| It is recognized that macropores are ubiquitous in soils and have a profound effect on soil water movement, although it has a small proportion of the soil volume. In this paper, three paddy soils under different long-term fertilizations, no fertilizer as a control (CK), chemical fertilizer (NPK), chemical fertilizer and organic manure (NPKOM), were selected to investigate pore structure at aggregate and soil column scales. Aggregates (3-5 mm in diameter) and intact soil cores (100 mm in diameter,120 mm height) were sampled from the plough layer of paddy soils. Based on micro-CT and image analysis, the three-dimensional characteristics of soil macropores of aggregates and soil cores were quantified at different scales. Using bromide as a tracer, the effect of pore structure on water flow was analyzed. Our aims were i) to analyze how long-term fertilization impact soil structure, ii) then to determine how soil structure affect water flow. From this study, we attempted to build up a relationship between pore characteristics and water movement, and provide a scientific knowledge for improving soil structure. The main results of this study were as follows:(1) The long-term application of chemical fertilizer with organic manure significantly decreased soil bulk density and enhanced porosity relative to CK and NPK treatments (p<0.05). Meanwhile, NPKOM also promoted the formation and stability of soil aggregates. Soil saturated hydraulic conductivity and the effective moisture under NPKOM were significantly higher than CK and NPK (p<0.05). Based on water characteristic curve, the porosity of the micropores (<0.2μm) were accounted for more than half of the total porosity under the three fertilizer treatments. The macroporosity (>120μm) of NPKOM treatment was accounted for 10% of the total porosity, three times higher than CK and NPK treatments.(2) Based on the micro-CT scanning and image analysis, we studied the soil microstructure of the soil aggregates and undisturbed soil columns. From the binary images and three-dimensional pore structure diagrams, the macroporosity (>120μm) of soil aggregates under CK, NPK, NPKOM treatments were 0.39%,0.10% and 1.07%, respectively; while the macroporosity (>120μm) of soil columns were 7.39%,5.85% and 7.71%, respectively. It indicated that the volumne of macropores were increased with the sample scale. For the characteristics of soil macropore, NPKOM significantly increased soil throat surface area and pore connectivity, whereas NPK did not, and enven decreased them, as compared with the pore characteristics under CK treatment. Pore structure parameters (e.g., surface area, number of pores, paths and throats) had a significant relation with saturated hydraulic conductivity (p<0.01). Effective moisture had a significantly positive correlation with porosity, surface area, number of pore throats, and average path length (p<0.05). Our results demonstrated that NPKOM treatment significantly improved soil structure under good aeration and permeability conditions.(3) The breakthrough curves (BTCs) of Br- in undisturbed soil column showed that the average flow velocity of NPKOM treatment was the fastest and the breakthrough process was the shortest, although the parameters of BTCs did not show significant difference between the three fertilization treatments. The two-region model fitted BTCs well (r>0.95). and the dispersivity was greatest in NPKOM treatment. The quick breakthrough, long tail and asymmetrical shape of BTCs for all of soil columns indicated preferential flow occurred in paddy field. The maximum of C/Co, finishing time of penetration, total time of penetration of BTCs parameters had a significantly positive correlation with wilting moisture. Soil saturated hydraulic conductivity had a significantly positive correlation with flux (r=0.901), while negatively with the peak relative concentration. A significant relationship was observed between solute transport parameters and soil pore structure properties. For instance, the number of pore throats showed a significant negative correlation with finishing time of penetration. The same good correlation was also observed between the number of pore and total time of penetration or maximum of C/Co.In a brief, this study demonstrated that NPKOM improved soil structure, with more porous structure and larger pore throat area, which made soil better aeration and higher plant available water pool. However, single NPK did not improve, even worsen soil structure, relative to CK. Thus, to obtain good soil structure needs the input of organic matter. |
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