Nitrogen addition regulates tradeoff between root capture and foliar resorption of nitrogen and phosphorus in a subtropical pine plantation

KEY MESSAGE : Increased nitrogen availability affects root capture and leaf resorption of nitrogen and phosphorus, and the corresponding acquisition-conservation tradeoff in slash pine forests. Plants may maintain their stoichiometric homeostasis via regulating root nutrient capture (acquisition) and leaf nutrient resorption (conservation). However, how increased nitrogen (N) availability affects root capture and leaf resorption of N and phosphorus (P), and the corresponding acquisition-conservation tradeoff remains unclear. We quantified N and P concentrations and their stoichiometric ratios of five nutrient pools (soils, absorptive fine-roots, transport fine-roots, green needles, and senesced needles) to investigate simultaneous responses of root capture (nutrient foraging potential and translocation) and needle resorption (efficiency and proficiency) to different rates and forms of N addition in a slash pine plantation in subtropical China. N addition increased N concentration in senesced needles, but reduced N capture capacity. N addition reduced P concentration in senesced needles (improved P resorption proficiency), but did not affect P capture capacity. N addition exerted no significant influence on resorption efficiency of N and P. N rate and form effects occurred only in nutrient foraging (N foraging was more responsive to high N rate and P foraging was more responsive to ammonium-based N addition), but not in nutrient translocation and resorption. The decreased root N capture capacity and increased senesced needle N concentration imply an alleviation of N limitation to plant growth and a behavior of plants to economize on cost for nutrient acquisition and conservation. P resorption rather than capture strategy was employed, indicating a more conservative use of P in response to exacerbated P limitation. These findings jointly suggest that the allocation of effort towards capture and resorption strategy depends on the strength and type of nutrient limitation. Our results have implications for understanding the acquisition-conservation tradeoff in plant nutrient economy and the mechanisms that plants maintain their stoichiometric homeostasis in the context of N loading.