Effects of nitrogen on the hydraulic characteristics and biomass allocation in Notopterygium franchetii in the Qinghai-Tibet Plateau

Nitrogen fertiliser changes the soil environment, regulates the water balance of plants, and directly affects nitrogen and water use efficiency. In order to identify the adaptation mechanism and water regulation strategy of Notopterygium franchetii to the soil nitrogen environment, this study was conducted. Two-year-old Notopterygium franchetii plants grown in farmland were used as the test material. The hydraulic characteristics, natural dehydration, water capacitance and biomass allocation were investigated at three nitrogen application levels (0, 40, 80 kg/ha) and four growth periods (seedling, jointing, flowering and harvesting period). Nitrogen content in roots, stems and leaves increased with different levels of nitrogen treatments at each growth stage, and the change of nitrogen content in stems was the most significant. The overall water capacitance of all tissues was root > stem > leaf. After medium nitrogen treatment and high nitrogen treatments, the average water capacitance of leaves was 122.00% and 129.26% of low nitrogen treatment, and the average water capacitance of stems was 125.78% and 134.59% of low nitrogen treatment, the average water capacitance of leaves was 112.28% and 116.37% of low nitrogen treatment. Nitrogen treatment can improve the water use efficiency of plant roots, stems and leaves, and the relative content of stable carbon isotope (δ13C) in the whole growing season increased by 0.77%, 1.96% and 5.27% of the control group after medium nitrogen treatment, and 1.60%, 2.18% and 1.61% of the control group after high nitrogen treatment. The ratio of root to canopy decreased significantly in each growth stage after nitrogen treatment, and the change was most significant at seedling period (from 1.25 to 0.66), and the change was the most least at jointing period (from 0.77 to 0.63). Our study thus indicates that nitrogen addition could regulate water balance in plants, which is beneficial to water storage and utilisation, but excessive nitrogen fertiliser supply will also affect the distribution of plant biomass.