| The root system is an important tissue organ that reflects the ability of plants to adapt to the habitat and is the key link between plants and the surrounding environment to achieve material circulation and energy flow.To find out the growth adaptation law of the root growth configuration,fine root morphology,and chemical characteristics of different order fine roots on the water-salt heterogeneity of the groundwater-soil system in the muddy coastal zone.Tamarix chinensis forest at different groundwater levels（high: GW1 and GW2,medium:GW3,low: GW4 and GW5）in the coastal beach of the Yellow River Delta were researched,full excavation method was adopted to excavate the root system of T.chinensis under different groundwater levels.The aboveground biomass,root biomass,root spatial distribution characteristics,root topology and fractal characteristics of T.chinensis were measured and analyzed.The fine roots were classified by the Pregitzer fine root classification method,and the fine root morphological parameters of each order level were determined in combination with the fine root analysis software.Measured the nutrient and chemical function characteristics of fine roots of each order（C,N content,non-structural carbohydrate content,soluble sugar content,starch content,soluble sugar/starch）.Discuss the differences in the root morphology and architectural characteristics of T.chinensis under different groundwater levels,as well as the morphology and chemical functions of different order fine roots.The main results of this study are as follows:The groundwater level had a significant effect on soil moisture content,salinity,the absolute concentration of soil solution,and salt ions（P<0.05）.With decreasing groundwater level,soil moisture content and salinity both showed a downward trend.The soil salinity at medium（GW3）and low groundwater level（GW4,GW5）was 19.05%,43.92%,and 46.30%lower than that of high groundwater level（GW1）,respectively.The soil moisture content was32.25%,61.21%,and 59.10% lower than the high groundwater level（GW1）,respectively.Groundwater level had a significant effect on soil bulk density,total porosity,and noncapillary porosity（P<0.05）,but had no significant effect on soil capillary porosity（P>0.05）.The groundwater level affected the root growth of T.chinensis by affecting the soil water and salt content.The root-shoot ratio of T.chinensis under different groundwater levels was less than 1,ranging from 0.59 to 0.93.The root-shoot ratio of T.chinensis under low groundwater level（GW4,GW5）was significantly higher than that at high（GW1,GW2）and medium groundwater levels（GW3）（P<0.05）.With the decrease of groundwater level,the root extinction coefficient β of T.chinensis gradually increased,and the proportion of root in the deep soil gradually increased.The root biomass has a negative logarithmic relationship with the deepening of the soil depth.The roots of T.chinensis tend to be distributed on the surface,showing the characteristics of horizontal roots.At high groundwater levels（GW1,GW2）,the topological index of the root system of T.chinensis was small（TI=0.67,0.70）,the topological structure was close to the dichotomous,and the fractal dimension and fractal abundance value were both large,which can be improved by increasing the branching complexity.Realize space expansion and increase plant stability.At medium（GW3）and low groundwater levels（GW4,GW5）,the root topological index was relatively large（TI=0.85,0.95,0.95）,the topological index was close to the herringbone-like pattern.The fractal dimension and fractal abundance values were smaller,the secondary branches were fewer and the structure was simple.The topological structure and fractal characteristics of the root system of T.chinensis respond synergistically to different groundwater levels.Among them,topological index,fractal dimension,main root length,and aboveground biomass parameters can well characterize the root morphology and structure characteristics of T.chinensis under different groundwater levels.At high groundwater levels,the specific root length and specific surface area of fine roots were small,root tissue density was high,fine root growth rate was slow,and nutrient use efficiency was higher than at low groundwater levels,and the absorption of water increased with increasing specific surface area.With decreasing groundwater level,the N content,C/N ratio of fine roots first decreased and then increased,and the soluble sugar,starch and nonstructural carbohydrate contents of fine roots first increased and then decreased.At high and low groundwater levels,the metabolisms of fine roots of T.chinensis were enhanced,and their adaptability to high salt content and low water content soil environments improved.Principal component analysis showed that fine root tissue density,specific surface area,soluble sugar content,C content,and starch content could well characterize the adaptability of T.chinensis.The first-and second-order fine roots of T.chinensis were mainly responsible for water and nutrient absorption,while the higher-order（from third-to fifth-order）fine roots were primarily responsible for transportation and storage of carbohydrates.The responses of fine root morphology and functional indicators of T.chinensis to groundwater level were complex.The fine root morphology,nutrients,nonstructural carbohydrate characteristics and other aspects of the water and salt environment heterogeneity cooperated in a synergistic response and trade-off adjustment.In conclusion,at high groundwater levels,the root system of T.chinensis can achieve spatial expansion and increase plant stability by increasing branch complexity and coping with high-salt environments by reducing root biomass and other avoidance mechanisms.At low groundwater levels,T.chinensis adapts to water deficit by expanding root distribution and absorption area.T.chinensis fine roots can improve its adaptability to high salinity and low water content soil environment by enhancing its metabolism.Based on the differences in root growth,architecture,and fine root morphology,nutrition,and non-structural carbohydrates,the roots of T.chinensis have strong phenotypic plasticity and adaptability to the water-salt heterogeneous habitat of the groundwater-soil system. |
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