Carbon sequestration in topsoil for forage-based cropping systems : an analysis of Swedish long-term experiments and SOC modelling

Abstract: C sequestration in soils is a potential way to mitigate climate change and increase the fertility of arable soils. Ley has long been known to deliver more C to its root systems compared to annual crops. Therefore, it has been suggested as a potential C sequestration strategy. My objective was to evaluate the SOC sequestration potential of ley in Nordic countries by studying three Swedish long-term experimental sites. From each site a ley rotation and bar-ley monoculture rotation were selected, and mineral- was compared to combined fertilization. Two of the sites (in N. Sweden) were measured for SOC stocks, while corresponding data was available for the third site (in S. Sweden). Higher topsoil SOC stocks was found for ley rotation than barley rota-tion for all sites. The effect from SOC stocks on soil aggregate stability was determined on the soil samples by analysing readily dispersible clay (RDC) and water stable aggregates (WSA). Ley might have influenced RDC positively, while mineral fertilization and WSA was found to correlate positively. As a second part of the study, I modelled SOC stock changes with ICBM, a SOC model used in National SOC Inventory reporting system and in the consultant tool OP. OP uses a simplified version of ICBM with default values. Samples from the soil archive from each of the northern experiments were included in the modelling. I used yield data records to estimate belowground C inputs from plant residues using shoot to root ratios. The predictions with OP matched in many cases better to measured SOC stocks than ICBM, sug-gesting that the default values in OP worked relatively well. The models matched the measured SOC stocks in the southern site. However, the models did not always apply to the northern sites. One explanation lies in the high uncertainty when estimating initial SOC stocks and imprecision in estimations of belowground C. Although the SOC stocks decreased in the northern sites (all treatments), the SOC stocks remained higher in ley rotation than in barley rotation. Combined fertilization had a positive effect upon the SOC sequestering potential of ley at the northern sites. This could however not be evaluated for the southern site. Finally, the SOC stocks increased for ley rotation in the southern site and the SOC stocks in ley rotation in one of the northern sites did not start to decrease until after 30 years, suggesting a SOC preserving potential in addition to the observed SOC sequestering potential of ley.