Increases of Soil C, N, and P Pools Along an Acacia Tree Density Gradient and Their Effects on Trees and Grasses
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Nitrogen (N) fixing trees including many species of Acacia are an important though variable component of savanna ecosystems. It is known that these trees enrich the soil with carbon (C) and N, but their effect on the combined C:N:P stoichiometry in soil is less well understood. Theory suggests that they might reduce available phosphorus (P), creating a shift from more N-limited conditions in grass-dominated to more P-limited conditions in tree-dominated sites, which in turn could feed back negatively on the trees’ capacity to fix N. We studied the effects of Acacia zanzibarica tree density upon soil and foliar N:P stoichiometry, and the N2-fixation rates of trees and leguminous herbs in a humid Tanzanian savanna. Foliar N:P ratios and N2-fixation rates of trees remained constant across the density gradient, whereas soil C, N and organic P pools increased. In contrast, the N:P ratio of grasses increased and N2-fixation rates of leguminous herbs decreased with increasing tree density, indicating a shift towards more P-limited conditions for the understory vegetation. These contrasting responses suggest that trees and grasses have access to different sources of N and P, with trees being able to access P from deeper soil layers and perhaps also utilizing organic forms more efficiently.
Keywordscarbon sequestration legume nitrogen-fixing trees nutrient limitation N:P stoichiometry plant–soil feedback phosphorus savanna tree–grass interactions woody encroachment
We thank Stefanie Karrer, Marc-Jacques Mächler, and Anneke Valk for their help in the field and Sabine Güsewell for her statistical advice. We acknowledge Britta Jahn-Humphrey, Marilyn Gaschen, and Adolphe Munyangabe for their help in the lab and Stefano Bernasconi for carrying out isotopic analyses. We thank Annette Stähli and the authorities of Saadani National Park for their support in Tanzania, and John Williams and Benjamin Donald for their assistance in the field. Helpful comments of Werner Suter, Cory Cleveland and two anonymous reviewers significantly improved the manuscript. Research was conducted with permission and support given by the Tanzanian Wildlife Research Institute (TAWIRI), Commission of Science and Technology (COSTECH) and Tanzania National Parks (TANAPA). This study was financed by the Swiss National Science Foundation Grant No. 2-77321-08.
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