Nitrogen uptake preference and allocation in Populus cathayana in response to drought stress

The preference for different nitrogen (N) forms may enhance plant survival and fitness. However, little information is known about how soil water availability, N addition, and their interaction dynamically impact plant N uptake preference and allocation. Here, we conducted a pot experiment using Populus cathayana cuttings, consisting of soil water regime (well watering control/mild drought/intensive drought: 80/60/40 % of field capacity) combined with N addition (N0 as control/N4/N8: 0/4/8 g N m−2 yr−1) treatments based on 15N isotope tracer technology. Two tracer solution (15NH4NO3/NH415NO3, both at 10.2 atom%) were used in N4/N8 treatments. Root, stem and leaf were collected at 6 h, 1, 3, 7, 14, and 31 days after labeling. Results showed that N addition significantly increased plant biomass and N accumulation, and decreased N allocation to leaf. Drought significantly decreased plant biomass, N accumulation, and N uptake (indicated by δ15N), and intensive drought reduced more than mild drought did. Nitrogen allocation to leaf was increased by drought. NO3- uptake by P. cathayana (indicated by δ15N in NH415NO3 treatment) was correlated significantly positively with total root length (TRL), total root surface area (TRA), specific root length (SRL), specific root area (SRA), root vitality, and root nitrate reductase (NR) activity. Intensive drought significantly reduced TRL, TRA, SRL, SRA, root vitality, and root NR activity under N4 and N8 treatments, which resulted δ15N in NH415NO3 treatment reduction and lower than in 15NH4NO3 treatment, so P. cathayana exhibited ammonium preference under intensive drought treatment. Our study provides an essential insight into how plants adapt and prefer different N forms under drought stress in nature.