Root morphological and physiological plasticity of perennial grass speciesand the exploitation of spatial and temporal heterogeneous nutrient patches

Root morphological and physiological characteristics of four perennial gras s species were investigated in response to spatial and temporal heterogeneo us nutrient patches. Two species from nutrient-rich habitats (i.e. Holcus l anatus and Lolium perenne) and two species from nutrient-poor habitats (i.e . Festuca rubra and Anthoxanthum odoratum) were included in the study. Patc hes were created by injecting equal amounts of nutrient solution into the s oil either on one location (i.e. spatial heterogeneity) or on several, alte rnating locations (i.e. temporal heterogeneity) within the pot. The consequ ences of changes in root morphology and the implications for the exploitati on of the nutrient patches by individual plants were quantified by the amou nt of N-15 captured from the enriched patches. The effects of nutrient hete rogeneity on the acquisition of nutrients by species were determined by com paring the total nitrogen and phosphorus acquisition of the species in the two heterogeneous habitats with the total nitrogen and phosphorus acquisiti on in a homogeneous treatment. In this homogeneous treatment the same amoun t of nutrient solution was supplied homogeneously over the soil surface. Th e experiment lasted for 27 days and comprised one harvest. In response to the spatial enrichment treatment, all species produced signi ficantly more root biomass within the enriched patch. The magnitude of the response was similar for species from nutrient-rich and nutrient-poor habit ats. In contrast to this response of root biomass, root morphology, includi ng specific root length, branching frequency and mean lateral root length w as not affected by the treatments. In response to the temporal enrichment t reatment, all species were able to increase the nitrogen uptake rate per un it of root biomass. The species from nutrient-poor habitats had, on average , higher uptake rates per unit root biomass than the species from nutrient- rich habitats, but the magnitude of the response did not differ between the species. These results question the general validity of the assumptions th at root foraging characteristics differ among species from nutrient-rich an d nutrient-poor habitats. As a result of these root responses, all species captured an equal amount o f N-15 from the spatial and temporal enriched nutrient patches and all spec ies acquired significantly more nitrogen in the heterogeneous treatments th an in homogeneous treatment. Hence, the ability to exploit local and tempor al nutrient heterogeneity does not appear to differ between species from nu trient-rich and nutrient-poor habitats, but is achieved by these species in different ways. The ecological implications of these differences are discu ssed.