Nitrogen conservation via resorption across two fertility gradients

Plant nutrient resorption has often been proposed to provide greater nutrient conservation in infertilite than fertile habitats because of the presumed advantage when nutrients are scarce. However, previous literature reviews provide weak and inconsistent empirical support for this hypothesis, although few have been made examining intraspecific variation on 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 N availability. For the seven non-N fixers, either resorption proficiency or efficiency decreased modestly in treatments with higher N availability. Additionally, the legume Amorpha canescens had higher N levels in green and senesced leaves than the non-fixers, resorbed N weakly, and did not respond in terms of proficiency or efficiency to N availability. Further support for nutrient resorption as a conservation measure is given utilizing a mathematical model developed in the literature to explicitly test this hypothesis. Our results suggest that species can have modest resorption responses reflective of soil nutrient availability that may provide a mechanism of nutrient conservation in nutrient-limited soils.