Mechanism And Quantitative Characterization Of Changes Of Sodic Soil Structure Under The Effect Of Reclamation Using Flue Gas Desulphurized Gypsum

Soil structure composed of soil particles, aggregates and pore system is critical to water, air and nutrients movement in soils and biological activities, and an important factor of soil quality. The ultimate goal for sodic soil reclamation using Flue Gas Desulphurized (FGD) gypsum is to develop the soil quality including soil structure. So it is necessary to investigate the impact of different reclamation method on soil structure characteristics not only to reveal the dynamics of soil quality enhancement process, but to develop the methods of reclamation. This study was mainly based on a two-year field experiment lied in Hetao irrigation district in China. The dosage of FGD gypsum (G) and quantity of leaching water (W) were considered as the two main factors and nine treatments consisting of the two factors at three different levels (1:low level;2:high level;3:none) were applied in experiment plots. The chemical properties of sodic soil under different treatmetns at different depths, and the growth and yield of sunflower planted in2012were monitored during the experiment. Soil samples after two-year reclamation were collected to investigate the charactersic of soil particle size distribution under different reclamation treatments based on fractal and multifractal theory. Undisturbed soil cores in nine representative treatment plots at four depths and intact soil columns with three different soil texture were collected for X-ray computed tomography(CT) scanning, which provides digital images for quantifying the changes of soil pore system under the effect of reclamation treatments. A sodic soil quality evaluation model based on soil structure parameters was constructed using the principal component analysis (PCA) method. The main conclusions were as following:(1) Combined reclamation treatments with both FGD gypsum and leaching water applied significantly (p<0.05) decreased ESP and pH at the0to40cm depth; treatments with FGD gypsum applied alone showed significant but less effect while treatments with leaching water applied alone exhibited no impact. Compared with the control treatment, all reclamation treatments significantly decreased EC at the0to20cm depth, but treatments with either FGD gypsum or leaching water applied alone showed higher EC after reclamation. The treatment with high level of FGD gypsum (14.5t/hm2) and low level of leaching water (1.52×103t/hm2) applied had the biggest improvement of the growth and yield of sunflower.(2) Combined reclamation treatments significantly (p<0.05) decreased soil bulk density and fractal dimension D of soil aggregates, while increased the mean weight diameter (MWD), geometric mean diameter (GMD) and macro-aggregate (diameter>0.25mm) content (DR0.25and WR0.25) measured by both drying-sieving and wet-sieving methods. Compared with the control treatment, the treatments with leaching water applied alone had higher drying-sieving GMD, MWD and DR0.25, but lower D value at the0to40cm depths. The treatments with FGD gypsum applied alone showed lower drying-sieving GMD, MWD and DR0.25than the control treatment at the0to10cm depth, but higher than that at the10to40cm depths. Its D value was lower than the control treatment across the profile.(3) Combined reclamation treatments showed no impact on the clay, silt and sand content and the fractal dimension D of soil particle size distribution at0to60cm depths, but significantly increased the entropy dimension1l based on multifractal spectra.(4) Combined reclamation treatments at0to20cm depths significantly increased the image-based total porosity, macroporosity, mesoporosity, irregular porosity, regular porosity and fractal dimension D of pore size distribution from2D images, and total number of pores, macroporosity, horizontal porosity, pore length density and number of junctions from3D reconstructed pore system. Treatments with either FGD gypsum or leaching water applied alone only positively influence pore structure at the10to40and0to10cm depths, respectively.(5) Reclamation with both FGD gypsum and leaching water applied significantly increased macroporosity, tortuosity and number of junctions of the image-based3D pore system in silt, silt loam and sandy loam sodic soils. Particularly, reclamation showed more pronounced impact on the vertical porosity, horizontal mesoporosity and horizontal macroporosity in silt, silt loam and sandy loam sodic soils, respectively.(6) Based on the constructed sodic soil quality evaluation model combined with soil structure parameters using the PCA method, the quality of sodic soil under the treatment with high level of FGD gypsum (14.5t/hm2) and low level of leaching water (1.52×103t/hm2) applied showed the most significant improvement.