1 Introduction

The Earth Critical Zone (ECZ) is a complex integrated system encompassing various components of the Earth’s surface, such as bedrock, groundwater, pedosphere, atmosphere and biosphere [1]. It is also a natural reactor which characterized by intricate exchanges of matter and energy among these components [2]. Horizontally, the ECZ extends across different ecosystems, while vertically, it spans from the upper boundary, including the plant canopy, to the underlying soil layer, aquifer, and lithosphere [1, 3]. This region plays a crucial role in regulating the vital dynamics of the Earth’s surface, maintaining the natural ecological environment, and providing valuable societal resources [4]. Given that the advancements in technology on a global scale, the study of the ECZ has been emerged as an important and cutting-edge research direction within the field of Earth sciences in the twenty-first century [2, 5].

As the most active and dynamic region of the Earth’s surface system, the ECZ significantly influences the biogeochemical cycling of elements, the movement and migration of water and carbon (C) processes [1]. Most of researches on the ECZ primarily focused on unraveling the physical, chemical, and biological processes that control material cycling within this zone, as well as analyzing the interconnections among these processes [6, 7]. Within the dynamic core of this region, soil organic matter (SOM) serves as a key component, enabling the proper functioning of the critical zone. It acts as an essential energy and nutrient source for heterotrophic microorganisms and functions as a significant complexing agent or adsorbent for environmental pollutants [8, 9]. The decomposition and transformation of SOM play a pivotal role in various physical, chemical, and biological processes within this zone.

Loess, which covers approximately 10% of the Earth’s land area, is widely distributed worldwide (Fig. 1A). Now, research on loess has transitioned from qualitative descriptions to quantitative and interdisciplinary investigations. In China, the loess region represents around 5% of the global loess area (Fig. 1B). In the typical of this region (Loess Plateau), the vegetation coverage increased from 21 to 71% in recent 40 yrs due to the grain for green project. The mean annual temperature gradually increased from 1950s. The mean annual precipitation first decreased before 2000, and then increased after 2000 (Fig. 2). Chinese research on the structure and evolution of loess has surpassed that of European and American countries, reaching an advanced international level [10,11,12,13]. According to the new global development paradigm and China’s major national strategic goals of “high-quality development of the Yellow River” and “dual-carbon targets” the soil C sink within the Loess Critical Zone (LCZ) presents significant opportunities alongside formidable challenges. Therefore, examining the dynamic changes, trends, and controlling processes of soil organic C (SOC) within LCZ is crucial for understanding the patterns of material transport and energy transfer, thereby contributing to the sustainable development of human society.

Fig. 1
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The loess distribution and partition in China and the world (modified from [10,11,12,13])

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Fig. 2
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Observed mean annual temperature (MAT) and precipitation (MAP) variations between 1950 and 2020 in the Loess Critical Zone (a). The slight orange dashed line (MAT) is fitted to the data with the slop = 0.05 (p < 0.001). The slight blue dashed line (MAP) is fitted to the data with the slop = -0.23 (before 2000) and slop = 0.24 (after 2000). Vegetation coverage from 1982 to 2020 in the Loess Critical Zone (b-e)

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2 The development of the Loess Critical Zone

Since the establishment of the first three Earth’s Critical Zone (ECZ) observation stations in the United States in 2007, approximately 64 international Critical Zone observation stations have been gradually established worldwide by 2015, forming an observational network that monitors changes in environmental gradients [14]. However, studies on the ECZ in China began relatively late. In 2008, an article titled “Hydropedology: an emerging interdiscipline” was published in “Science & Technology Review” which introduced the concept of the Earth’s critical zone in a simplistic manner [6, 7]. In 2010, the “International Academic Conference on Hydropedology and Forefront Research and Applications in the Earth’s Critical Zone” took place at Bei**g Normal University, promoting the development of research on the ECZ in China. In the same year, Lin [1] reviewed the concept of the Earth’s Critical Zone and published an article entitled “Earth’s Critical Zone and hydropedology: concepts, characteristics, and advances.” During the “International Workshop on Hydropedology and Sustainable Use of Natural Resources” in 2013, ECZ was considered as a significant topic. Chinese academician Congqiang Liu presented on “Processes in the Earth’s Critical Zone and biogeochemical cycles” at the forum with the theme “Frontiers in Earth Biology,” which garnered substantial attention for the public. In 2014, the “Shuangqing Forum”, organized by the National Natural Science Foundation of China, highlighted the urgent need for research on the ECZ in China and discussed its future development project. Then, it was announced that five ECZ observation stations would be officially established, including the red soil region of southern China, suburban areas in Ningbo city, the Karst region of Southwest China, and the Loess Plateau, among others. Subsequently, the other ECZ observation stations were established, such as Qinghai Lake, Jianghan Plain, North China Plain, the Circum-Bohai Sea coast, and the Yanshan mountainous area [5]. In 2015, the National Natural Science Foundation of China and the UK’s Natural Environment Research Council initiated a major international research project in the field of the ECZ. This project aimed to understand the sustainability of land and water resources, the dynamic processes and timescales of evolution, and the ecosystem service functions within the Karst region, Loess Plateau, red soil region of Southern China, and rapidly changing suburban areas. Based on the Chinese ecosystem research network, this project officially commenced scientific research on the typical Critical Zone.

