Spatiotemporal Differentiation of Deep Soil Organic Carbon and its Response to Human Activities: An Empirical Analysis Based on Long-Term Monitoring Data from China's Typical Farmland

Abstract

Understanding the dynamics of soil organic carbon (SOC) in farmlands is crucial for evaluating soil health, quantifying the potential for ecosystem carbon sequestration, and guiding sustainable agricultural management. Research on farmland SOC sequestration and mineralization has predominantly focused on the topsoil layer (0-20 cm), and there remains a paucity of systematic research on the response of deep SOC (>20 cm) to human activities and strategies to enhance deep organic carbon sequestration. This study utilizes long-term fixed-site monitoring data from 120 farmland plots across 21 typical farmland and farmland complex ecosystem stations in the Chinese Ecosystem Research Network (CERN) spanning 17 years (2004-2020). By combining spatial analysis with econometric models, this study systematically elucidates the spatiotemporal differentiation characteristics of SOC below 20 cm in typical farmland ecosystem soil profiles across seven major geographical zones in China. Furthermore, it empirically analyzes the differentiated impacts of various fertilization and straw management practices, tillage modes, and types of farmland on SOC in the 20-40 cm, 40-60 cm, and 60-100 cm soil layers by constructing a panel fixed-effects model. The research results indicate that SOC exhibits significant vertical heterogeneity across different soil layers below 20 cm in typical farmland profiles in China, and the regional pattern exhibits a spatial gradient characterized by "higher values in the north and south, lower values in the east and west". The 60-100 cm soil layer exhibited a significant carbon accumulation effect during the investigation period, with a cumulative increase of 4.07%. Compared to single organic material inputs, the carbon accumulation efficiency in deep soil is enhanced when organic and inorganic materials are applied together. Compared to reduced tillage and no-tillage, conventional tillage is less conducive to SOC accumulation in the soil layer above 60 cm, but it has a significant positive impact on SOC in the 60-100 cm soil layer. In comparison to dryland or irrigated land, paddy fields are less favorable for SOC accumulation in soil layers deeper than 20 cm, with their carbon sequestration advantage being confined to the surface layer. This study provides novel insights into the response of deep SOC in farmland to human activities, underscoring the importance of investigating and actively implementing agricultural practices that "store carbon in deep soil".

Keywords: Soil organic carbon (SOC), Deep soil, Carbon sequestration, Human activities, Typical farmland ecosystem, Fertilization and straw management