Does soil nitrogen availability mediate the response of grassland composition to water regime?

Questions How is the composition of wet meadow plant communities influenced by water regime? What controls soil nitrogen (N) availability along a gentle soil water regime gradient? Is vegetation response to subtle changes in soil water regime mediated by N availability? Location Cricklade, Wiltshire, southwest England. Methods A field survey of community composition and soil N availability was conducted along a gentle gradient of water table depth (mean water table depth between 25 and 70cm) at a species-rich hay meadow in southwest England. This study was complemented by a controlled mescosm study undertaken at water table depths between 5 and 45cm, and a controlled laboratory N mineralization study with water tension varying between 0 and 100cm (010kPa), the latter augmented with a microbial community study. Results The field, mesocosm and laboratory studies showed that the soil N mineralization rate and N availability responded to water regime. In particular, the N mineralization rate strongly declined when the soil air-filled pore space dropped below 10% of total soil volume. In the field, plant cover distribution (individual species as well as functional groups) correlated with increase in soil water table depth (r=0.86 and r=0.89 for graminoids and forbs, respectively) and soil N availability (r=0.77 and r=0.85, respectively). A switch in dominance between forbs and graminoids occurred at a mean water table depth of 55cm and mean soil inorganic N of 20mg center dot kg1). Study of the soil microbial community composition in the mesocosms, using phospholipid fatty acid (PLFA) signatures, showed that microbial community composition was also significantly influenced by soil water status. Conclusions We show that soil moisture content and soil aeration control the availability of N, a limiting nutrient, in species-rich wet meadows. The mechanism is linked to changes in suitability of the soil water regime for microbially mediated N mineralization. Maintaining the water regime is therefore critical because adjustment of the hydrological niche also conditions species' responses along the N availability gradient. This could subsequently prove a useful tool for conservation managers trying to sustain a target vegetation type.