1 Description Title A method for measuring biomass of tree-roots by ground-penetrating radar FIELD OF THE INVENTION The present invention generally relates to a method for measuring biomass of tree-roots by ground-penetrating radar (GPR). BACKGROUND OF THE INVENTION Roots play a crucial role in plant ecosystem, with two principal functions: the acquisition of soil-based resources (mainly water and nutrients); and the provision of stability to maintain the plant's upright structure. In addition, the contribution of roots to the ecosystem carbon budget is also significant and indispensable, accounting for 20%-40% of the total forest carbon and driving rhizosphere phenomena associated with soil respiration. Recently, root responses to global change have also been demonstrated: coarse-root biomass increased with elevated CO 2 , suggesting that coarse roots have implications for long-term sequestration and storage of excess atmospheric C02 in the rhizosphere. With the recognition of the important functions of roots in ecosystems, especially in the global change context, such root-related parameters as root size, biomass, distribution, structure and 3D architecture are urgently required for a better understanding of root functions, biogeochemical cycles, and various aspects of global change. However, measurements are difficult, because roots are hardly visible and difficult to sample. Thus, quantitative estimation of root parameters is still inadequate and limited. Estimating root parameters accurately by new root analysis methods is consequently becoming a critical task in ecological studies. Ground Penetrating Radar (GPR), also named as geological radar, is a geophysical technology to detect the distribution of the subsurface objects. The basic structure of GPR is shown in Fig.1. Electromagnetic wave can reflect on the interface of the different mediums with different permittivity. Therefore, through analyzing the reflective signals, the underground targets can be detected. Different mediums with different permittivity can form an electrical interface, where electromagnetic wave can be reflected. When scanning by GPR, radar transmitter 1 produces the high radiofrequency wave and it is sent into underground as broadband short pulse by 2 transmitting antenna 11. Then, the wave is reflected by stratum 2 or underground objects 3 and received by receiving antenna 12. After that, the detected signals are sampled and processed in the receiving machine. When electromagnetic wave is propagating in the underground medium, the propagating path, intensity and waveform are varied with the change of the electronic characteristic and geometric shape of the mediums. Therefore, the depth, structure, characteristic and spatial distribution of underground targets can be interpreted according to the delaying time, waveform and frequency characteristic of the echo. However, study on the mapping distribution of roots by GPR is only qualitative; study on the quantitative analysis is still on-going. SUMMERY OF THE INVENTION The objects of present invention are to provide a method for measuring biomass of tree-roots by ground-penetrating radar to avoid the above problem. In detail, the present invention provides a method for measuring the biomass of tree-roots by GPR, which can be used to precisely measure biomass of tree-roots by GPR and the measured result is highly correlated with actual biomass of roots. In order to solve the above problem, the present invention disclose a method for measuring biomass of tree-roots by ground-penetrating radar, includes following steps: a) according to the scanned data of GPR, determine the location of tree-root and acquire the corresponding reflected waveform; b) select the reflected waveform crossing the center of target root; c) extract five waveform parameters from the acquired waveform, which are respectively the maximum amplitude (Max-A), the minimum amplitude (M/n-A), the times between successive zero crossing corresponding to the two amplitudes of the reflected waveform (A and 4 2 ), and the round trip time of radar wave from ground surface to root (); d) formulate three comprehensive parameters derived from five waveform parameters, Pmax_A, PminA and Pampd, defined as followed: PmaxA=MaxAxvt/2 Pmin A=Min Axvt/2 Pamp_d=Pmax_Axt1/ 2 +Pmin_AXt2/ 2 e) acquire the biomass of target root through equation W=KxPampd , where W 3 is the biomass of tree-roots, K is the characteristic coefficient of the area. Preferably, the method for determining the said characteristic coefficient K including following steps: dig out the tree-root and measure the actual biomass W 1 ; the dug out root located in the same area of the target roots; derive the comprehensive parameter Pamp_dl; acquire characteristic coefficient through K=W1/Pamp_d1. Preferably, the total biomass of target roots is: JWi=K*YPampdi wherein ZWi is the total biomass, K is the characteristic coefficient, and ZPampdi is the sum of comprehensive