In this case, Chinese academician Ming’an Shao proposed the word of “Loess Critical Zone” (LCZ). In fact, LCZ has only been in use for the past years. However, researches and investigations related to this concept can be traced back to studies on biogeochemistry and the C-water cycle conducted several decades ago [15,16,17,18]. In recent years, there have been extremely global climate changes and intense human activities, leading to considerable alterations in the SOC transformation and other processes of the LCZ [16, 19, 20]. Previous studies on the ecosystem C stocks of the LCZ primarily focused on C output, lacking the underlying C driven processes [19, 20]. Consequently, it is of great significance to explore the sources, transport pathways, and transformation characteristics of SOC in the LCZ.

3 Distribution of the Loess Critical Zone

Geographically, loess distributed sporadically in the arid and semi-arid mid-latitude regions of both the northern and southern hemispheres [11]. The largest area covered by loess in the world is found in Asia, followed by Europe, North America, and South America (Fig. 1A). In Europe, the loess coverage ratio is about 16.6%, while Asia accounts for 10.6%, North America for 6%, and South America for 2.6%. Loess sediments can be found at altitudes ranging up to 5,300 m above sea level in areas north of the Kunlun Mountains in China, as well as in locations such as Argentina and New Zealand [21, 22]. With the exception of moist frozen loess in northeastern Siberia and northern Alaska, loess is generally found above the water table [23]. The thickest and most continuous loess sediments are located in China, with an average thickness of 300 m and a maximum thickness reaching 505 m, and the calculated mean loess thickness is 105.7 m [22]. In Siberia and Central Asia, the thickness of loess generally ranges from 0 to 200 m [11], while in Europe and North America, it is usually less than 20 m. However, in certain regions such as downstream of the Danube, the Palouse area, Nebraska, and Alaska, the thickness can reach several dozen meters or even approximately 100 m. In South America, loess has an average thickness of 50 m, while in New Zealand, Africa, and the Arabian Peninsula, it is generally around 20 m, and in Australia, it is approximately 3 m.

The LCZ exhibits a wide distribution in China and encompasses the interaction of five spheres: rock, water, soil, atmosphere, and biota [13]. The horizontal boundaries primarily correspond to the loess region. Through in-depth studies and improved understanding of the LCZ, significant progress has been made in its classification, making it an important tool for managing natural resources within the Earth’s surface system [16, 24,25,26]. Among the major regions of the LCZ, the Loess Plateau represents a typical and distinctive critical zone, and stands out as an area with the most fully developed loess deposits globally, preserving comprehensive records of the age and climatic information of the LCZ [22, 27]. A recent study has established an index system and methodological framework for the regional classification of the LCZ, and the entire Loess Plateau has been classified into eight Critical Zone (Fig. 1C) [13]. Within the national strategies such as the “high-quality development of the Yellow River” and “dual-carbon targets”, the soil C stock of the CLZ will encounter significant opportunities and challenges [24, 25]. Soil organic C plays a vital role in maintaining the proper functioning of the LCZ, and all interfaces within the zone are active sites for the substantial decomposition and transformation of SOC (Fig. 3). Currently, a preliminary network of observation stations has been established in the Loess Plateau, leading to research advancements in the structure, processes, evolution, and simulation of the LCZ [5, 23,24,25, 28]. However, due to vegetation greening project and extremely climate change, the structure and soil C cycling processes within the LCZ are undergoing significant changes compared to other areas. Therefore, gaining a correct understanding of the structure of the LCZ and the SOC cycling is crucial for achieving ecological goals and sustainable development in the Loess Plateau.

