The cropland ecosystem is an important component of terrestrial ecosystems, and its carbon source and sink processes are highly sensitive to climate change. However, the contributions and driving mechanisms of climate change and carbon-nitrogen coupling to cropland ecosystem carbon source and sink processes remain unclear. This study, based on Gross Primary Productivity (GPP), Net Primary Productivity (NPP), and Net Ecosystem Productivity (NEP) as carbon sink indicators, as well as Total Respiration (Rs), Autotrophic Respiration (Ra), and Heterotrophic Respiration (Rh) as carbon source indicators, designed six scenarios to reveal the spatiotemporal evolution characteristics of cropland ecosystem carbon sources and sinks in China. The Lindeman-Merenda-Gold model was used to quantitatively assess the contributions of climate change and carbon-nitrogen coupling to cropland ecosystem carbon source and sink changes. The results show that from 2000 to 2020, the cropland ecosystem carbon sinks and sources in China exhibited a fluctuating growth trend. The average growth rates of GPP, NPP, and NEP were 4.27, 1.65, and 0.15 g C m?2 a?1, respectively, while those of Respiration (Rs), Autotrophic Respiration (Ra), and Heterotrophic Respiration (Rh) were 4.12, 2.62, and 1.50 g C m?2 a?1, respectively. The cropland ecosystem was overall a weak carbon sink, primarily due to high respiration rates. During this period, except for a slight increase in Rs/GPP at a rate of 0.12% per year, the ratios NEP/NPP, NEP/GPP, and NEP/Rs showed declining trends, with rates of -0.28%, -0.39%, and -0.46%, respectively, indicating that the dominant role of respiration in carbon processes was strengthening, weakening the role of carbon sinks. CO? fertilization and nitrogen deposition contributed 28.98% to the increase in net carbon sinks in cropland ecosystems, primarily controlling 25.30% of the cropland carbon sink areas. However, these factors also enhanced carbon sources, with a relative contribution of 32.41%, primarily controlling 26.75% of the carbon source areas. A weak GPP dominant mechanism drove the increase in NEP, with GPP being the primary driver of carbon sink growth in 77.14% of cropland sink areas (KNEP > 0). Conversely, a strong Rs dominant mechanism led to the decline in NEP, with Rh being the main driver of carbon source enhancement, controlling 46.98% of carbon source areas (KNEP < 0). These findings provide theoretical guidance for consolidating and enhancing cropland ecosystem carbon sinks, thereby supporting the achievement of carbon neutrality goals.