parameters of all the roots in the area. The features and advantages of the present invention are that it can measure the biomass of tree-roots by GPR precisely, and the measured result is well correlated with actual biomass of roots. DESCRIPTION OF THE DRAWING The following drawing is only for the purpose of description and explanation but not for limitation, wherein: Fig. 1 illustratiing the measuring principle of GPR; Fig. 2 illustrating the typical single pulse waveform of underground object; Fig.3 illustratiing the reflected waveform crossing the center of target root; Fig.4-6 illustratiing the relationship between actual biomass and the comprehensive indices derived from three frequencies (2GHz, 900MHz, 500MHz). DESCRIPTION OF PREFERRED EMBODIMENT In order that the present invention can be more readily understood, reference will now be made to the accompanying drawing to illustrate the embodiments of the present invention, and the like numerals refers to the like component. EXAMPLE 1: MEASURING METHOD In this invention, we apply the one-dimensional measurement by GPR for precisely measuring the biomass of tree-roots. GPR records a single pulse reflective waveform at a certain location (xi, yi) of plant root. Fig.2 shows a typical single pulse reflective waveform of underground target. The only variable of the waveform is time. In the following description, the scanned data is named as 'scanned reflected 4 waveform data'. The scanned reflected waveform data acquired from Fig.2 is the unprocessed data. As Fig.2 shows, the reflectance amplitude of target is much weaker than reflectance amplitude of air-ground interface, which is regarded as the background noise wave, which usually covers the reflective signal of targets. Consequently, it should be removed in the processing for enhancing the characteristic of targets. The reflective waveform after background noise removal is shown in Fig.3. In the case application of this invention, we located the plant root firstly, and then select the waveform crossing center of target root (Fig.3). The waveform shown in Fig.3 has been conducted the noise removal, which compress the ground signal and enhance the target signal. The method of locating underground objective can be referenced in 'Research on Automatic Target Detection and Orientation of Ground Penetrating Radar in Shallow Subsurface Application' (Zhang Chun-cheng, Zhou Zheng-ou, JOURNAL OF ELECTRONICS & INFORMATION TECHNOLOGY, 2005, 7, 1065-1068) .This patent referred the full article for reference. According to the waveform in Fig.3, we extracted five wave form parameters from the acquired wave form. The five parameters are respectively the maximum amplitude (Max-A), the minimum amplitude (M/n-A), the times between successive zero crossing corresponding to the two amplitudes of the reflected waveform (A and 4 2 ), and the round trip time of radar wave from ground surface to root (t). Although the defined parameters were extracted from noise removal waveform, however, the technicists should understand that the parameters can also be extracted from unprocessed data. However the parameters are difficult to be extracted for the unprocessed data because the noise wave can cover the reflective signal of targets. Therefore it is reasonable to process the original data before parameter extraction, and this procedure can not affect the reliability of data. The reason why we select these five parameters includes: (1) the traces passing through the center of each root in each profile were collected when the radar was passing right above the root; (2) It is easy to extract parameters of the reflective waveform on this line. More important, the amplitude denotes the energy power reflected by the roots and it is related with the characteristics of detecting target. Then, formulate three comprehensive parameters derived from five waveform parameters, Pmax_A, PminA and Pampd, defined as followed: PmaxA=MaxAxvt/2 5 Pmin A=Min Axvt/2 Pampd=PmaxAXt1/ 2 +Pmin_AXt2/ 2 wherein, MaxA and MinA denote the maximum amplitude and the minimum amplitude, t1 amd t2 denote the times between successive zero crossing corresponding to the two amplitudes of the reflected waveform, and tis the round trip time of radar wave from ground surface to root. As Fig.3 shows, v is the velocity of radar wave in the position of root. It is seen that vt/2 denotes the depth of detected root (h), therefore, the parameters can be rewritten as: PmaxA=MaxAxh Pmin A=Min Axh Pamp d=PmaxAXt/ 2 +Pmin_AXt2/ 2 However, the depth of root can only be acquired indirectly. In this invention, the depth of root (h) is expressed using the waveform parameters for clearer description. There are many methods to estimate the wave velocity at the postion of root. For example, referencing ' Research on Wave Velocity Estimation for Ground Penetrating Radar' (author, Hu Jin-feng, Kong Ling-jiang, Zhou Zheng-ou, Joural of Electronics & Information Technology, 2006, 28 : 