Journal of Vegetation Science 24 (2013) 506–517 Does soil nitrogen availability mediate the response of grassland composition to water regime? Yoseph N. Araya, David J. Gowing & N. Dise Keywords Air-filled pore space; Grassland composition; Ion exchange resin; Meadow vegetation; Microbial community composition; Nitrogen mineralization; Phospholipid fatty acids (PLFA) Nomenclature Rodwell (1992) Received 13 September 2010 Accepted 18 August 2012 Co-ordinating Editor: Helge Bruelheide Araya, Y.N. (corresponding author, y.n.araya@open.ac.uk) & Gowing, D.J. (d.j.gowing@open.ac.uk): Department of Environment, Earth and Ecosystems, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK Dise, N. (n.dise@mmu.ac.uk): Department of Environmental and Geographical Sciences, Manchester Metropolitan University, Manchester, M1 5GD, UK Abstract 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 mineral- ization 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 min- eralization 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, respec- tively) 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 mecha- nism 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. Introduction Plant communities in wet meadows are strongly influenced by the depth and annual variation in the water table (Hayati & Proctor 1990; Gowing & Youngs 1997; Dwire et al. 2004). Experimental and observational field studies have shown that minor shifts in soil water regime may play a significant role in the composition of plant communities (e.g. Gowing & Spoor 1998; Silvertown et al. 1999; Van Oorschot et al. 2000; Leyer 2005). In spite of this recognition, relatively little attention has been paid to the mechanism of how the water regime influences plant composition, and thus the distribution of species along gra- dients in soil water regime (Levine et al. 1998; Yu & Eh- renfeld 2010). It is known that in many natural environments soil nutrient availability is the major factor Journal of Vegetation Science 506 Doi: 10.1111/j.1654-1103.2012.01481.x © 2012 International Association for Vegetation Science