Improving our understanding of carbon cycling in agroecosystems by studying δ¹³C signatures in soil carbon stocks and fluxes

Soil carbon (C) storage has gained much attention in the past decade due to its potentially huge impact on climate change mechanisms. Particular focus has been paid to possible feedback mechanisms, whether soil can be considered a sink or source of C and how soils can be managed in order to mitigate climate change. However, there is still a lack of knowledge about the contribution of different C sources to soil organic carbon (SOC) formation, particularly in the subsoil. As the subsoil can store approximately as much C as the topsoil, such information is crucial. In a series of studies described in this thesis, data from the Ultuna long-term field experiment established in 1956 were used to study the effects of different organic amendments and mineral fertilizers, which were found to have influenced topsoil and subsoil in several ways. Cultivation of maize, a C₄ crop, began in the experiment in 2000, but prior to that only C₃ crops were grown. By exploiting the natural difference in C isotopic signature between the metabolisms in these two plant types, the role of these two sources in soil organic matter turnover could be quantified. The isotopic composition of C fluxes and stocks in the Ultuna experiment revealed the contribution of different sources to different C pools. Old (>10 years) SOC resulted contributing as much as young (