Weathering Process of Parent Material and Accumulation Process of Organic Carbon on the Black Soil Profile: A Case Study from the Keshan Area of the Songnen Plain in Northeast China

Black soil is essential for maintaining regional food security and promoting global agricultural production. Understanding the weathering process of parent material and the accumulation of organic carbon is crucial to comprehending the developmental history and future trends of black soil, especially against the background of large-scale global cultivation and climate change. Although the importance of black soil formation and evolution cannot be ignored, the relevant research is still very scarce. In this study, a typical eight-meter-deep soil core was collected from the Keshan area of the Songnen Plain, Northeast China, where surface black soil developed on paleo-sediments. Using ¹⁴C dating, the formation age of the black soil was determined. Based on the characteristics of the geochemical composition, grain size and the magnetic susceptibility of the sediments, it was demonstrated that the black soil and its parent material originated from reworked loess. Furthermore, the mass transfer coefficient (τ) of some elements was determined, in order to explore the soil weathering process. By calculating the transported amount of alkaline and alkaline-earth elements, the weathering rate of parent material to black soil was found to be weak, at 0.16 kEq·ha^–1·year^–1. Combining the results of dating and carbon density in the different layers of black soil, the accumulation rate of organic carbon was determined as follows: rapidly increasing in the initial period of 13.2–2.2 ka, reaching its maximum average value of 34.0 g·cm^–2·a^–1 at 2.2–0.8 ka, then showing a decreasing trend with an average value of -77.5 g·cm^–2·a^–1. Compared with regional climate change, Keshan black soil has developed under a colder and wetter climate during the Holocene. Predictably, ongoing global warming may lead to the degradation of black soils in the Songnen Plain, as well as in other regions. Our results will enrich geological knowledge of black soil formation and future evolutionary trends.