Mountain grassland SOC stocks decline for 30 years with low N mineral fertilisation, independent of yield size and temperature increase

The carbon (C) stocks of grassland soils represent one of the largest potential sinks for the greenhouse gas (GHG) CO 2 . This makes knowledge of the changes in the soil organic C (SOC) stock essential. In a long-term experiment, we studied mountain grassland yields and topsoil SOC stocks in a one-level N and six-level PK-mineral fertiliser trial between 1990 and 2022, following the cessation of farmyard manure (FYM) fertilisation. To distinguish C-input from C-output pathways, we quantified the CO 2 exchange and the net ecosystem C balance (NECB) in 2022. During 33 years of NPK fertilisation, mean plant yield declined at a rate of 0.03 t C ha −1 yr −1 , and SOC stocks declined at 0.5 t C ha −1 yr −1 . In response to NPK treatments, plant yields showed a 1.6-fold difference, but the corresponding SOC stocks did not differ. In 2022, reference plots under continuous FYM fertilisation, still had the same SOC stocks as NPK plots in the zero year (1989). Short-term changes in SOC stock were not correlated with any climate indices or plant yields, but extreme weather conditions in 2022 coincided with abrupt changes in SOC stock. We conclude that in our experiment, grassland yield and long-term temperature increase were not modulators of SOC stock decline. Only under C and N import conditions, through FYM application, were historical SOC stocks maintained. In single years, hot and dry weather may cause large SOC stock losses of ca. 5 t C ha −1 yr −1 in the form of CO 2 .