Affiliation: | (1) Area d’Ecologia Vegetal i Botànica Forestal, Centre Tecnològic Forestal de Catalunya, Pujada del Seminari s/n, E 25280 Solsona, Spain;(2) Departament Productes Naturals, Biologia Vegetal i Edafologia, Universitat de Barcelona, Avgda, Joan XXIII s/n, E 08028, Barcelona, Spain;(3) Fundación Centro de Estudios Ambientales del Mediterráneo (CEAM), Parque Tecnológico c/ Ch, 14, E-46980 Darwin, Paterna, Spain;(4) Departament Biologia Vegetal, Facultat de Biologia, Universitat de Barcelona, Diagonal 645, E 08028 Barcelona, Spain |
Abstract: | Fires may greatly alter the N budget of a plant community. During fire nitrogen is lost to the atmosphere. Although high light availability after fire promotes N2-fixation, the presumably high soil N availability could limit N2-fixation activity. The latter limitation might be particularly acute in legume seedlings compared with resprouts, which have immediate access to belowground stored carbon. We wished to learn whether early post-fire conditions were conducive to N2-fixation in leguminous seedlings and resprouts in two types of grassland and in a shrubland and whether seedlings and resprouts incurred different amounts of N2-fixation after fire. We set 18 experimental fires in early autumn on 6 plots, subsequently labelling 6 subplots (2 × 2 m2) in each community with 15NH4+-N (99 atom % excess). For 9 post-fire months we measured net N mineralisation in the top 5 cm of soil and we calculated the fraction of legume N derived from the atmosphere (%Ndfa) in seedlings and resprouts. We used two independent estimates of the amounts of N derived from non-atmospheric sources in potentially N2-fixing plants: mean soil pool abundance and the 15N-enrichment of non-legumes. Despite substantial soil net N mineralisation in all burned community types (about 2.6 g Nm−2 during the first nine months after fire), the %Ndfa of various legume species was 52–99%. Legumes from both grasslands showed slightly higher N2-fixation values than shrubland legumes. As grassland legumes grew in more belowground dense communities than shrubland legumes, we suggest that higher competition for soil resources in well established grass-resprouting communities may enhance the rate of N2-fixation after fire. In contrast to our hypothesis, legume seedlings and resprouts from the three plant communities studied, had similar %Ndfa and apparently acquired most of their N from the atmosphere rather than from the soil. |