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

摘要

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 70 cm) 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 45 cm, and a controlled laboratory N mineralization study with water tension varying between 0 and 100 cm (0-10 kPa), 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 55 cm and mean soil inorganic N of 20 mg.kg-1). 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.