Characteristics of NH(4)(+) and NO(3)(−) Fluxes in Taxodium Roots under Different Nitrogen Treatments

To understand the characteristics of net NH(4)(+) and NO(3)(−) fluxes and their relation with net H(+) fluxes in Taxodium, net fluxes of NH(4)(+), NO(3)(−) and H(+) were detected by a scanning ion-selective electrode technique under different forms of fixed nitrogen (N) and experimental conditions. The results showed that higher net NH(4)(+) and NO(3)(−) fluxes occurred at 2.1–3.0 mm from the root apex in T. ascendens and T. distichum. Compared to NH(4)(+) or NO(3)(−) alone, more stable net NH(4)(+) and NO(3)(−) fluxes were found under NH(4)NO(3) supply conditions, of which net NH(4)(+) flux was promoted at least 1.71 times by NO(3)(−), whereas net NO(3)(−) flux was reduced more than 81.66% by NH(4)(+) in all plants, which indicated that NH(4)(+) is preferred by Taxodium plants. T. ascendens and T. mucronatum had the largest net NH(4)(+) and total N influxes when NH(4)(+):NO(3)(−) was 3:1. (15)N Atom% and activities of N assimilation enzymes were improved by single N fertilization in the roots of T. distichum. In most cases, net H(+) fluxes were tightly correlated with net NH(4)(+) and NO(3)(−) fluxes. Thus, both N forms and proportions could affect N uptake of Taxodium. These findings could provide useful guidance for N management for better productivity of Taxodium plants.