A Decade of Continuous Greenhouse Gas Exchange Measurements and Soil Carbon Monitoring At Two Temperate Grassland Sites.

Managed grassland ecosystems form a significant part of the European and global land cover. However, long-term greenhouse gas flux monitoring on grassland sites is still rare in comparison to forest sites. Beside climatic factors, also the management regime (and history) have an influence on the carbon budget of agricultural ecosystems. We have monitored the greenhouse gas exchange together with the carbon and nitrogen cycle of two sown meadow sites on the Swiss Central Plateau since 2002. The experimental field had been divided in two plots, one undergoing intensive management (high nitrogen input), the other non-intensive management (no fertilization). Continuous eddy covariance measurements of the CO2 exchange and the quantification of carbon export and import by harvest and manure application allowed for the assessment of the complete carbon budget of both plots which was compared to the temporal change of soil carbon measured by repeated soil inventory. Over the entire experiment, the intensive management led to a carbon sequestration, while the non-intensive management caused a net carbon loss. Repeated radiocarbon measurements in different soil layers confirmed different soil carbon dynamics of the two fields. A positive correlation between C and N sequestration was observed for the two fields corresponding approximately to the soil C/N ratio. The carbon sequestration of the intensive field was counterbalanced to a minor part by an increased N2O emission mainly related to fertilizer applications. Therefore and due to the very small field methane fluxes, the total greenhouse gas budget of the grassland fields was clearly dominated by the carbon exchange.