Application of salt-isolation materials to a coastal region: effects on soil water and salt movement and photosynthetic characteristics of Robinia pseudoacacia

Soil salinization is a major obstacle to the optimal utilization of land resources. Salt-affected soils are widely distributed throughout the world. The present extent of salt-affected soils substantially restricts plant growth in these areas. It has been demonstrated that leaching with water,chemical amendment,surface mulching with straw and phytoremediation are the most often used approaches to ameliorate saline soils. Engineering measures are also an effective solution to control salt movement in the saline soil. Establishing salt-isolation interlayers beneath the surface of saline soil is one of the most widely used engineering measures. Even though there are many studies dealing with salt-isolation interlayers establishment,very little is known about their effects on saline soil. Here,we used three salt-isolation materials to restrict the water andsalt movements in saline soil: zeolite,ceramsite and river sand. The aim of this study was to assess the effects of saltisolation interlayers application on soil water and salt movements in saline soil and on the photosynthetic characteristics of Robinia pseudoacacia in the coastal regions of Tianjin city. We also anticipate that the outcome of the study could form part of the basis for selection of salt-isolation materials in improving coastal saline soil with consideration of their costs. The research was conducted from April 2010 to August 2013 at the Coastal Salt-tolerant Plant Science and Technology Park,Dagang,Tianjin,China. An experiment was conducted with four treatments:( 1) No addition of slat-isolation interlayer( CK);( 2) Addition of zeolite( FS) at the bottom and side walls of the planting sites;( 3) Addition of ceramsite( TL) at the bottom and side walls of the planting sites;( 4) Addition of river sand( HS) at the bottom and side walls of the planting sites. All treatments were arranged in a randomized complete block design with four blocks. Each block was again divided into four plots. The four treatments were randomly assigned to each plot within the individual blocks with a separate randomization for each block. The salt-isolation materials were added to nine planting sites in each plot. The nine sites were evenly distributed based on a planting spacing of 3 m× 3 m. Each planting site was 1 m× 1 m× 1 m. The results indicated:( 1) Salt-isolation interlayers could significantly increase soil water content of tree planting site. FS treatment performed best and had the lowest salt content and salt solute concentration,followed by TL. Relative to the CK,HS had no significant effects on salt content and salt solute concentration.( 2) FS treatment significantly increased the leaf photosynthetic rate( Pn),transpiration rate( Tr),stomata conductance( Gs) and leaf water use efficiency( LWUE),and reduced intercellular CO2concentration( Ci). TL treatment can also significantly enhance Pn、Tr,but it had no significant effects on Gs、Ci and LWUE. HS treatment had significantly increased Pn and Tr,and reduced LWUE. We concluded that the zeolite was an optimal salt-isolation material in controlling salt movement in saline soil and improving tree growth in the coastal regions.