On May 15, 2021, after 296 days of space travel, the “Zhu Rong” rover and its landing assembly carried by the “Tianwen” Mars rover developed by the Chinese themselves successfully arrived in China The key stop on the road to human beings is Mars. One of the main tasks of “Zhu Rong” is to answer the scientific question of “whether there has ever been life on Mars or an environment that can support life”. To this end, it will use the relevant instruments it carries to explore the surface of Mars and the superficial environment. Different from the desolate and silent surface of Mars, the surface of the earth where we live is very lively: here there are gurgling streams, dry vegetation, and crowds of people. However, the Anthropocene lives here, and humans are also blind to it. Regarding this vibrant land under our feet, we often lose sight of Mount Tai.
The near-surface layer where we live is an area where the five major spheres (atmosphere, biosphere, soil sphere, hydrosphere, and lithosphere) converge: the processes of material circulation, energy flow, and biological information transmission are coupled and nested here. Whether it is a geological movement calculated in units of one hundred thousand years, a million years or even a hundred million years, or a short chemical reaction that changes quickly, everything has changed or is changing here. It is precisely with these seemingly non-stop response processes that the story of the sea turning into mulberry fields and the soil nurturing all things can be staged again and again, and the endless survival and reproduction of mankind can become possible.
The near surface layer is the most direct and most profound area on the earth’s surface where the earth’s environment and human society interact with each other. Sustainable development is extremely important. In order to deeply understand this complex and open system, geoscientists put forward the concept of “Earth’s Critical Zone”. So, what exactly is the key zone of the earth? Why study the key zones of the earth? What progress has been made in scientific research on the critical zones of the earth? What other issues are worth studying in the future?
Critical Zones of the Earth: Definition and Function
The critical zone of the earth refers to the continuum that extends from the bottom of the groundwater or the soil-rock interface to the top of the vegetation canopy [1], including the intersection of the five major spheres of the lithosphere, hydrosphere, soil sphere, biosphere, and atmosphere. Areas of heterogeneity. In the horizontal direction, it can be covered by forests, agricultural land, deserts, rivers, lakes, coastal zones, and shallow sea environments. Sexual change characteristics), and its composition shows strong surface differences. For example, the key karst zone in my country is multi-peak clusters and depressions, and the soil layer is very shallow; the key zone of the red soil in the south is undulating, and the soil is very developed and mostly acidic; the key zone of the Loess Plateau is thousands of gully and the thickness can reach hundreds of meters. However, no matter what kind of key zone, soil is always the core unit that connects other elements; substances are driven by water to participate in biogeochemical cycles, and then perform ecological functions and provide ecosystem services.
From a functional point of view, the critical zone is called the critical zone of the earth because it is essential to maintain the operation of the earth’s terrestrial ecosystem and the survival and development of human beings. Specifically, the functions of the key zone can be divided into four aspects: supply, support, regulation and cultural services. Provision of services means that the beneficiaries obtain beneficial products from the key belt system, such as fresh water, food, fiber and fuel; support services are a necessary prerequisite for other services to function, including plant growth, soil formation and evolution, and elemental organisms. Processes such as the geochemical cycle; regulation services refer to the regulation and control of various products obtained from the critical zone system, such as the regulation and response to the quantity and quality of fresh water, atmospheric composition and climate change; cultural services refer to the regulation and response of human beings from the critical zone system The sensory experience acquired in the system, such as leisure and entertainment, cultural education, travel check-in, etc. [2].
Just imagine, if there is no key zone, the earth will be no different from a desolate extraterrestrial body. How uninteresting is it? !
Earth’s Key Zone Science: A New Opportunity for Scientific Research on the Earth’s Surface System
The interaction and evolution of water, soil, atmosphere, organisms, and rocks in the earth’s surface system driven by the internal and external energy of the earth is not only the basis for maintaining the supply of natural resources, but also playing an irreplaceable ecological function. However, with the continuous development of human society, issues such as resource exhaustion, environmental degradation and ecosystem degradation have increasingly become bottlenecks restricting the sustainable development of society. For example, the black soil in Northeast China is the most fertile soil in my country, and it has the reputation of “Beidacang”. It plays an important role in maintaining my country’s food security. However, due to long-term irrational use, the soil continues to degenerate and the black soil is “thinner.” The phenomena of “thinning”, “hardening” and “polluting” are particularly prominent, which seriously threaten the sustainable development of local and even national agriculture. Another example is the vast red soil area in southern my country, which accounts for 23% of the country’s land area. It is rich in water and heat resources and feeds 40% of our population. However, due to poor management and utilization, problems such as the degradation of natural resources such as water and soil and the uncoordinated allocation of water and soil have become prominent [3 ]. For the arid regions in Northwest China, the shortage of water resources and the uneven distribution of time and space restrict economic and social development are prominent contradictions that need to be resolved.
