Impact of drought on the functioning of grassland systems

A rainfall manipulation experiment was conducted over a two-year period on two semi-natural grassland sites, in order to determine the effects of drought on both forage production (dry matter and nutritive value) and plant-soil relationships. Water stress simulations were performed with the aid of rainout shelters. Among the nutritional parameters, lignocellulose content (ADF) and water-soluble carbohydrates (WSC) showed the greatest variation in drought conditions. Variations in ADF content were strongly linked to the drought-induced yield losses (less fibre in the small plants). By contrast, crude protein (CP) content remained fairly constant under drought conditions due to antagonistic processes: the detrimental effects on nitrogen nutrition were offset by slower plant growth (i.e. higher plant N concentration due to reduced shoot biomass). The phosphorus cycle (P cycle) was adversely affected by the water restrictions, with quite different responses depending on soil P content. A late stress (i.e. one occurring after the grass growth peak) had more-pronounced effects than an early one (occurring during the peak). This experiment allows the effects of water shortage to be placed in a broader context by showing that the variations in yield and quality caused by drought are of same order of magnitude as the natural variations that can be observed between different years.