPP “Natural Forest Protection Project” GTGP “Grain to Green Project” ECP “Ecological Construction Program” PCM “Primary Carbonate Minerals” SCM “Secondary Carbonate Minerals” SOC “Soil Organic Carbon” DIC “Dissolved Inorganic Carbon” TGA “Thermo-gravimetric Analysis” XRD “X-ray Diffraction Technology” LTM “Limestone Tablet Method” IRGA “Dynamic Closed Gas Chamber Infrared CO2 Analysis”.

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Land 12 00133 g001 550
Figure 1. Schematic diagram of carbonate dissolution and carbon cycle process in loess.
Figure 1. Schematic diagram of carbonate dissolution and carbon cycle process in loess.
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Land 12 00133 g002 550
Figure 2. Schematic diagram of the concept of hydrochemical runoff method. Reprinted with permission from Ref. [85], Copyright 2009 Elsevier.
Figure 2. Schematic diagram of the concept of hydrochemical runoff method. Reprinted with permission from Ref. [85], Copyright 2009 Elsevier.
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Figure 4. Relationship between rock (mineral) weathering rate, rainfall, and temperature.
Figure 4. Relationship between rock (mineral) weathering rate, rainfall, and temperature.
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Table
Table 1. Comparison of HCO3- concentration and pH in surface water ecosystems in karst and loess areas, based on the results of this study and previous work.
Table 1. Comparison of HCO3- concentration and pH in surface water ecosystems in karst and loess areas, based on the results of this study and previous work.
Study AreaSurface Water TypeHCO3− (mg/L)pHReference
Daihai Lake
(Loess area)		groundwater274.57.50–8.23[52]
river256.28.16–8.55
lake628.39.00–9.03
Fen river
(Loess area)		river (2015)529.58.7 ± 0.4[53]
groundwater (2015)481.27.9 ± 0.4
river (2017)477.98.2 ± 0.2
groundwater (2015)682.28.0 ± 0.2
Qingliangsi river
(Loess area)		river 247.37.65–8.46[54]
groundwater 380.17.55–8.37
Puding (Karst area)river 120–1807.40–9.67[55]
groundwater 73.2–384.37.32–8.60[56]
Yellow River
(Via Loess area)		river200.1 [31]
Yangtze Riverriver133.8 [57]
Amazonriver 43.9 [58]
Global Median 30.5 [59]
Table
Table 2. CO2 consumption rate and flux of mineral weathering.
Table 2. CO2 consumption rate and flux of mineral weathering.
WatershedAnnual Average TemperatureAnnual Annual RainfallCarbonate (Mineral) Weathering RateSilicate (Mineral) Weathering RateRock (Mineral) Weathering RateCO2 Consumption RateReference
°Cmmt/(km2·a)t/(km2·a)t/(km2·a)103 mol/(km2·a)
Qingliangsi River (Loess area)8.8437.32.833.499.31144.1[54]
Sanchuan River9.2467.7——7.84120unpublished data
Yellow River——9.922.0236.46169[113]
Yangtze River——55.865.2564.99611[59]
Songhua River45005.152.237.38120[114]
Second Songhua River466413.504.7418.24268[114]
Nenjiang River34553.311.394.7075[115]
Pearl River201000~200074.536.87—620.36[116]
Wujiang River 14.61163656108.5902[114]
Yalong River16100042.06.5—281[117]
Qingshui River14105020.1611.77109.97725[118]
Bishuiyan River19.91685.581.5113.4693.10853.02[119]
Qin River14.4578.58.470.0716.92146[120]
Amazon River——11.0813.0449.15157[89]
Global Median————36246[59]
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Shao, M.; Adnan, M.; Zhang, L.; Liu, P.; Cao, J.; Qin, X. Carbonate Mineral Dissolution and Its Carbon Sink Effect in Chinese Loess. Land 2023, 12, 133. https://doi.org/10.3390/land12010133

AMA Style

Shao M, Adnan M, Zhang L, Liu P, Cao J, Qin X. Carbonate Mineral Dissolution and Its Carbon Sink Effect in Chinese Loess. Land. 2023; 12(1):133. https://doi.org/10.3390/land12010133

Chicago/Turabian Style

Shao, Mingyu, Muhammad Adnan, Liankai Zhang, Pengyu Liu, Jianhua Cao, and Xiaoqun Qin. 2023. "Carbonate Mineral Dissolution and Its Carbon Sink Effect in Chinese Loess" Land 12, no. 1: 133. https://doi.org/10.3390/land12010133

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