2003-2006); or' Estimation of EM Wave Velocity in Detection of Underground Target by GPR ' (Author: Yu Jing-lan, Wang Chun-he, Progress in Geophysics, 2003,18 :477-480). This patent referred the full articles for reference. In ' Estimation of EM Wave Velocity in Detection of Underground Target by GPR', it mentioned that the most simple method estimating radar wave velocity is the method based on the relative permittivity v=c/ ,where c is the velocity in the air, relative permittivity can be acquired by permittivity tester or empirically. Finally, we can estimate the root biomass W as followed W=KxPamp_d Wherein, K is the characteristic coefficient of the study area. K has the same value for the same plant roots in the same area. K has different values even for the same plant roots but in different areas because of the different climate and soil characteristics. K is totally different for the different plant roots in different areas. However, for a particular area, like grassland i[Mongolia, the species composition, climate and soil characteristic is similar in the whole area. Therefore, it is reasonable to regard K value is a constant in this specify area. The basic feature of this method is proposing a comprehensive parameter Pampd, 6 which is used to estimate the root biomass. The referenced technology documents used to extract the parameters is not features of this invention. This patent referenced the full articles for simplifying the description, making it easy to be understood by technicists. The referenced articles disclose all the technologies, therefore, the technicists can acquire the technologies without other labors. EXAMPLE 2: MEASURING PARAMETER K According to the example 1, the root biomass M is well correlated with comprehensive parameter Pamp_d. Therefore, K is a necessary parameter, which can be acquired using following method. Firstly, dig out the tree-root scanned in the example 1 and measure the actual biomass Wi. Then, derive the comprehensive parameter Pamp-dl from the corresponding scanned waveform. Finally, acquire characteristic coefficient through K=W1/Pamp_d1. In the other word, this invention formulate a index Pamp-d , which is positively correlated with root biomass. Therefore, the coefficient K could be acquired through measuring the actual biomass of plant roots. In the same region, K can be regarded as a constant. Therefore, after measuring all the plant roots in a specific area, the corresponding Pamp_di can be acquired. Finally, the total biomass in the specific area is: Wi= K*ZPampdi=W1/Pamp_d1 *YPampdi EXAMPLE 3 In this example, the correlation between actual root biomass and three parameters derived from radar wave were investigated and shown in Tablel and Table2. Table 1 shows the fresh biomass, while the table 2 shows the dry biomass. Table 1 PmaxA PminA Pamp_d R P R P R P 2GHz 0.743 <0.001 0.655 <0.001 0.806 <0.001 900MHz 0.902 <0.001 0.892 <0.001 0.908 <0.001 500MHz 0.900 <0.001 0.901 <0.001 0.929 <0.001 Table 2 PmaxA PminA Pamp_d R P R P R P 7 2GHz 0.722 <0.001 0.633 <0.001 0.786 <0.001 900MHz 0.894 <0.001 0.881 <0.001 0.897 <0.001 500MHz 0.892 <0.001 0.891 <0.001 0.920 <0.001 As the tables show, there are significant positive correlations between the parameters and actual biomass. R is ranged from 0.63 to 0.93 (P<0.001). It can be seen that the parameters are better correlated with fresh biomass than dry biomass. Pampd can indicate the biomass best compare with other two parameters for the same frequency. In the three frequencies, 500MHz is best correlated with biomass, indicating that 500Mhz is sensitive to the biomass. Therefore, the proposed parameter Pampd in this invention is effective for measuring the plant root biomass. EXAMPLE 4 In this example, /ampd was selected as the independent variable. Linear regression analysis was conducted on the relationship between root biomass and the corresponding Pampd on three frequencies (2GHz, 900MHz, 500MHz). It is seen that the correlation between them is good (determine coefficients R 2 are 0.629, 0.828 and 0.867 ). The determine coefficients R 2 increased with the radar frequency decreased. This invention proposed a comprehensive energy parameter Pamp for measuring the root biomass. From the examples, Pamdp on three frequencies are found to be effective for estimating single root biomass. The plant root biomass estimating model based on the index predicts a consistent biomass with actual measured biomass, indicating the outperformance of the new index. Especially for the 500MHz frequency antenna, Pampd performs best. Therefore, we recommended that 500MHz frequency antenna is the best choice for establishing the root biomass estimating model. Whilst the above has been given by way of illustrative examples of the present invention, many variations and modifications thereto will be apparent to those skilled in the art without departing from the broad ambit and scope of the invention as herein set forth in the following claims.