Abstract: Aims: Drought effects on the carbon balance are considered as the major factor of tree mortality and are as-sumed to be regulated by soil nutrient (e.g., nitrogen) availability. However, the effects of nitrogen addition on trees carbon and nitrogen utilization between aboveground and belowground and on coupling relationship of carbon and nitrogen in various organs in response to drought are still unclear in trees. Methods: A two-year full factorial microcosm experiment was set up with sessile oak (Quercus petraea). Nitrogen addition was performed in the first year, and drought was conducted in the second year. The isotope 15N labeling and 13C labeling were carried out before drought and during drought, respectively. Three consecutive samplings were conducted after dual labeling of 13C and 15N labelling in the second year, and the effects of ni-trogen addition on carbon and nitrogen allocation dynamics during progressive drought were tested. Important findings: Our results showed that previous nitrogen addition promoted aboveground photosyn-thetic carbon fixation and nitrogen allocation, increased root nitrogen uptake, reduced the non-structural carbo-hydrates (NSC) contents in all organs and changed the relationships of carbon and nitrogen in aboveground and belowground organs in sessile oak. However, drought had minor effects on nitrogen and carbon allocation be-tween aboveground and belowground, and had minor effects on the relationship of carbon with nitrogen in all organs (represented by the ratio of 13C to 15N in all organs). Drought only significantly reduced the content of NSC in sessile oak. During drought (from day 40 to 73), previous nitrogen addition led sessile oak to prioritize belowground carbon and nitrogen allocation. Our results indicate that sessile oak can change its carbon and ni-trogen allocation strategies to adapt to drought well, while previous nitrogen addition may increase its drought sensitivity.

Key words: Previous nitrogen addition, Drought, Carbon and nitrogen allocation, 13C isotope labelling, 15N isotope labelling