Assessing carbon sequestration along a natural carbon gradient impacted by short-term drought

Carbon sequestration in soils has been proposed as a strategy to mitigate greenhouse gas emissions in the agricultural sector. However, there is still much uncertainty regarding how carbon is sequestered and accumulated in soils. We conducted a six-month soil incubation study amended with ¹³C-labelled plant litter to investigate carbon sequestration along a hillslope in southeast Norway. The field, cultivated with cereals, exhibits a natural gradient in soil organic matter (SOM) content, pH and soil moisture. Eight rain exclusion shelters (excluding ~50% of the rain) were installed along the gradient for four months, after which soils were sampled to investigate the impact of soil conditions and short-term drought on carbon sequestration. Carbon and nitrogen contents, as well as stable isotope ratios, were measured in bulk soil and in particulate organic matter/mineral-associated organic matter fractions at both the beginning and the end of the incubation. ¹³C-CO₂ was measured continuously throughout the incubation and used for allocating ¹³C to a two-pool model. Exoenzymatic activity was also measured to provide insights into nutrient cycling in the soil. Litter decomposition was found to be highest in soils with low SOC, high pH and low moisture.