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

Abstract

Variation in stable nitrogen isotope ratios (δ15N) was assessed for plants comprising two wetland communities, a bog‐fen system and a flood plain, in central Japan. δ15N 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 δ15N 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 δ15N values; plant δ15N was positively associated with rooting depth. Moreover, an increasing gradient of peat δ15N 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.

Foliar 15N natural abundance indicates phosphorus limitation of bog species

“…At Kopuatai, L. scoparium had highly depleted d 15 N across all but the most nutrient-rich marginal peats (range=13.5&) and, on low nutrient peats, differed from co-existing Empodisma by an average of 13.2&, with a maximum separation of 16.5&. Large differences in d 15 N (up to 10-12&) in co-habiting species have similarly been recorded in other nutrient-poor environments such as subarctic tundra (Michelsen et al 1996(Michelsen et al , 1998Nadelhoffer et al 1996), subtropical wet heathland (Schmidt and Stewart 1997), and temperate bog-fen (Kohzu et al 2003). Changes in external P availability relative to internal plant demand have been shown to effect spatial changes in plant d 15 N across nutrient gradients in mangrove stands.…”

Advancing the use of minirhizotrons in wetlands

“…In contrast to woody species, sedges, which are found in the waterlogged northern tundra, across a variety of peatland ecosystems, and in marshes, develop gas exchange root features called aerenchyma that permit rapid transport of oxygen to deep roots (Moog and Janiesch 1990). As a result, sedges are often found deeper in the peat profile than roots of woody plants (Moore et al 2002;Sullivan et al 2007;Murphy et al 2009a), which can allow greater access to soil nutrients (Chapin et al 1988;Kohzu et al 2003). Forbs, although relatively rare in nutrient-poor wetlands, can also have aerenchymous root systems (Rydin and Jeglum 2006).…”

δ15N of soil N and plants in a N‐saturated, subtropical forest of southern China

“…However, an increase in the δ 15 N of EIN with increasing soil depth has been proposed 18, 39. In fact, a higher δ 15 N value of EIN in deeper soil depth has been frequently assumed in studies to determine the N sources of plants using plant δ 15 N 40–42. Consequently, we first surveyed the δ 15 N profile of available N in soil, including EON, to evaluate the variation in δ 15 N among different N forms and the vertical variation in δ 15 N. We hypothesized that the δ 15 N of all the available N in soil increases concomitantly with soil depth following the increase in the δ 15 N of bulk soil N. We also expected that the δ 15 N of EON is similar to that of bulk soil N because solubilization of organic N is expected to exhibit no isotopic fractionation 43, 44.…”