Drivers of Grassland Greenhouse Gas Budget: Management vs. inter-annual Weather Variations

Managed grassland ecosystems form a significant part of the global land cover. Beside climatic factors, also the management regime and history have an influence on the trace gas exchange of agricultural ecosystems. We have monitored the greenhouse gas (GHG) exchange together with the total carbon cycle of a sown grassland site in Central Europe since 2002. The experimental field consisted of two plots, one undergoing intensive management (high nitrogen input, 4-5 cuts), the other extensive management (no fertilization, 3 cuts). Within the 10-year measurement period, a grassland renovation was performed on the intensive field at the end of the sixth year. The field was ploughed in winter and reseeded in the following spring. 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. N₂O fluxes were continuously measured using automated static chambers.

Over the entire observation period, the GHG budget was dominated by the net carbon storage change (sequestration or loss). Although the different management led to a systematic difference between the two paired fields, it had only little influence on the inter-annual variation of the carbon budget apart from the renovation effect. The inter-annual variation showed a positive correlation with the net ecosystem productivity and the harvest yield. They were mainly weather driven with influences of the spring temperature and the length of the growing season as well as of the soil moisture content during summer.

While the extensive mostly showed a net carbon loss and a negligibly small N₂O exchange, the intensive field showed a continuous carbon sequestration in the six years before the renovation that was counterbalanced to a minor part by N₂O emissions. Contrastingly, following the renovation the carbon budget changed its sign from sequestration to loss. This was partly due to respiration losses during the fallow phase between ploughing and reseeding, but smaller carbon losses persisted for about two years. In addition, the N₂O emission was also considerably enhanced (about threefold) after the renovation. Overall, management induced differences in the annual GHG budgets of the two paired fields exceeded the weather induced inter-annual variability, especially when considering the strong renovation effect of the intensively managed field.