Fig. 3
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The structural model of the Loess Critical Zone

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4 The stock and dynamic of SOC in the Loess Critical Zone

Soil organic C (SOC) in the LCZ experiences significant decomposition, transformation, and migration across various media (atmosphere, vegetation, soil, and rocks) [26, 29, 30] (Fig. 3). In the atmosphere, it exists in the form of inorganic C, while in terrestrial ecosystems, SOC stock is the most abundant. The estimated values for SOC stocks in the atmosphere, terrestrial vegetation, and oceans proposed by different researchers are approximately consistent [31,32,33]. However, there are considerable discrepancies in the estimated values of SOC [15, 17]. These differences primarily arise from variations in data, research methods, sampling techniques, and the considerable spatial variability of soil [15, 26]. In the LCZ, the SOC stock is immense and the C cycling processes are complex. Furthermore, the combined impacts of climate changes and human activities subject the SOC stock to dynamic changes. Additionally, due to the vast area of the Loess Plateau, data collection becomes challenging, and differences in sampling technologies and processes further contribute to the uncertainty in estimating SOC stocks [29, 30]. These uncertainties significantly affect SOC stock estimation in evaluating the total C budget in China.

Loess Plateau are the significant C sink in the Earth. To provide a complete picture of the C sequestration in the Loess Plateau, we documented C pools and the changes for all C sectors (aboveground biomass, ecosystem C, and SOC) in the Loess Plateau (Table 1), by synthesizing the studies in the special feature and other recent studies [26, 29, 30]. Former estimates for C stocks of the Loess Plateau changed substantially because of diversity in data sources and inconsistency in methodologies [31,32,33]. These studies have shown that the SOC density in the topsoil ranges from 0.66 to 12.18 kg·m–2 within the 0–20 cm depth, with most values falling between 1 and 4 kg·m–2 [34]. Li et al. [7] pointed out that the total C stock of the Loess Plateau is approximately 2.29 Pg, and the C stocks of farmland, grassland, shrubland and forest are nearly 0.98, 1.09 and 0.21 Pg. In our previous study, the total C stock in these four ecosystems (forest, shrubland, grassland, and cropland) was 2.84 Pg C, among which 29.63% was stored in soil (0–20 cm), 53.23% in aboveground biomass, and 17.14% in belowground biomass [26]. The 0–10 and 10–20 cm soil layers on the Loess Plateau, covering almost 6.5% territory in China, held around 5% total SOC stocks at the mentioned layers across the country [34,35,36,37]. Furthermore, the C stock in grassland and farmland soils were higher than belowground biomass, which is similar to the estimates for the continental China (3.9 g/m2) [38], United States (3.0 g/m2) [39] and Europe (3.5 g/m2) [40]. However, higher SOC stock does not necessarily lead to greater ecosystem C stock. Fox example, the C stocks in the soil of forestland and shrubland were lower than the biomass due to the “Grain-for-Green” project in China, which had built numerous forestland and shrubland regions [7, 26].

Table 1 Statistics of soil C density and stock in the Loess Plateau and China derived from various studies
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In addition, the area-weighted mean biomass C densities in forestland (72.1 Mg ha−1), and grassland (1.02 Mg ha−1) across the Loess Plateau (Fig. 4) were substantially lower than the global means [94.2 Mg ha−1 in forestland and 7.2 Mg ha−1 in grassland] [38, 51]. Large-area young forests, extensive grazing and soil water limitation are the possible contributors of low biomass C density of the Loess Plateau [24, 25]. Specifically, almost 90% forests are aged below 60 years, and corresponding biomass is below 60 Mg ha−1, obviously lower compared with that (104.7 ± 30.3 Mg ha−1) in old forests (≥ 100 years) and the average value across China’s forests [52]. The higher proportion of young and middle-aged forests of the Loess Plateau suggests enormous future potential of C sinks [25]. Future biomass C density may surmount our estimates after forest areal expansion, vegetation restoration and protection and soil C increasing needs to be examined [53]. Vegetation restoration measures, ecological improvement projects, as well as natural conservation policies also may enable C gains to continually increase in this region [54].

