2019-2020 electrical capacitance of cotton roots

We conducted two rounds of plastic pot experiments (in 2019 and 2020), one PVC tube experiment (in 2020) and one field experiment (in 2020) at the Luancheng Agro-Eco-Experimental Station of the Chinese Academy of Sciences (37°53′N, 114°41′E, altitude: 50.1 m) located in Shijiazhuang, China.The ECroot and soil electrical capacitance (ECsoil) were measured with a Passive Component LCR Meter (EXTECH LCR200, USA) in a parallel equivalent circuit at 100 Hz, 1 kHz, and 10 kHz AC frequencies with 1 V terminal voltage. For Pot + Field topsoil experiments, we measured the ECroot before and after severing the root with a sharp knife (the knife was inserted at an angle of approximately 45 degrees from the horizontal plane and approximately 5 cm from the stem.). Except for the Tube + Mixed topsoil experiment, all the ECroot and ECsoil measurements followed similar steps as the other three experiments. When measuring ECroot, the negative electrode was inserted into the soil to 12 cm through a stainless steel rod (13 cm long and 3 mm in diameter), which was positioned 10 cm away from the stem base. The positive electrode (round rod stainless steel and 1 mm in diameter) was inserted into the center of the residual plant stem at 1 cm above the soil surface. Two stainless steel electrodes with a diameter of 3 mm and a distance of 20 cm were inserted into the soil to a depth of 12 cm to measure ECsoil.For the Tube + Mixed topsoil experiment, the same positive electrodes were used as in the other three experiments. The negative electrode was inserted into the soil at the side of the pipe wall and 10 cm from the top end of the PVC pipe. After the measurement was completed, the plant electrode was fixed, and the ground electrode was sequentially moved 10 cm downward for the next measurement. Each PVC pipe was measured 6 times down to 60 cm. We pretested the effects of varying the distance of the soil electrode (at the same soil layer) on the capacitance measurements.All the roots were separated from the soil by rinsing through a stainless steel sieve (0.5 mm mesh). We carefully washed soil from the roots and retained most of the root material in the pot and PVC tube experiments. Root samples were stored in a freezer (-20 °C) until they could be washed and separated. The debris was manually removed from washed root samples. The cleaned roots were scanned on a flatbed scanner (Epson Expression 12000XL, USA), and root length, surface area and volume were analyzed with WinRhizo software (ver. 2019a, Reagent Instruments Inc., Quebec, Canada). In the Pot + Field topsoil experiment, only the tap root and the attached lateral roots were scanned. Some roots broken during cleaning were not scanned. In the Pot + Mixed topsoil, Tube + Mixed topsoil and Field experiments, all roots were scanned. All roots were subsequently oven-dried to a constant mass (75 °C for 72 h), and the dry weight was recorded.At the time of root harvest, a subsample of soil was collected, and the soil water content was measured by the oven-drying method (drying for 24 h at 105 °C).

本研究于中国石家庄市的中国科学院栾城农业生态系统试验站（北纬37°53′，东经114°41′，海拔50.1 m）开展了2组塑料盆盆栽试验（2019年、2020年）、1组聚氯乙烯（PVC）管试验（2020年）以及1组田间试验（2020年）。采用美国产EXTECH LCR200型无源元件电感电容电阻（LCR）测试仪，以平行等效电路模式，在100 Hz、1 kHz、10 kHz的交流频率下，以1 V端电压测定根系电容（ECroot）与土壤电容（ECsoil）。针对盆栽+田间表层土试验，我们在使用利刃切断根系前及切断后测定了ECroot（切割时刀具与水平面呈约45°夹角，插入位置距茎秆约5 cm）。除管+混合表层土试验外，其余3组试验的ECroot与ECsoil测定步骤均保持一致。测定ECroot时，将直径3 mm、长13 cm的不锈钢棒作为负极，插入距茎秆基部10 cm处的土壤中，插入深度达12 cm；将直径1 mm的圆形不锈钢棒作为正极，插入土面以上1 cm处的残留植物茎秆中心。测定ECsoil时，将2根直径3 mm、间距20 cm的不锈钢电极插入土壤至12 cm深度。针对管+混合表层土试验，正极电极与其余3组试验保持一致；负极电极插入PVC管壁侧、距管顶端10 cm处的土壤中。完成一组测定后，固定植物电极，将土壤电极依次向下移动10 cm开展下一次测定，每根PVC管向下共测定6次，直至深度60 cm。本研究预先测试了同一土层内土壤电极间距变化对电容测定结果的影响。所有根系均通过0.5 mm孔径的不锈钢筛冲洗以与土壤分离，仔细洗去根系附着的土壤，在盆栽与PVC管试验中保留了绝大多数根系样品。根系样品保存于-20 ℃冰箱中，待后续清洗与分离操作。手动去除清洗后根系样品中的杂质。将清洗后的根系置于平板扫描仪（美国爱普生Expression 12000XL型）上扫描，并采用WinRhizo软件（2019a版，加拿大魁北克省Reagent仪器公司）分析根系长度、表面积与体积。在盆栽+田间表层土试验中，仅扫描主根及附着的侧根，清洗过程中断裂的部分根系未纳入扫描。在盆栽+混合表层土、管+混合表层土及田间试验中，所有根系均被扫描。随后将所有根系置于烘箱中烘干至恒重（75 ℃下烘干72 h），记录其干重。在收获根系时，采集土壤子样品，采用烘干法（105 ℃下烘干24 h）测定土壤含水量。