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

Sammanfattning: 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 (<10 years) SOC to soil respiration. A set of soil respiration measurements with a cavity-ring-down spectrometer directly in the field at different times over the growing season allowed the relative contributions of autotrophic and heterotrophic respiration to be determined. Another set of measurements on soil samples from 0 to 50 cm depth was used to estimate the contribution of different SOC sources within the profile, finding a high contribution of root-derived C on total SOC in the topsoil and upper subsoil. By utilizing the topsoil C isotope and SOC data the root humification coefficient with two different methods could be calculated, suggesting that contribution of root-derived C on total SOC is often underestimated. The measured changes in the δ¹³SOC signature over the years in European bare fallow plots receiving virtually no C input during several decades allowed the natural δ¹³C enrichment occurring in SOC to be measured and modelled. The results validated those of the natural abundance-based techniques over the mid- to short-term. The natural δ¹³SOC enrichment was modelled by calibrating a Rayleigh distillation function. The enrichment kinetic resulted similar in all the sites, and the general calibration could be extended to other studies. This thesis studied the C cycle in a managed long-term field experiment from several perspectives and across different scales, so providing valuable information of potentially high importance for understanding the general dynamics of C in soil. During the 53 years of the Ultuna long-term field experiment, significant differences between the treatments have developed, with clear C accumulation or depletion in soil to 35 cm depth. Thus, SOC changes below ploughing depth should be considered in SOC balance studies and as a viable C sequestration strategy.