BackgroundExtensively managed grasslands in temperate biomes are capable of harboring a big variety of plant and invertebrate species. Yet, they have suffered from a strong decline in the past decades mainly due to agricultural intensification. Agri-environment schemes have been introduced in Europe in order to promote farmland biodiversity, but they were only little effective, especially so in grasslands. Not surprisingly, grassland restoration and recreation through active seed addition has thus gained in importance in the recent years. The most common methods used rely either on the addition of commercial seed mixes, on the addition of seeds collected from a speciose donor grassland or on transferring hay from a speciose donor grassland after the soil of the receiver site has been prepared (ploughing, harrowing or topsoil removal). While there is evidence that these restoration methods may contribute to improve the biodiversity of grasslands, especially plant diversity, their relative effectiveness remains poorly known. The aim of this systematic review is to scrutinize the peer-reviewed literature for scientific evidence about comparative effectiveness.MethodsWe will search for peer-reviewed journal articles in bibliographic databases and grey literature in search engines and organizational websites dealing with at least one of the above mentioned seed addition methods. We will only include studies which were carried out in temperate Europe. Through a scoping exercise a search string was developed which was based on a previously prepared test-list. The search string was then tested for validity with two independent reference lists. Screening will be done on the title, abstract and full-text level and consistency checking will be done on a random subsample by a second reviewer. After critically appraising internal validity of the retained studies, data on the responses of plants and invertebrates as well as all relevant meta-data will be extracted and coded. A meta-analysis will be conducted on studies with high internal validity whereas a narrative analysis will be done with descriptive statistics on studies with medium internal validity. Potential effect modifiers like study duration, former land use or local climate will be included in the analysis as moderators.