Plant nutrient resorption, a ubiquitous mechanism of nutrient conservation, has often been proposed to be more pronounced in infertile than fertile habitats, and in species common to infertile compared to fertile habitats, because of the presumed advantage when nutrients are scarce. However, previous studies provide weak and inconsistent empirical support for these hypotheses, although few have examined intraspecific variation across well-quantified resource gradients. This study addresses intraspecific patterns of nutrient resorption for eight species across two N availability gradients on similar soils in an N-limited oak savanna ecosystem: a long-term fire frequency gradient with a negatively correlated N fertility gradient and a long-term N fertilization gradient. We hypothesized that both resorption proficiency (the minimum nutrient level retained in a senesced leaf) and efficiency (the proportional change in leaf nutrient concentration) would decrease with increasing soil N availability and plant N status. For the seven non-N fixers, either resorption proficiency or efficiency decreased modestly in treatments with higher N availability. In contrast, the legume Amorpha canescens Pursh had higher N levels in green and senesced leaves, and resorbed N much more weakly than the non-fixers, and did not respond in terms of proficiency or efficiency to soil N availability. Across all species and sites in each N fertility gradient, a scaling analysis showed greater resorption efficiency in plants with lower N concentrations. Our data suggest that species can have modest resorption responses reflective of soil nutrient availability and differences in resorption related to their N economy that represent mechanisms of nutrient conservation in nutrient-limited soils.
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Acknowledgements We are grateful to Autumn Sabo and Fernando Silla for helpful discussion in experiment implementation and initial sample collection. Chris Bolton and Casey Fagre helped with sample collection and processing. Nutrient analyses were performed by Cathleen McFadden at the University of Nebraska. Two anonymous reviewers provided comments which contributed to the clarity of the manuscript. Financial support was provided by the National Science Foundation Long Term Ecological Research Program (DEB-0080382).
- Leaf nutrient status
- Nutrient conservation
- Soil fertility gradients