Classification of agricultural critical zones in the North China Plain

The Earth’s critical zone is an area where water and solutes, as well as energy, gases, solids, and organisms, are exchanged among the atmosphere, hydrosphere, biosphere, lithosphere, and pedosphere, creating a life-sustaining environment for human society. In the vertical direction, the Earth’s critical zone goes up to the plant canopy and down through soil layers, unsaturated vadose zones, and saturated aquifers. Laterally, the Earth’s critical zones include not only weathered loose strata but also lakes, rivers, shallow marine environments, and vegetation. Earth’s critical zone studies mostly focus on the interaction between air, water, organisms, soil, surface rocks, and soil, integrating aboveground and belowground, time and space, and living and abiotic factors. This provides a basis for a comprehensive analysis of the evolution of complex terrestrial ecosystems and interdisciplinary research. The critical zones of the Earth are classified according to the differences in land use, among which the agricultural critical zone is the most strongly affected by human activities. However, most studies only consider surface function elements, without considering the important elements in the vadose zone and aquifers. The North China Plain (NCP) is a highly productive region where groundwater overexploitation and pollution are the major concerns. The crop-soil-aquifer critical zone perspective can provide new ideas for groundwater protection. Classification of the agricultural critical zone is the early basis for the study of regional groundwater volume/quality evolution and spatial differences. However, there has been little research on the classification of critical agricultural zones in the NCP. The classification of the agricultural critical zones in the NCP refers to the scheme for a comprehensive natural zone to a certain extent. By analyzing the hydrogeological conditions of the area and other information, combined with regional characteristics, the new classification followed the principles of comprehensiveness and dominant factors were developed with a three-level classification scheme for agricultural critical zones in the NCP by comprehensively considering the quaternary geology and geomorphology, shallow groundwater salinity, groundwater table depth, and agricultural land use factors. Taking the NCP as an example, agricultural critical zone zoning and mapping were carried out using the superposition method to superimpose and merge the classification elements, and an agricultural zone classification scheme was proposed. Finally, the results of this study divided the agricultural critical zone in the North China Plain into three first-level units, 13 second-level units, and 38 third-level units. This study has important reference significance for promoting the development of Earth critical zones, systematically understanding agricultural activities and their impacts on critical zone processes, and conducting research on the integrated management of regional groundwater and natural resources based on protection.