Abiotic and biotic factors determining the nutrient stoichiometry of contrasting terrestrial ecosystems
- Autores: Ifigenia Urbina Barreto
- Directores de la Tesis: Josep Peñuelas Reixach (dir. tes.), Oriol Grau (codir. tes.), Jordi Sardans Galobart (codir. tes.)
- Lectura: En la Universitat Autònoma de Barcelona ( España ) en 2019
- Idioma: español
- Tribunal Calificador de la Tesis: Joan Romanyà Socoró (presid.), Xavier Domene Casadesús (secret.), Emma J. Sayer (voc.)
- Programa de doctorado: Programa de Doctorado en Ecología Terrestre por la Universidad Autónoma de Barcelona
- Materias:
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- Resumen
Abstract Everything on Earth is based on chemistry. This statement has profound implications for ecological interactions. Living organisms generate and control fluxes of energy and matter among the atmosphere, lithosphere and the hydrosphere, shaping the chemistry of the Earth in many different ways. Ecological stoichiometry aims to explore the balance and role of multiple chemical elements in ecological interactions and help us to understand patterns and processes in nature. It represents the link between the biogeochemistry and the ecosystems’ function and allows to describe processes across different levels of biological organization, from cellular structures to ecosystems.
In this Thesis I use ecological stoichiometry to describe processes at organism and ecosystem levels in three contrasting terrestrial environment conditions. Autotrophs’ stoichiometry is established when these organism use light to fix carbon (C) and simultaneously assimilate nutrients. Plants are able to store nutrients in the cells’ vacuole and in different organs, which make them highly flexible (less homeostatic) in terms of their elemental composition. This feature explains the high adaptability of plants to different environments, including soil nutrient limitation conditions. Furthermore, plant-soil interaction could be explored through the foliar stoichiometry, because it has been shown that the foliar N:P is positive correlated with the N:P of soil in all terrestrial ecosystem, suggesting that foliar stoichiometry is a good indicator of the resource availability.
Plant adaptations to soil nutrient limiting conditions are quite common in all terrestrial ecosystems, such as nitrogen fixation, mycorrhiza association, production of phosphatases and nutrient resorption before leave abscission. The species’ chemical composition is affected by all these abiotic and biotic interactions, and these exchange of chemical elements between the species and the abiotic part of the system determine the elemental composition of different components of the ecosystems.
In Chapter 2, we explore the biotic effect of the community composition on the species foliar stoichiometry, taken as a proxy of the species’ biogeochemical niche. We found that each species has its own biogeochemical niche and is able to readjust its chemical composition in response to different biotic conditions. We conclude that plants can readjust their foliar element composition when they grow in communities with contrasting plant composition through the biogeochemical niche displacement, suggesting a differential use of the resources when the patterns of species coexistence change.
In Chapter 3 we explore the plant-soil stoichiometry changes due the shrub expansion into the subalpine grassland in the Pyrenees. Shrub expansion had a clear impact on the plant-soil stoichiometry spectrum. This expansion represents the transition from pure grassland to shrubland. The grassland is an ecosystem dominated by short-lived species, fast nutrient turnover between the plant-soil compartments, high nitrogen (N), phosphorus (P) and potassium (K) concentrations in the plant-soil system, high productivity but low biomass stocks. The shrubland is an ecosystem characterized by long-lived species with more conservative strategy, slow nutrient turnover (low N and P concentrations in the plant-soil compartments, high C:nutrient ratios in the aboveground biomass) and high stocks of C and nutrients in the plant aboveground biomass. Shrub encroachment increase the acquisition of N through mycorrhizal associations. The changes in storage and elemental composition of the plant soil system along the succession from grassland to shrubland suggests that there is a slowdown of the biogeochemical cycle in the subalpine mountain areas where shrub encroachment occurred.
In the Chapter 4, we describe the distribution of C and the most important nutrients for the plant development (N, P, K) in the plant and soil compartments in old-growth tropical forests growing in nutrient-poor soil in French Guiana. We also studied the nutrient resorption from senescent leaves, a poorly explored mechanism that plants use to avoid losing nutrients in this ecosystem. Our results showed that P was the scarcest nutrient in the leaf, leaf-litter and soil. Resorption efficiencies were higher for K and P than for N, and only K resorption efficiency was affected by seasonality. P resorption showed a negative and weak correlation with P in soil (total and available). Relationships between nutrient resorption and species functional characteristics (growth rate, wood density, diameter at breast height and specific leaf area) were weak and varied among the nutrients, and phylogenetic relatedness did not account for the variability in resorption efficiencies. Our results suggest that high K and P resorption from senescent leaves is an adaptive strategy allowing species to cope with soil nutrient scarcity. Furthermore, the level of nutrient immobilization in foliar compounds (N > P > K) seem to significantly determine the resorption process. We conclude that nutrient resorption from senescent leaves is a key process for plants to conserve nutrients in tropical forests of French Guiana, especially for K and P, where soil availabilities are low and depend mainly on soil parent material and leaching process.
To sum up, in this Thesis we have demostrated how the elemental composition of the plant-soil system reflects ecological interactions and processes, such as intra and inter specific plant interactions (Chapter 2), poorly explored physiological processes such as nutrient resorption (Chapter 4) and the importance of stoichiometry studies for describing changes at ecosystem level and predicting future scenarios (Chapter 3). These studies add new knowledge to the ecological stoichiometry field and highlights the importance of this approach in the ecological studies.
