Study site
This study was conducted in Shenmu County of Shaanxi Province in China (110° 05′–110° 30′ E, 39° 27′–39° 15′ N), which is located in the continental arid and Semiarid areas. The annual average temperature is 8.9 °C. The annual frost-free period is 130 days, the mean annual precipitation of the area is about 396 mm, and potential evaporation is 1,790 mm.
The research plot is in the heartland of Shendong coal mining subsidence area in Shenmu County, typical steppe landscape, soil impoverishment, and fragile ecological environment. The basic physical soil properties in the test site were measured (Table 1). According to the SL237-1999 engineering classification standard of the Geotechnical Test Regulations, the soil in the test area was named as low liquid limit silt (ML). Major plant species under natural conditions in the study area include S. psammophila, Caragana microphylla Lam., H. rhamnoides L., Artemisia ordosica Krasch., Agriophyllum squarrosum (Linn. ) Moq., and Lespedeza bicolor Turcz.
Root sampling
The straight roots of 4 years old of S. psammophila and H. rhamnoides L. were used as materials, and applied instantaneous axial small injured force (corresponding to 30% of the average ultimate force of the radial level, less than the elastic ultimate force, and the deformation is recoverable elastic deformation) and instantaneous axial large injured force (corresponding to 70% of the average ultimate force of the radial level, greater than the elastic ultimate force, and the deformation is irreversible plastic deformation) without leaving the plant body, to understand the survival rate, the change in root diameter and tensile strength of straight roots withstand transient tensile injured after self-repair.
As the layers of soil could interfere with the anti-tension force and anti-tension strength of roots, we excavated the roots without leaving the plant body and selected the roots which are distributed in the same soil layer. Straight roots with uniform diameter ranged from 1 to 4.5 mm, roots segments of 100 mm were selected from the root systems. To sufficiently attribute the tensile ability of roots, the selected roots were divided into seven diameter classes with 0.5 mm interval. To ensure the parallelism of the test, each diameter class selected eight test roots and eight control roots.
In the test, the soil around test roots was removed and the position of test roots was kept unchanged so that the exposed length of the roots reached the test requirements, the test roots were shaded, and sprayed water to maintain moisture. And each test root length was greater than 100 mm (Fig. 1), three points of A, B and C was selected along the root, and the diameters were measured by the cross method using an electronic Vernier caliper with an accuracy of 0.01 mm. B was the midpoint of the test root, A, C were the ends of the 30 mm from the midpoint. The diameters of control roots were measured in the same way.
The schematic diagram of test root.
Root treatments
At the beginning of the plant growing season, in 2019 May. Test roots were applied two instantaneous axial injured forces without leaving the plant body, then covered soil growth. The process of excavation and covering soil was also carried out on the control roots in the same way (Fig. 1). By August, after a 3-month growth period, the test roots self-repaired for 3 months and excavated the test roots and control roots again, observed the survival rate, measured the root diameter and the tensile strength of test root (Fig. 2).
Excavation phase of test roots.
Root tensile tests were conducted by a homemade portable instrument (Fig. 3). The instrument is composed with a platform (Part A), a root clap (Part B) fixed on the platform, a HG 100 digital display type push–pull meter (Part C), a moveable root clap (Part D) which is connected with Part C, a crank handle (Part E) which is used to move Part C and a Vernier caliper (Part F) which is connected with Part C to control the loading rate of the load. The test root is clapped by the two root pads. To make sure the test root not slip, we put a rubber pad inside each of the root clap. When the crank handle is turned, Part C is moved away from Part B and the force acted on the test root is recorded. The accuracy of HG 100 digital display type push–pull meter is 0.05 N. After selecting the test root, carefully excavated the soil under the test root and placed the instrument (length 50 cm, width 13 cm, height 20 cm). Fixed the points a and c of the test root at the jaws of the clamp so that the test root was in tension. The axial direction was pulled, and the length of the instrument was placed in the same direction as the root growth direction. Using 50 mm/min loading rate applied force injury by reading the Vernier caliper moving rate, stopped after the degree of injury urging force of the design, marked the ends of the test root segment and backfilled them, and marked on the ground to be dug again. The treatment method of the parallel control test roots was the same.
HG 100 digital display type push–pull meter and self-made portable test instrument.
Determination of injury force
The recoverable elastic deformation occurs in the root system before the elastic limit point. After the elastic limit point, the root system undergoes irreversible plastic deformation. Previous researches indicate that the elastic limit of the 0–8 mm straight root of S. psammophila is about 40% of the ultimate tensile strength, and that of H. rhamnoides L. is about 60% of the ultimate tensile strength30,31. Tests have shown that when the injury force reaches 80% of the average ultimate tensile strength, more test roots break when applying the injury forces. To observe impact of varied injury force on the self-repair of roots, we selected two levels of injury force in this study, the small injury force was 30% of the ultimate tensile force (less than the elastic limit point), and the large injury force was 70% of the ultimate tensile force (greater than the elastic limit point).
Due to the uneven root diameter along the axial direction, it was impossible to determine the fracture point at which the test root may be damaged before the test. To guarantee data quality, the measured number was not recorded when the test root was fractured in the experiment. The ultimate force was calculated from the regression equation according to the average root diameter of each test root test segment, the average root diameter of each test root was the mean of the root diameters of the three points A, B, and C. According to the root diameter of each test root, the ultimate tensile force was calculated by the regression equation, and the corresponding small injury force and large injury force were determined (Table 2).
Data analysis
The data were analyzed using SPSS 15.0 for Windows. The test roots and the parallel control roots were excavated after self-repaired for 3 months, observed the root shape, color and elasticity. If the root turned black, dry and begins to fall off, the root was dead. For the roots that survived, the root diameter and ultimate tensile strength were measured again, and the tensile strength was calculated using Eq. 1.
$$ {text{P}} = 4{text{F}}/left( {uppi {text{D}}^{2} } right) $$
(1)
where P is the tensile strength (MPa), F is the tensile force (N), D is the root diameter (mm).
Source: Ecology - nature.com
