Organic carbon contents of mineral grassland soils in Switzerland over the last 30 years

Long-term monitoring data on the evolution of soil organic carbon (SOC) content and soil stocks are crucial considering climate change and carbon sequestration. Due to the slow processes involved, it takes longer time spans until changes in SOC stocks become detectable. Thus, long-term monitoring is essential. Beside measurements, models are often used to report changes in SOC stocks for greenhouse gas (GHG) inventories. Consistent field data over extended periods are essential for validating such models whereby data from long-term monitoring is of great value. We present SOC measurements from the topsoil (uppermost 0.2 m) of 24 grassland sites of the Swiss Soil Monitoring Network (NABO) for six sampling campaigns from 1985 to 2014 and compare their evolution with predictions by the RothC model, which, for example, is used for the Swiss GHG Inventory. While there was no general temporal trend of the measured data over all sites, from a single site perspective, SOC stocks decreased/increased on 8% and 25% of the sites respectively. When comparing the measured data with the predictions from modelled values from RothC, a good fit was achieved in two thirds of the cases. However, at 5 out of the 24 sites (21%), the modelled SOC changes over time were lower than the measured ones. For 3 out of the 24 sites (12%) the model predicted higher SOC stock increases than were measured. Overall the modelled data fit well together with the measured ones. Most of the sites revealed stable values over time or increasing trends between 1985 and 2014, which might indicate that the SOC stocks potentially are in equilibrium and that the farming management used in Switzerland is suited to maintain these stocks in topsoils. There might not be a big potential for C sequestration, however, it is also very important to maintain SOC stocks over time. For future climate change scenarios accurate long-term SOC data and modelling are important for gathering the required information.