Impact of saline irrigation and application of N and P on growth and nutrient distribution of Tamarix chinensis planted in coastal saline-alkali soil

Saline irrigation is a highly effective method of elimination of salt and drought stress in spring in the coastal severe saline-alkali soils. Tamarix chinensis, a salt-tolerant plant, plays an important role in reforming the ecological landscape of the coastal wetlands in northern China. However, the responses of plant growing rhythm of T. chinensis to long-term saline irrigation-associated fertilization remain unclear. To promote the rapid growth of T. chinensis for carbon (C) fixation, landscape construction, and saline-alkali soils reclamation, an experiment involving irrigation with local phreatic saline resources and application of nitrogen (N) and phosphorus (P) was conducted to alleviate spring drought and salt stresses, and solve the prominent problems of soil N and P deficiencies in the coastal severe saline-alkali soil. The experiment was consisted of two factors of saline irrigation and N and P fertilization over a four-year period in the coastal saline-alkali soil in the low plain of North China. Six treatments included no fertilizer as control (CK, WCK), only applying N (N, WN), applying both N and P (NP, WNP), each corresponding to no irrigation (the first) and irrigation with local phreatic saline water containing 8.02−9.34 g·L⁻¹ salt (the second), respectively. The results showed that the growth and resprouting of T. chinensis occurred mainly in spring till to summer, but plant height and diameter increase and resprouting did not synchronized simultaneously. The trends of rates of plant height growing, resporuting and stem diameter thickening of T. chinensis during the growth season were as sloping shapes, triangle, and up fast following down slowly, respectively. In the first year of saline irrigation, soil water content, fresh weight of plant and leaf dry weight of T. chinensis was reduced significantly (P<0.01); but the stem dry weight was not impacted. However, after three-year saline water irrigation, the plant height and diameter growth was inhibited, the N content of stem and leaf were decreased significantly (P<0.05), but the P content of leaf increased by 11.8% on average, and branch numbers of T. chinensis increased significantly (P<0.05). The WNP treatment, i.e. saline irrigation and applying NP, retarded the decline of growth rate of plant height and stem diamter during spring to summer (May−June), not only accelerated the resprouting rate, but also increased the N and K contents of leaves, and promoted the transfer of Ca²⁺ and Mg²⁺ from stem to leaves, which resulted in a distribution change of ion in stem and leaf. WNP can alleviated the negative effect of continuous saline irrigation on the growth of T. chinensis by accelerating resprouting, promoting nutrient absorption and ion transport. The results provides a support for rational saline water irrigation and fertilization for T. chinensis in the costal serious saline-alkali soil.