Influence of climate and land management on water, carbon and nitrogen cycling in grasslands of the pre-alpine region of southern Germany

Pre-alpine grasslands provide important economic value through forage for milk and meat production. Grassland soils also support ecosystem services such as carbon and nitrogen storage, water retention, erosion control and biodiversity. These functions are currently threatened by climate change, which is likely to accelerate in the coming decades. In addition to climate change, management decisions such as mowing and fertilisation frequency have a major impact on grassland yields, biodiversity and soil C and N dynamics. In this presentation we will summarise results from long-term monitoring of control and translocated grassland soil monoliths (1m2; 1.4m height) as operated in TERENO and studied in detail in the SUSALPS project.

From 2012, moderate climate change (plus 2°C) has increased grassland productivity, unless water stress has reversed the temperature stimulating effect. However, this increase in plant growth is only possible because increased N mineralisation rates under climate change allow increased N demand to be met. As plant N uptake is already in the range of total N fertilisation rates under current climate conditions, N losses to the environment, such as microbial N₂O emissions and nitrate leaching from montane grassland soils, are comparatively low. If other ecosystem N losses such as NH³ and N² emissions are considered, it becomes clear that even under the present climatic conditions substantial N has to be provided by mineralisation of soil organic N, indicating soil N (and C) mining. As the latter is associated with negative effects on soil fertility/productivity, C sequestration and GHG exchange, as well as filtering functions to protect water bodies, this trend poses risks to key soil functions in the long term. The detailed investigations from long-term monitoring sites were essential for testing a process-based model (LandscapeDNDC), which was used together with remote sensing information for spatial and temporal upscaling of the results.