Understanding the status quo, evolution process and interaction of various elements in the earth’s surface system is a necessary prerequisite for realizing process regulation and sustainable use of resources in the critical zone. Traditionally, for the study of surface systems, there are disciplines that specialize in each single element, such as hydrology, soil science, atmospheric science, life science, petrology and mineralogy. These disciplines study the elements of the earth’s surface relatively independently, laying a solid foundation for a full understanding of their nature, current situation, and functions. However, this element-centered research paradigm limits to a certain extent the overall understanding of the composition and function of the entire system and the interaction between various elements. In 2001, the National Research Council of the United States formally put forward the concept and methodology of “Critical Zone of the Earth” in the “Opportunities for Fundamental Earth Science Research”, which opened up a new path for the study of the above problems and provided a feasible way for scientific research on the Earth’s surface systems. The physical framework of the above-mentioned earth sciences has since added a bridge to facilitate communication between the various sub-disciplines of the earth sciences, thus greatly promoting the multidisciplinary comprehensive research of the surface circle. Earth’s critical zone science is considered to be the key field of earth science research in the 21st century, and also the priority development field of my country’s environmental geosciences in the new era. In 2020, the National Academy of Sciences, the Academy of Engineering, and the School of Medicine issued a report entitled “The Earth in Time: National Science Foundation Earth Science Decade Vision”, suggesting that the issue of “How does the Earth’s critical zones affect the climate?” One of the funding directions.
A systematic study of the key areas of the earth as a whole can break through the limitations of traditional research. Take the biogeochemical cycle of soil nitrogen as an example. For a long time, soil scientists and agronomists have only paid attention to the cycle process of nitrogen in the root zone of crops (0 to 1 meter underground), and there is little research on the range below the root zone. . However, long-term excessive fertilization and unreasonable management measures have caused the problem of nitrogen surplus in the soil in many areas. When conducting research on the nitrogen balance, due to insufficient knowledge of the whereabouts and fate of surplus nitrogen, it is generally called “disappeared” nitrogen. In fact, these nitrogen have not really disappeared. Under the action of leaching, most of the surplus nitrogen flows out of the soil root zone with water, accumulates in the depths of the vadose zone, and may even enter the groundwater, threatening human drinking water safety [ 4]. Therefore, in order to fully understand the nitrogen cycle, it is necessary to study it from the perspective of all elements on the earth’s surface [5]. In this way, it is possible to have a more comprehensive understanding of the biogeochemical cycle process of nitrogen in the key zone of the earth.
Earth’s critical zone science: scientific issues and research platforms
Earth’s critical zone science is a systematic integration of multidisciplinary research, which can solve scientific problems that cannot be solved by a single discipline. The overall goal of the key zone research is to observe various biogeochemical processes coupled in the Earth’s surface system, try to understand the formation and evolution of this life support system, its response to climate change and human disturbance, and finally predict its future changes. S. Banwart et al. summarized the six key scientific research issues and divided them into short-term and long-term aspects [6].
Short-term scientific questions
(1) What controls the resistance, response and resilience of the critical zone and its coupling functions (including geophysical, geochemical and ecological functions), and the ability to cope with climate change and human disturbance? How to quantify the above processes and functions through observations, and use mathematical models to predict the interactions and future changes of these processes?
(2) How to integrate sensor technology, electronic networked information infrastructure and models to simulate and predict the basic variables of terrestrial ecosystems?
(3) How to integrate theories, data and mathematical models of natural sciences, social sciences, engineering, and technical applications to simulate, evaluate and manage key products and services that are beneficial to human society?
Long-term scientific issues
(1) How do geological evolution and paleontology construct and maintain the function of the ecosystem in the key zone and the basis for sustainable development?
(2) How does the critical zone process at the molecular scale dominate the material circulation and energy transfer between the various elements in the vertical space (including above-ground plants, soil, aquifer, and weathering layer) in the critical zone? How does it affect the evolution of watersheds and aquifers?
(3) How to integrate theories and data from molecular to global scale to understand the evolution process of the earth’s surface and predict future changes and planetary effects?
The key zone observation station of the earth is an important platform for carrying out scientific research in the key zone. By establishing field laboratories at the watershed scale to monitor hydrology, meteorology, vegetation, rock weathering and soil and other elements in the watershed to obtain observation data, it is possible to study various biogeochemical processes coupled with each other in the earth’s surface system [7], And finally simulate and predict its future dynamics [8]. In recent years, great progress has been made in the construction and research of international key zone observatories. Since the first true earth key zone observatory was officially established in the United States in 2007, Germany, France and other countries have begun to establish their own key zone observatory (network), and the total number is expected to reach more than 65.
