Conference Agenda

Soil organic carbon (SOC) is crucial for soil fertility and climate regulation. Understanding its turnover helps predict responses to management. This study examines the turnover of organic carbon and phosphorus pools in soils with contrasting textures. We analyzed two agricultural experiments where vegetation shifted from C₃ to C₄. We hypothesize that the SOC pool will turn over more slowly in the clay-rich soil than in the clay-poor soil due to stronger sorption in the fine-textured soil.

We studied two replicated experiment sites in southeastern England; Rothamsted (Chromic Luvisol, 25% clay) and Woburn (Cambic Arenosol, 10–15% clay). The treatments included: (1) continuous barley (C₃ vegetation) (2) continuous maize (C₄ vegetation) and (3) continuous maize with additional incorporation of aboveground biomass (stubbles) after harvest. Soil samples were collected four times (1997–2015), and δ¹³C was analyzed to calculate SOC turnover times.

We found that the SOC content was higher at Rothamsted (1.24 g kg^-1) than Woburn (0.79 g kg^-1), likely due to the silty clay loam texture. Treatments had no significant effect on the SOC content. Bulk δ¹³C values were generally more enriched at Rothamsted (Δ¹³C = 1.54‰) than Woburn (Δ¹³C = 1.94‰). A significant difference between both C₄ treatments was observed at Woburn, where the δ¹³C of the total soil organic carbon pool was up to 1.8 times more enriched with the stubble incorporation.

The soil at Rothamsted had a larger C stock (386 Mg·ha^-1 vs 305 Mg·ha^-1 in Woburn) and slower C turnover (mean turnover time = 192 years), likely due to the stabilization of organic matter on clay minerals. In contrast, the sandy soil texture at Woburn allowed for faster turnover and lower organic C retention (121 years). These differences highlight the influence of soil texture on carbon dynamics, with clay-rich soils promoting long-term carbon storage.

ATES (Aquifer Thermal Energy Storage) is a CO₂-free technology for seasonal heating or cooling of buildings based on the storage and recovery of thermal energy in the aquifer. Since aquifers in urban areas are often contaminated with organic pollutants, the application of ATES raises the research question whether this type of heat management can accelerate natural attenuation processes. In the KONATES project, we are investigating how storage of hot water in an aquifer contaminated with trichloroethene (TCE) impacts the aquifer’s microbiome structure and its potential for reductive dehalogenation of TCE. In laboratory experiments we could demonstrate that the native microbiome of the contaminated aquifer can reductively dehalogenate TCE within a temperature range typical for low-temperature ATES (12°C to 25°C). However, these processes are significantly inhibited or entirely absent at temperatures characteristic for intermediate- to high-temperature ATES (30°C to 70°C). In a pilot test, hot water (70°C) was injected into the TCE contaminated aquifer. The microbial community composition in the groundwater was investigated, with specific focus on thermophiles and organohalide respiring bacteria. To allow distinguishing the extent of biotransformation of TCE via reductive dehalogenation from e.g. dilution effects, dual-element (δ¹³C/δ³⁷Cl) compound-specific stable isotope analysis was applied. However, as low concentrations of TCE at the pilot test site were observed, which were further decreased due to injection of hot water and mixing effects, a cryo-trap was integrated into the GC-analysis allowing a > six-fold increase in volume of injected gas samples, thus decreasing the limit of detection significantly.