Development of a negative plant-soil feedback in the expansion zone of the clonal grass Ammophila arenaria following root formation and nematode colonization

1 The feedback between individual plants and their soil communities is a major driver of plant community processes. We analyse the rate at which plant-soil feedback develops in the root zone of the clonal dune grass Ammophila arenaria. 2 Ammophila arenaria grows vigorously when it can form new roots in newly deposited windblown beach sand. The colonization zone is hypothesized to provide an enemy-free space, with root pathogens and parasites possibly contributing to degeneration of A. arenaria when deposition stops. 3 We quantified root biomass and plant parasitic nematode densities in new and 1-year-old root zones, as well as in degenerate stands, in the field at monthly intervals. Each month, we studied the biomass production of Ammophila seedlings in controlled conditions in sterilized and non-sterilized soil from the different sampling sites, with and without nematicides. 4 Colonization of newly deposited sand by the endoparasitic nematodes Heterodera arenaria and Pratylenchus spp. within 1 month of root formation coincided with a negative feedback in the bioassays. Nematicides counteracted growth reduction significantly, but their effectiveness decreased in soil samples collected later in the growing season. 5 In older layers, roots and nematodes were already present before the sampling started. Growth reduction in unsterilized sand was observed at all sampling events, but was not counteracted by nematicides. 6 In the field, root biomass of A. arenaria in the newly colonized sand increased throughout the growing season, despite the development of a negative plant-soil feedback. 7 We conclude that the development of negative plant-soil feedback in the root zone of A. arenaria closely follows the colonization of newly deposited sand by roots and then by the endoparasitic nematode species Heterodera arenaria. There seems to be a shift in organisms causing this feedback during the first growing season, but root biomass nevertheless increases. However, in older root layers and in the degenerate stand, ongoing negative plant-soil feedback may contribute to a gradual decrease in root biomass.