Fig. 4
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Distribution of soil C density in the Loess Critical Zone (a revised from [26]; b revised from [37])

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In terms of vertical distribution (Fig. 4), the soil layer in the LCZ is thick, and there are notable variations in the distribution of SOC stock, with significant differences in SOC content across different soil layers [36, 55]. Previous studies on SOC stock and influencing factors in the LCZ have primarily focused on the shallow layer within 0–1 m, while the C stock in deep soil layer is enormous [11, 56] (Table 1). Generally, the content of SOC decreases with soil depth, with the SOC content in the shallow layer (0–10 cm) being 4–10 times higher than that in the deep layer (80–100 cm) [57]. The SOC stock in 0–20 cm, 0–50 cm, and 0–100 cm are estimated to be 1.68 Pg, 3.47 Pg, and 5.32 Pg, respectively. The SOC stock in the 0–20 cm and 0–50 cm layers accounts for 32.0% and 65.0% of the 0–100 cm layer, respectively. This is primarily due to the generation, transformation, and decomposition of aboveground vegetation biomass, as well as the predominance of roots in the surface layer [55]. Consequently, the contribution rate of SOC stock in the 0–50 cm soil layer is as high as two-thirds. The SOC stock is relatively low in boggy soil, gray desert soil, and solonetz, while loessal soil and gray cinnamonic soil, which have a relatively large area, are the main regions with SOC distribution [26]. Other study suggests that the SOC stock in the shallow layer (0–20 cm) of the Loess Plateau is 1.64 Pg, which increases to 2.86 Pg in the 0–40 cm soil layer. In the deep soil layer (0–100 cm), the SOC stock is estimated to be 4.78 Pg, and it reaches 5.85 Pg in the 0–200 cm soil layer. The SOC stock in the 0–100 cm and 0–200 cm layers accounts for 8.21% and 5.32% of the total SOC stock in China, respectively [58].

5 The microbial turnover of SOC in the Loess Critical Zone

Soil microorganisms act as the engine of biogeochemical processes in the LCZ, serving as important links between the input and output of soil C [59, 60]. They mediated crucial metabolic processes of C cycling, driving the exchange and transfer of active matter among various interfaces in the critical zone, such as the conversion of CO2 to organic compounds, the production and oxidation of CH4, and the decomposition of organic matter [61]. In alkaline loess soils characterized by high heterogeneity and calcium richness, the coupling mechanism among C in plant residue, C in microbial residue, and microorganisms requires further investigation.

Soil microorganisms determine the transport of SOC to the underground, the absorption and transport of nutrients by plants, and the micro-channels for nutrient transfer in underground ecosystems [9, 62,9.2 Binding the multi-process of C and hydrologic process

The SOC cycling in the LCZ is intricately linked to hydrological processes and nutrient cycling. However, due to their complex interactions and interdependencies, accurately quantifying the contributions of these processes to the C cycling remains challenging. In order to address this knowledge gap, it is crucial to conduct comprehensive research in typical small watersheds within the LCZ. Future study should focus on soil C flux, turnover, average retention time, and exchange rates among different C pools under different hydrological conditions, climates, and human activities. Such studies will help elucidate the mechanisms and characteristics of changes in SOC, nutrient components, and hydrological processes, as well as their primary influencing factors. Furthermore, it is important to enhance our understanding of the impacts of human activities on SOC migration and transformation by conducting investigations on the identification and tracing of C caused by human activities in the LCZ.

9.3 Paying attention to the roles of microorganisms in C cycling

Advancements in technology, such as third-generation sequencing techniques, stable isotopic tracer methods, DNA-stable isotope probes, and high-throughput sequencing, provide the opportunities to investigate the mechanisms by which microorganisms decompose SOC pool and drive its transformation. In addition, metagenomics can provide valuable information on the structure and function of soil microbial communities, enabling the determination of species types, metabolic functions, and their association with C cycling processes. In our future study, it is necessary to employ a comprehensive approach using techniques from molecular biology, isotopic tracing, and earth system science, with a focus on microbiology and soil C cycling models. This interdisciplinary approach will enhance our understanding of the biogeochemical processes involved in soil C cycling and facilitate the evaluation of C sequestration potential in the LCZ, ultimately aiding in achieving C-related targets.