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Complementarity in mineral nitrogen use among dominant plant species in a subalpine community¹

Laboratoire Evolution et Diversité Biologique, CNRS-UMR 5174, Université Paul Sabatier, 31062 Toulouse cedex 4, France; Centre d'Etudes Spatiales de la Biosphère, CNES-CNRS-IRD-UMR 5126, Université Paul Sabatier, 31401 Toulouse cedex 4, France

ABSTRACT

The underlying mechanisms that enable plant species to coexist are poorly understood. Complementarity in resource use is among the major mechanisms proposed that could favor species coexistence but is insufficiently documented. In alpine soil, low temperatures are a major constraint for the supply of plant nitrogen. We carried out ¹⁵N labeling of soil mineral N to determine to what extent four major species of a subalpine community compete for N, or develop ionic (NH₄⁺ vs. NO₃^–) or temporal complementarity. The Poaceae took up much more ¹⁵N per soil area unit than the ericaceous species, and all species displayed three major strategies in exploiting ¹⁵N: (1) uptake mainly early in the growing season (Vaccinium myrtillus), (2) uptake at a slow and similar rate throughout the growing season (Rhododendron ferrugineum), and (3) uptake at high rates over the growing season (Festuca eskia and Nardus stricta). However, while F. eskia used ¹⁵NH₄⁺ mainly early and ¹⁵NO₃^– mainly late in the growing season, the reverse was observed for N. stricta. Taking into account ¹⁵N dilution in soil NH₄⁺ and NO₃^– pools, we calculated that NH₄⁺ provided more than 80% of the mineral N uptake in Ericaceae and about 60% in grasses. Together, such ionic and temporal complementarity would reduce competition between species and could be a major mechanism promoting species diversity.

Key Words: ionic and temporal complementarity • mineral N • ¹⁵N uptake • plant interaction • subalpine community

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