Soil abiotic and biotic controls on plant performance during primary succession in a glacial landscape

Plant-soil interactions are temporally dynamic in ways that are important for the development of plant communities. Yet, during primary succession, the degree to which changing soil characteristics (e.g. increasing nutrient availabilities) and developing communities of soil biota influence plant growth and species turnover is not well understood. We conducted a two-phase glasshouse experiment with two native plant species and soils collected from three ages (early, mid- and late succession) of an actively developing glacial chronosequence ranging from approximately 5 to <100years in age. The first phase assessed how soil abiotic characteristics and soil biota immediately influenced plant performance. In the second phase, we used conditioned soils to grow the same plant species and assessed how the direction and strength of plant-soil feedback (PSF) varied through succession. We used high and low nutrient andsoil sterilization treatments to isolate effects of abiotic and biotic factors on plant performance and to assess how patterns of PSF related to soil nutrients. For both species, our experiments revealed that the initial effect of soil biota on plant performance depended on successional stage. In soils collected from early-, mid- and late-succession stages, we observed neutral to negative, negative and positive effects of soil biota on plant performance, respectively. Plant-soil feedback varied with successional stage and depended on whether microcosms were supplied with nutrients. Under low nutrient conditions, soil conditioning by plants, unrelated to initial soil biota, was an important mechanism for positive PSF. Under high nutrient conditions, PSFs differed by plant species in early- and mid-succession soils, but were consistently neutral in late succession.Synthesis. Soil biota can affect the performance of pioneer plants over short gradients of ecosystem development, but the strength and direction of interactions depend on nutrient availability. Conceptual frameworks describing mechanisms of primary succession should acknowledge soil conditioning by previous plants as a potential mechanism of positive feedback.