基于成像光谱技术预测氮素在土壤剖面中的垂直分布
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国家自然科学基金项目 (40801082, 41471179)、中国博士后基金面上项目(2014M561772)资助


Prediction of vertical distribution of soil nitrogen content in soil profile using spectral imaging technique
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    摘要:

    可见—近红外(vis-NIR)高光谱成像技术应用于土壤科学是当前数字土壤研究的新方向。本研究考察了该技术预测土壤剖面氮素垂直分布的可行性。深达1 m的土壤整段剖面(1 000 mm × 170 mm × 65 mm)采自湖北崇阳县,成像光谱仪配备了25 μm狭缝,视场角13.1°的35 mm焦距镜头和1 004 × 1 002像素的面阵CCD,拍摄得到剖面vis-NIR高光谱影像(400 ~ 1 000 nm共753个波段)。对获取的影像先通过几何校正解决影像形变问题,再采用监督分类方法识别提取有效土壤像素,剔除阴影裂缝等无效像素。最后利用室内土样vis-NIR反射光谱建立的土壤全氮校正模型,对3个土壤整段剖面的高光谱影像数据进行全氮(TN)预测制图。结果表明,vis-NIR成像光谱技术对土壤整段剖面TN含量预测效果达到甚至优于经标准制样处理后所建模型精度。但存在纵向局限性,其良好地还原了浅层土壤氮素的分布规律,0 ~ 600 mm为较佳预测深度。

    Abstract:

    The study on soil profiles and the various soil horizons they consist of are of great significance to the studies on soil genesis and development, soil classification, and some other disciplines of soil science. Traditional soil information acquisition methods are both time-and labor-consuming, however, the proximal soil sensing technology can be used to provide soil information of various scales rapidly and periodically, and has been widely applied to researches such as soil resource survey, land quality evaluation, soil classification, soil mapping, etc.. Traditional methods for measuring soil physical and chemical properties are complicated, time-consuming and costly, and can hardly meet the demands for rapid monitoring of changes in soil property. In recent years, the spectrometric technology has extensively been used to quantitatively analyze samples in a fast, simple and non-destructive way in various fields with results. The data acquired with using the imaging technology combined with the spectroscopic technology are high in both spatial resolution and spectral resolution, and contain very rich soil remote sensing information and hence can provide a solid foundation for quantitative monitoring and mapping of soils properties in the horizontal dimension. However, in the light of the researches done by scholars both at home and abroad, it appears that the study of soil science still lacks an imaging technique high in spatial and spectral resolution, specifically for measuring soil total nitrogen (TN) contents in entire soil profiles. At present, quantitative analyses of soil properties mostly use soil samples collected in the topsoil, 0 ~ 150 mm or 0 ~ 200 mm in depth; studies on soil point samples in the profile 0 ~ 1 000 mm in depth are rarely reported, and little has been found in literature on dot samples in the 0 ~ 1 000 mm soil profile, let alone reports on mapping of vertical distribution of soil TN contents in entire soil profiles. In view of this situation, this study is oriented to explore the feasibility of using the technology of vis-NIR imaging spectroscopy in instant prediction of vertical distribution of TN contents in soil profiles. A total of 3 soil profiles (0 ~ 1 000 mm) were collected from Chongyang, Hubei Province, China. Vis-NIR hyperspectral images (753 spectral bands in 400 ~ 1 000 nm) of the profiles were taken with an imaging spectroscoper quipped with a 25 μm slit, a 35mm focus lens 13.1° in angle of field and an area array CCD of 1 004 pixels × 1 002 pixels. First, the digital photos with fixed scenical grid scale taken by a digital camera were put to undergo geometric correction with reference to the hyperspectral images of the profiles to solve the problem of image deformation caused by technological limitations of the spectrometer and the shooting platform, and then modified to 1mm in image precision. Through spatial and spectral dimensional clipping, pixels of the wooden frame and platform background were removed leaving only soil image data (160 pixels×980 pixels) and valid spectral bands (470 ~ 1 000 nm). After geometric correction and clipping, the images were processed with a variety of supervised classification methods. Results show that the minimum distance method is the best at distinguishing invalid data (e.g., shadows and cracks) from soil data. A "sampling panel" method was proposed for strip-sampling with the panel in line with the specified, averaging the samples similarly to ROI and finally solving the problem of scale inconsistency between point samples and profile spectra. Furthermore, a PLSR calibration model was built up based on the spectral data of 10 spot soil samples, and used to predict TN contents in three complete soil profiles based on their spectral images. Results show that the technology of vis-NIR imaging spectroscopy could be used to inverse and map soil TN vertically in profiles with good prediction results. Verification with measured data demonstrates that R2 and RPD was 0.56 and 1.41, respectively, for the 0 ~ 1 000 mm soil layer, which indicates that the prediction method reached the range of rough estimation. And for the 0 ~ 600 mm soil layer, the effects were better with R2 and RPD being 0.87 and 1.76, respectively, which indicates that the technology of vis-NIR imaging spectroscopy might have some limitation in the vertical direction, though it can well restore the soil TN distribution patterns in the topsoil layer, especially in the 0 ~ 600mm soil layer. The above findings demonstrate that this study has preliminarily established a set of procedures for soil TN inversion and mapping using the technology of vis-NIR spectroscopy, and the method is applicable to rough estimation of soil TN contents in whole soil profiles.

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李 硕,汪善勤,史 舟.基于成像光谱技术预测氮素在土壤剖面中的垂直分布[J].土壤学报,2015,52(5):1014-1023. DOI:10.11766/trxb201409030442 Li Shuo, Wang Shanqin, Shi Zhou. Prediction of vertical distribution of soil nitrogen content in soil profile using spectral imaging technique[J]. Acta Pedologica Sinica,2015,52(5):1014-1023.

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  • 收稿日期:2014-09-03
  • 最后修改日期:2015-05-01
  • 录用日期:2015-05-27
  • 在线发布日期: 2015-07-01
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