This is a synthetical and critical presentation of the papers of the session n°4 of the European Grassland Congress (La Rochelle, 27-30 May 2002). The three plenary papers dealt with : 1) the amplitude and diversity of effects on grasslands of the present increase in atmospheric carbon dioxide (on grassland production and its distribution, on forage quality, on soil water content and the nitrogen cycle...), 2) the key elements of the nitrogen cycle on animal farms (evaluation of pollution risks ; codes of recommended practices...), and 3) the main mechanisms explaining the co-existence of species and/or of vegetation dynamics induced by the agricultural practices ('clonal' strategies, interest of the 'functional group' concept). As a whole the papers showed an increased interest towards the effects of management on bio-diversity, mainly considered from the botanical point of view ; nutrient balances on the farm scale and a larger use of legumes were discussed ; the studies of environmental balances were enlarged to include the climatic change and the greenhouse effect. Three types of problems stand out at the end of this congress : the necessary progress in modelling the bio-geo-chemical cycles in grasslands ; the necessity of integrating approaches to the grassland eco-systems (coupling sward dynamics to the bio-geo-chemical cycles especially, and reinforcing interface studies ; linking together the various approaches) ; and thirdly the development of the changeing of scales in time and space as an investigational tool.