Biodiversity provides insurance against loss of ecosystem functioning. Understanding of functional relationships between various biodiversity components is critical to predict ecosystem responses under global change scenarios or during ecosystem restoration. Human activities such as industry and agriculture release unprecedented amounts of active nitrogen (N) into the environment, which alter the global N-cycle and result in the eutrophication of large parts of the biosphere. Significant modifications of ecosystem services such as biogeochemical cycles and biodiversity loss may occur. It is therefore crucial to understand the linkages between different components of biodiversity (soil/vegetation) in order to maintain or restore ecosystem functioning when the carbon/nitrogen balance is severely altered. Mountain grasslands (MG) constitute a suitable model for such studies, as they have a high ecological and agronomic value and may be substantially influenced by global change. We study experimentally the fate of N in both the above- and belowground compartments following carbon (C) addition on MG under increased N-conditions due to cattle concentration. Any engineering method to restore these sites should first reduce plant-available N. A method to lower plant-available N is the addition of labile C to the soil. In the lowland C-addition stimulated soil microbial activity, resulting in a higher immobilisation of inorganic N. However, C-addition may also affect other soil compartments containing N (organic N, mineral and gaseous N, N in plant and microbial biomass) and mineralization through changes in microbial activities. Here we present results of a short-term study at three sites in the French Alps at an altitude between 1650 and 1900m. We added sawdust as a C-source to sheep night enclosures and evaluated its effectiveness to decrease the plant-available soil-N concentration. Quantities of sawdust were adjusted to attain a similar C/N ratio as in adjacent undisturbed reference communities. To evaluate microbial enzymatic pools involved in N transformations we measured nitrifying and denitrifying enzyme activities. The soil respiration was measured to quantify microbial mineralization, and pools of total C and organic/ mineral N contents in soil, litter and vegetation were quantified. From an applied perspective, our study will provide information on a low-cost ecological engineering method to restore ecosystem functioning of highly disturbed MG.