Investigating Soil Organic Carbon Dynamics in Long-Term Lysimeter Systems

The balance between soil organic carbon (SOC) input and mineralization is fundamental to understanding the potential of agricultural soils as carbon sinks. Isotopic signatures (δ¹³C, δ¹⁵N) are effective tools for investigating carbon turnover, particularly when distinct differences exist between SOC fractions or inputs and outputs, such as those between C3 and C4 plant-derived carbon. This study utilizes long-term data from a large field lysimeter system to explore SOC dynamics in loess and sandy soils. Established in 1980, the lysimeter station includes large lysimeters (3 m in diameter) with loess and sandy soils. For the four loess lysimeters, SOC-free, CaCO₃-containing loess sediments were used. In the three sandy lysimeters, the top 30 cm of soil was mixed with peat, incorporating 3.5% organic material by volume. Maize, a C4 plant, was cultivated in all lysimeters until 2024, with complete removal of above-ground biomass after harvest. Soil samples from 1980, 1995, and 1998 provide a timeline to track SOC changes. In 2024, soil sampling was conducted at three depths (0–30 cm, 30–60 cm, and 60–90 cm) using Göttinger Bohrstock probes, with five subsamples per depth composited into a mixed sample. Samples were dried, sieved, and analyzed for C and N content, as well as δ¹³C and δ¹⁵N, following carbonate removal. The lysimeters also recorded water leachate volumes, revealing significant differences in groundwater seepage rates between loess and sandy soils. Preliminary results indicate that loess lysimeters show SOC accumulation, while sandy lysimeters exhibit a net loss of SOC, likely due to peat mineralization. Leachate from loess lysimeters was free of dissolved organic carbon (DOC), whereas leachate from sandy lysimeters consistently showed a brownish color and substantial DOC content. Isotopic analysis enables the partitioning of SOC sources and provides estimates of peat loss.In 2024, Silphium perfoliatum was planted in half of the lysimeters to assess its influence on SOC accrual and isotopic dynamics.