1. Understanding the mechanisms that underlie the generation of beta-diversity remains a challenge in ecology. Underground plant adaptations to environmental gradients have received relatively little attention.2. We studied plant nutrient-acquisition strategies and nutrient-use efficiency at three stages of pedogenesis in infertile soils from campos rupestres and on less infertile soil from cerrado sensu stricto in Brazil. All soils support very high plant diversity with high species turnover between soil types at small spatial scales (metres). We expected that differences in nutrient-acquisition and -use strategies would be associated with this high species turnover. With severely decreasing phosphorus (P) availability, we expected the effectiveness of arbuscular mycorrhizal (AM) symbioses for plant P acquisition to decrease, and reliance on nonmycorrhizal strategies (NM) to increase, while maintaining efficient nutrient use.3. Concentrations of total soil P and nitrogen (N) were greater in soils in cerrado than in those from campos rupestres, and the more weathered soils from campos rupestres were severely P and N impoverished. The proportion of the root length colonized by AM fungi was 71% in the soils from the cerrado and <1% in the most P-impoverished soil type from campos rupestres. Conversely, the proportion of species with nonmycorrhizal P-acquisition strategies such as rhizosheaths was greater in the most P-impoverished soils. Leaf [P] and [N] were very low and decreased with decreasing soil [P] and [N]. Leaf N:P ratios suggest P limitation of plant productivity in the campos rupestres but N-P colimitation in the cerrado.Photosynthetic rates decreased with increasing P impoverishment, but photosynthetic P-use efficiency was very high and photosynthetic N-use efficiency moderately high on all soils. Most species had very high P-remobilization efficiency during leaf senescence (>70%), but only moderate N-remobilization efficiency (~50%).
In old, phosphorus (P)-impoverished habitats, root specializations such as cluster roots efficiently mobilize and acquire P by releasing large amounts of carboxylates in the rhizosphere. These specialized roots are rarely mycorrhizal. We investigated whether Discocactus placentiformis (Cactaceae), a common species in nutrient-poor campos rupestres over white sands, operates in the same way as other root specializations. Discocactus placentiformis showed no mycorrhizal colonization, but exhibited a sand-binding root specialization with rhizosheath formation. We first provide circumstantial evidence for carboxylate exudation in field material, based on its very high shoot manganese (Mn) concentrations, and then firm evidence, based on exudate analysis. We identified predominantly oxalic acid, but also malic, citric, lactic, succinic, fumaric, and malonic acids. When grown in nutrient solution with P concentrations ranging from 0 to 100 μM, we observed an increase in total carboxylate exudation with decreasing P supply, showing that P deficiency stimulated carboxylate release. Additionally, we tested P solubilization by citric, malic and oxalic acids, and found that they solubilized P from the strongly P-sorbing soil in its native habitat, when the acids were added in combination and in relatively low concentrations. We conclude that the sand-binding root specialization in this nonmycorrhizal cactus functions similar to that of cluster roots, which efficiently enhance P acquisition in other habitats with very low P availability.
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