Nutrient resorption and stoichiometric responses of poplar (Populus deltoids) plantations to N addition in a coastal region of eastern China

Aims Leaf nutrient resorption is sensitive to changes in soil nutrients. However, the effects of N deposition on nutrient resorption efficiency (NuRE) in plant macro-nutrients remain unclear. Poplar (Populus deltoids) is one of the most extensively cultivated hardwood species worldwide. We explored general patterns and dominant drivers of NuRE and stoichiometry of poplar plantations in response to N addition. Methods We conducted a 4-year N-addition experiment to explore NuRE and stoichiometric responses to N addition in two poplar (P. deltoids) plantations (8- and 12-year-old stands) in a coastal region of eastern China. We measured soil and foliar (green and senesced leaves) concentrations of nitrogen (N), phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg) for a series of N addition treatments including N-0 (0 kg N ha(-1) yr(-1)), N-1 (50 kg N ha(-1) yr(-1)), N-2 (100 kg N ha(-1) yr(-1)), N-3 (150 kg N ha(-1) yr(-1)) and N-4 (300 kg N ha(-1) yr(-1)). Important Findings Consistent for (both) 8- and 12-year-old stands, N addition did not affect the NuRE and stoichiometry with the exception of CaRE and CaRE:MgRE ratio). N resorption efficiency-P resorption efficiency (NRE-PRE) scaling slopes were consistently less than 1.0 under N addition. These results suggest that NRE generally decouples from PRE within each N treatment. Moreover, these results point to robust control of green leaf nutritional status on nutrient resorption processes as indicated by the positive relationships between NuRE and green leaf nutrient concentrations. Our findings provided a direct evidence that growth in 12-year-old poplar plantations was N-limited in the coastal region of eastern China.