Influence of soil water regime on nitrogen availability and plant competition in wet meadows

Abstract

Identified ecological drivers controlling the equilibrium of species coexistence in wet meadows include site hydrology, soil nutrient availability and grazing. Of these, depth and annual variation of the water table has been considered as a primary factor and been frequently used in management decisions. A mechanistic understanding of how water regime influences species coexistence is thus vital for guiding conservation practices. In this context, this thesis explores the involvement of nitrogen availability, an often limiting resource which may be dependent on soil water regime. Laboratory and mesocosm experiments alongside field observations were undertaken to explore the interrelationships between water regime, nitrogen availability and plant competition.Three coexisting meadow species: meadow fescue, Festuca pratensis, common sedge, Carex nigra and greater burnet, Sanguisorba officinalis, were used to study the consequences in plant competition.Study of soil nitrogen mineralization revealed a depression in mineralized nitrogen asmatric potential approached zero. This depression coincided with soil air-filled porespace of less than 10%. The changes in water tension were accompanied by changes in soil microbial community composition as indicated by their phospholipid fatty acid signatures. Mesocosm study of F. pratensis and C. nigra grown on a gradient of constant water regime showed Significant differences in biomass production and tissue nitrogen concentration. Individually and in competition the species responded by modifying resource allocation to reproductive/vegetative as well as shoot/root tissues. Nitrogen fertilization removed the influence of water regime on biomass production and tissue nitrogen concentration of C. nigra and S. officinalis. However, it did not significantly negate the influence of water regime on plant competitive response. Direct field observation in a species-rich meadow confirmed species richness, biomass production and tissue nitrogen concentration were correlated to both soil water regime and nitrogen availability. A multivariate ordination of all recorded species along measured gradients of soil water regime, nitrogen availability and plant tissue nutrient concentrations indicated evidence of niche separation between species