Leaf nutrient resorption across contrasting soil parent materials: it is not just about efficiency

In theory, nutrient resorption from leaves should be higher in sites with lower soil nutrients, increasing the retention and reuse of nutrients that are limited in supply. However, evidence for this relationship has remained inconsistent for decades, particularly where the percent of nutrient resorbed is concerned. Furthermore, plant resorption responses to soil phosphorus (P) concentration remain poorly understood. Leaf nitrogen (N) and P concentrations and resorption percentages (efficiencies) were measured for 68 Australian plant species sampled across four nutrient-rich vs nutrient-poor site-pairs representing contrasting soil parent materials. N and P concentrations in senesced leaves were significantly lower on nutrient-poor than on nutrient-richer sites, enabling greater internal conservation of these nutrients on nutrient-poor sites. Resorption efficiencies averaged 45% for N and 69% for P and did not differ between contrasting soils. Though not sensitive to site, P resorption efficiency (P_RE) was positively related to the proportion of labile P in leaves (R² = 0.54). We demonstrated for the first time that senesced leaf P concentrations and the corresponding P_RE are distinctly shaped by plant adaptation to soil nutrient scarcity, and internal labile P pools, respectively. The high P_RE of Australian species informs an upper bound for this process in global vegetation models.