Significance of rooting depth in mire plants: Evidence from natural 15 N abundance

Variation in stable nitrogen isotope ratios (δ 15 N) was assessed for plants comprising two wetland communities, a bog‐fen system and a flood plain, in central Japan. δ 15 N of 12 species from the bog‐fen system and six species from the flood plain were remarkably variable, ranging from −5.9 to +1.1‰ and from +3.1 to +8.7‰, respectively. Phragmites australis exhibited the highest δ 15 N value at both sites. Rooting depth also differed greatly with plant species, ranging from 5 cm to over 200 cm in the bog‐fen system. There was a tendency for plants having deeper root systems to exhibit higher δ 15 N values; plant δ 15 N was positively associated with rooting depth. Moreover, an increasing gradient of peat δ 15 N was found along with depth. This evidence, together with the fact that inorganic nitrogen was depleted under a deep‐rooted Phragmites australis stand, strongly suggests that deep‐rooted plants actually absorb nitrogen from the deep peat layer. Thus, we successfully demonstrated the diverse traits of nitrogen nutrition among mire plants using stable isotope analysis. The ecological significance of deep rooting in mire plants is that it enables those plants to monopolize nutrients in deep substratum layers. This advantage should compensate for any consequential structural and/or physiological costs. Good evidence of the benefits of deep rooting is provided by the fact that Phragmites australis dominates as a tall mire grass.