Mineral nutrition in two biodiversity hotspots : campos rupestres and kwongan = Nutrição mineral em dois hotspots de biodiversidade: campos rupestres e kwongan

Species distribution along gradients of resource availability depends on their capacity to acquire and use these resources to survive and reproduce in each environment. Soil nutrients are among the most important drivers of the selection of species capable of establishing in each environment, and nutrient-acquisition strategies vary according to their availability. In phosphorus(P) impoverished soils, carboxylates are more efficient than mycorrhizas for P acquisition, while at intermediate P availability mycorrhizas play an important role. Symbiotic nitrogen (N) fixation has high P costs and is expected to occur when growth is N-limited. Each strategy represents a significant carbon (C) cost and creates a trade-off in C allocation for P and N acquisition which can change according to nutrient availability. The prevalence of the different P-acquisition strategies can be assessed through leaf [Mn], because carboxylates also mobilise soil Mn, which then accumulates in leaves, while mycorrhizas intercept Mn. Once taken up, nutrients can be allocated to increase photosynthetic nutrient-use efficiency and be remobilised from senescing tissues. In this thesis, I studied the shifts in nutrientacquisition and -use strategies along fertility gradients in the nutrient-impoverished Brazilian campos rupestres and in the south-western Australian kwongan. One third of the biodiversity hotspots occur in old and P-impoverished soils. Understanding how species in these environments acquire and use nutrients is an important step to unveil the mechanisms that generate and maintain species diversity in different scales. In the first study I investigated nutrient-acquisition and –use strategies of species occurring along a gradient of soil development that also represents a gradient of P availability. I hypothesised that the less Pand N-impoverished soils in campos rupestres would have a greater proportion of mycorrhizal species and that in the most impoverished soils a greater proportion of species would be non-mycorrhizal, with sand-binding roots (rhizosheaths). I also expected these specialisations to be associated with rapid rates of exudation of carboxylates and high leaf [Mn]. Finally, I expected the species from the most impoverished soils to exhibit greater photosynthetic Pand N-use efficiency (PPUE and PNUE) and greater Pand N-remobilisation efficiency. I observed a clear decrease in the prevalence of mycorrhizal species, and an increase in the abundance of species with rhizosheaths in the most impoverished soils. Leaf [Mn] was high, regardless of the presence of rhizosheaths, indicating rhizosheaths are not essential for carboxylate release. We observed similar PPUE and PNUE in species growing in the different soils and with different nutrient-acquisition strategies. Finally, P-remobilisation efficiency was very high in plants growing on all soil types, and N-remobilisation efficiency was only moderate, reflecting the greater Pthan Nlimitation for growth in these habitats. In the second study, I investigated shifts in nutrient-acquisition and –use strategies within two legume species along a soil chronosequence in the south-western Australian kwongan and grown in a glasshouse at varying P and N supply. I expected the proportion of the root length colonised by mycorrhizal fungi to decrease with increasing P supply, the amounts of rhizosphere carboxylates to increase at the lowest P supply, and leaf [Mn] and nodule formation to increase at low N supply, as long as enough P was available. Rather than investing in mycorrhizas or carboxylate exudation, the two species invested in differential biomass allocation to the roots at low-P when grown in the glasshouse. The two species increased nodulation at low N supply. Additional P supply stimulated plant growth, indicating growth was P limited, but they were not N-limited due to N-fixation. Overall, the strong association I observed between plant and soil traits that led to variation in species abundance among plant communities and to the phenotypic plasticity in nutrientacquisition and -use strategies within species may help explain how mechanisms underlying plant species distribution operate at different scales. TABLE OF CONTENTS (SUMÁRIO) CHAPTER 1. GENERAL INTRODUCTION . 14 1.1. Nutrient-acquisition strategies change with soil nutrient availability . 14 1.2. Nutrient-use efficiency in nutrient-impoverished soils . 18 1.3. Long-term soil development in nutrient-poor soils . 19 1.3.1. Study systems . 20 1.4. Significance of research . 24 1.4. 1. Aims . 25 1.5. Thesis outline . 26 1.6. References . 27 CHAPTER 2. NUTRIENT-ACQUISITION AND -USE STRATEGIES IN A MOSAIC OF SOIL TYPES IN CAMPOS RUPESTRES AND CERRADO . 37 2.1. Abstract.37 2.2. Introduction . 38 2.3. Methods.43 2.3.1. Study area . 43 2.3.2. Soil sampling and analyses.44 2.3.3. Species selection . 45 2.3.4. Root sampling for nutrient-acquisition strategies . 46 2.3. 5. Leaf collection for nutrient analyses . 47 2.3. 6. Photosynthesis, photosynthetic nutrient-use efficiency and leaf mass per area . 48 2.3. 7. Statistical analyses . 48 2.4. Results . 49 2.4. 1. Soil data . 49 2.4. 2. Nutrient-acquisition strategies . 50 2.4. 3. Leaf P and N concentrations . 50 2.4. 4. Photosynthetic nutrient-use efficiency . 51 2.5. Discussion . 51 2.5. 1. Soil nutrient availabilities . 52 2.5. 2. Nutrient-acquisition strategies . 52 2.5. 3. Conservative nutrient use . 53 2.5. 4. Photosynthesis, photosynthetic Pand N-use efficiency and functional traits . 54 2.5. 5. Nutrient remobilisation efficiency and proficiency . 55 2.5. 6. Conclusions . 56 2.6. Acknowledgments . 57 2.7. References . 57 CHAPTER 3. PHOSPHORUSAND NITROGEN-ACQUISITION STRATEGIES IN TWO BOSSIAEA SPECIES (FABACEAE) ALONG TWO CHRONOSEQUENCES IN THE SOUTH-WESTERN AUSTRALIAN KWONGAN . 98 3.1. Abstract.98 3.2. Introduction . 99 3.3. Methods.102 3.3.1. Field collections . 102 3.3.2. Glasshouse experiment . 104 3.3.3. Statistical analyses . 107 3.4. Results .