Accumulated temperature rather than nitrogen fertilization is the main factor determining growth of young citrus trees in the field

Tree growth is influenced by both, the availability of nutrients in the soil and temperature. However, how the interaction of nitrogen (N) fertilization and accumulated temperature (>= 10 degrees C) determines the growth of evergreen citrus trees is still unknown. To characterize this interaction, we studied the biomass distribution of young citrus trees in a four-year field experiment (2018-2021, Chongqing city, China) and used the results to build an allometric growth model. Based on a completely randomized block design (Two-way ANOVA and Duncan ' s test), three N levels were established, namely, N0 (no N fertilizer), N1 (80 % of the farmer's usual fertilization), and N2 (farmer's usual fertilization). Our results indicate that excessive N fertilizer application increases the inorganic N content in deep soil, whereas the aboveground biomass distribution was largely dependent on tree age, but independent of N fertilization. An allometric growth model of citrus aboveground biomass was established at different N levels and provided a realistic quantitative relationship between accumulated temperature and growth. It indicates that canopy volume (power function), stem diameter (linear function) and total aboveground biomass (exponential function) are highly positively correlated with accumulated temperature, rather than N fertilization. Since the canopy volume and total aboveground biomass are also significantly positively correlated with stem diameter, this parameter seems to be suitable for estimating the biomass of young citrus trees in the field. Further research is required to characterize the relationship between growth of citrus trees and other environmental factors.