Nitrogen addition elevated autumn phosphorus retranslocation of living needles but not resorption in a nutrient-poor Pinus sylvestris var. Mongolica plantation

Abstract

Continuously increasing atmospheric nitrogen (N) deposition has aggravated phosphorus (P) limitation in many forests worldwide, but how trees change nutrient use efficiency to adapt to the aggravated P limitation induced by N deposition, and what is the role of understory vegetation in mediating the responses are largely unknown. Here, the primary nutrient use strategies, i.e., nutrient resorption during leaf senescence and seasonal nutrient retranslocation in living tree leaves, were investigated in response to 5 years of N addition (none vs. 10 g N m−2 year−1) and understory removal (understory vegetation was intact vs. removed) in a nutrient-poor Mongolian pine (Pinus sylvestris var. mongolica) plantation in semiarid northern China. Soil available P concentration was significantly decreased due to N addition, but such decrease was counteracted by the understory removal. Neither P resorption efficiency nor P proficiency was significantly affected by these treatments, with an average P and N resorption efficiency of 60% and 57%, respectively. N addition decreased P concentration by 17% but did not affect total P content in a needle of all aged classes, regardless of the understory vegetation manipulation. Furthermore, N addition significantly elevated the autumn P retranslocation of all aged living needles only when understory vegetation was intact, with an average of 55% of autumn P retranslocation, compared to an average of 15% in the other treatments. The autumn P retranslocation of Mongolian pine was positively correlated with soil available P concentration. Understory removal alone marginally decreased leaf P concentration only in the current-year needles, and significantly attenuated the spring and autumn P retranslocation. Our results highlight the importance of seasonal P retranslocation in elevating tree P use efficiency under the condition of enhanced P deficiency, and role of understory vegetation in mediating seasonal P dynamics and their responses to increasing N enrichment.