In 2014, with funding from the National Natural Science Foundation of China and the British Natural Environment Research Council’s major international cooperative research project “Research on the Maintenance Mechanism of Ecological Service Functions of Water and Soil in the Earth’s Critical Zones”, China used the National Ecosystem Research Network as an example. Based on the foundation, 5 key earth belt observation stations have been officially established, covering 4 different environments in the Loess Plateau, Southwest Karst Region, Ningbo City Suburb and Southern Red Soil Region. In recent years, observation stations in some other regions of the earth have gradually been established, such as Qinghai Lake, Jianghan Plain, Black Soil, Bohai Sea, North China Plain, and Yanshan Mountains. In the future, it is necessary to continue to establish key zone observation stations in typical areas such as desert-oasis areas, temperate grasslands, tropical islands and the Qinghai-Tibet Plateau, forming a more complete observation network of key earth zones with Chinese characteristics, in order to further study key zone scientific issues and Provide an important platform for training relevant talents.
Earth’s Key Zone Science: Research Progress and Prospects
At present, with the intensification of climate change and human activities’ impact on natural ecosystems, the natural evolution process of key areas of the earth has been further interfered, resulting in a series of ecological and environmental problems. In order to meet these challenges, geoscientists’ research on key zones is also being further strengthened.
Earth’s critical zone science generally follows the research paradigm of “structure-process-function-service”, so current research is mainly carried out around the above four aspects. In terms of structure, it mainly conducts research on structural variation of key zones and multidisciplinary characterization methods. For example, by combining geophysical observation techniques such as classic profile surveys, dynamic drilling, and ground penetrating radar, the author’s team revealed that the underground structure of a typical red soil key zone consists of a homogeneous red soil layer, a reticulated red soil layer and a semi-weathered sandstone layer[4 ]. The structure is the basis for understanding the key zone and can profoundly affect the material migration process. However, due to the complexity of the surface environment, the structure of the key zone in different places is very different, which also brings great challenges to the research on the classification of the key zone. Therefore, how to combine traditional drilling surveys and emerging detection technologies to more finely and accurately characterize the structure of the key zone is one of the current hot areas of research on the key zone. In terms of processes, the focus is on the formation and evolution of key zones and their response and feedback to climate change and human activities, as well as the biogeochemical cycle process of important substances (such as carbon, nitrogen, phosphorus, and sulfur) in key zones. Research in this area is mainly through the monitoring and simulation of solutes, water, gas, soil and sediments and other key belt substances [9]. On the basis of understanding the structure of the key zone and the material circulation process, by developing a new prediction model to quantitatively characterize the structure of the key zone and predict the future changes of the key zone process, it can serve to evaluate the function of the key zone. In terms of functions, the main focus is on the improvement and trade-off of key functions. For example, in the context of “carbon peak” and “carbon neutrality”, how to assess the potential of atmospheric carbon dioxide fixation in key zones and regulate it is worthy of further study. Finally, the critical zone service provides a set of indicators for measuring the supply products and benefits of the critical zone process, and is developing into an environmental assessment standard for evaluating the critical zone process [10]. Although most researches on key areas of the earth use watersheds as the basic unit, no clear boundary is given in the horizontal dimension. In order to solve this problem, the author’s team comprehensively considered factors such as climate, soil-forming parent material, soil type, groundwater depth, landform type and land use, and constructed a three-level classification scheme for the key areas of the earth, dividing China into 44 first-level Units, 100 second-level units and 1448 third-level units. The proposal of this plan lays the foundation for the future expansion of research on key areas from small watersheds or stations to larger scales [11].
my country has a large population, limited natural resource endowments and extremely uneven regional distribution. How to achieve the coordinated allocation and sustainable use of natural resources is a key issue that needs to be solved urgently. Earth’s critical zone science has opened up a new path to solve this problem, but the research on the formation, evolution, structure, coupling process and function of different types of critical zones, especially the characteristics of changes under the influence of human activities and climate change, Still need to explore further. Looking at the starry sky over the sands of the Ganges, the “Zhu Rong” is pacing on the surface of the red planet Mars, looking at this possible future home for us. With the advancement of science and technology, it seems that mankind’s landing on Mars in the near future is no longer an unattainable dream. Similarly, in order to support the survival and development of human beings on the surface of outer planets such as Mars, the formation and evolution of “planetary critical belts” may also become an important research direction.
