Effects of direct saline irrigation and nitrogen and phosphorus application on a coastal saline-alkali soil planted Tamarix chinensis

In order to cope with the outstanding problems of spring drought, salt return, and lack of both N and P in coastal severe saline-alkali region, we utilized the rich underground saline water resources and the Tamarix’s characteristics of both fixation carbon (C) and discharge salt to achieve the goals of elimination of salt and drought stress, deficiency of both soil N and P, and improvement green ecological landscape in coastal severe saline soils as well. Tamarix chinensis Lour., a shrub species, is a species with strong salt-tolerant and water conservation ability that is widely planted in saline-alkali soils. This species plays an important role in improving the regional ecological landscape reformation and maintaining the stability of the coastal ecosystem, like the coastal wetlands in northern China. A two-factor experiment was conducted about saline water and fertilization in the Tamarix chinensis soil over a four-year period in 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 salt content in the 0−30 cm soil layer of Tamarix chinensis land decreased by 32.7% on average after the first saline irrigation, which had a significant desalination effect on the severe saline-alkali coastal soil, but after three consecutive-year saline irrigation, the salt content in 0−30 cm soil layer averagely increased by 11.2%−18.3%; yet the available P (Olsen-P) content in 0−90 cm soil layer decreased by 19.4%−32.1%, the contents of microbial carbon (Cmic) and microbial nitrogen (Nmic) in 0−30 cm soil decreased by 15.5% and 19.7% respectively. However, compared with the control WCK, the WNP treatment, i.e. saline irrigation and applying NP, decreased the salt content of 0−30 cm soil by 36.1%, in the meanwhile significantly decreased the total N, water content and contents of 0−30 cm soil layer, Olsen-P of 30−60 cm layer as well. WNP was conducive to increase of 0−30 cm Cmic, soil organic matter (SOM) and ammonium acetate extractable K (Kex) content. Our experiment provides evidence that P was a strongly limited nutrient, saline irrigation-associated fertilization of N and P can effectively alleviated the risk of soil secondary salinization caused by continuous saline irrigation, relieve the limitation of key nutrients deficiency like P and N, and promote the soil microbial activity, with the strongest evidence for saline irrigation and soil nutrient management for artificial Tamarix chinensis in severe saline-alkali coastal soil.